Add presubmit script to verify that scaled resources are correct dimensions.

ui/resources/resource_check/third_party/pypng/png.py

+#!/usr/bin/env python
+
+# $URL$
+# $Rev$
+
+# png.py - PNG encoder/decoder in pure Python
+#
+# Copyright (C) 2006 Johann C. Rocholl <johann@browsershots.org>
+# Portions Copyright (C) 2009 David Jones <drj@pobox.com>
+# And probably portions Copyright (C) 2006 Nicko van Someren <nicko@nicko.org>
+#
+# Original concept by Johann C. Rocholl.
+#
+# LICENSE (The MIT License)
+#
+# Permission is hereby granted, free of charge, to any person
+# obtaining a copy of this software and associated documentation files
+# (the "Software"), to deal in the Software without restriction,
+# including without limitation the rights to use, copy, modify, merge,
+# publish, distribute, sublicense, and/or sell copies of the Software,
+# and to permit persons to whom the Software is furnished to do so,
+# subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+#
+# Changelog (recent first):
+# 2009-03-11 David: interlaced bit depth < 8 (writing).
+# 2009-03-10 David: interlaced bit depth < 8 (reading).
+# 2009-03-04 David: Flat and Boxed pixel formats.
+# 2009-02-26 David: Palette support (writing).
+# 2009-02-23 David: Bit-depths < 8; better PNM support.
+# 2006-06-17 Nicko: Reworked into a class, faster interlacing.
+# 2006-06-17 Johann: Very simple prototype PNG decoder.
+# 2006-06-17 Nicko: Test suite with various image generators.
+# 2006-06-17 Nicko: Alpha-channel, grey-scale, 16-bit/plane support.
+# 2006-06-15 Johann: Scanline iterator interface for large input files.
+# 2006-06-09 Johann: Very simple prototype PNG encoder.
+
+# Incorporated into Bangai-O Development Tools by drj on 2009-02-11 from
+# http://trac.browsershots.org/browser/trunk/pypng/lib/png.py?rev=2885
+
+# Incorporated into pypng by drj on 2009-03-12 from
+# //depot/prj/bangaio/master/code/png.py#67
+
+
+"""
+Pure Python PNG Reader/Writer
+
+This Python module implements support for PNG images (see PNG
+specification at http://www.w3.org/TR/2003/REC-PNG-20031110/ ). It reads
+and writes PNG files with all allowable bit depths (1/2/4/8/16/24/32/48/64
+bits per pixel) and colour combinations: greyscale (1/2/4/8/16 bit); RGB,
+RGBA, LA (greyscale with alpha) with 8/16 bits per channel; colour mapped
+images (1/2/4/8 bit).  Adam7 interlacing is supported for reading and
+writing.  A number of optional chunks can be specified (when writing)
+and understood (when reading): ``tRNS``, ``bKGD``, ``gAMA``.
+
+For help, type ``import png; help(png)`` in your python interpreter.
+
+A good place to start is the :class:`Reader` and :class:`Writer` classes.
+
+Requires Python 2.3.  Limited support is available for Python 2.2, but
+not everything works.  Best with Python 2.4 and higher.  Installation is
+trivial, but see the ``README.txt`` file (with the source distribution)
+for details.
+
+This file can also be used as a command-line utility to convert
+`Netpbm <http://netpbm.sourceforge.net/>`_ PNM files to PNG, and the reverse conversion from PNG to
+PNM. The interface is similar to that of the ``pnmtopng`` program from
+Netpbm.  Type ``python png.py --help`` at the shell prompt
+for usage and a list of options.
+
+A note on spelling and terminology
+----------------------------------
+
+Generally British English spelling is used in the documentation.  So
+that's "greyscale" and "colour".  This not only matches the author's
+native language, it's also used by the PNG specification.
+
+The major colour models supported by PNG (and hence by PyPNG) are:
+greyscale, RGB, greyscale--alpha, RGB--alpha.  These are sometimes
+referred to using the abbreviations: L, RGB, LA, RGBA.  In this case
+each letter abbreviates a single channel: *L* is for Luminance or Luma or
+Lightness which is the channel used in greyscale images; *R*, *G*, *B* stand
+for Red, Green, Blue, the components of a colour image; *A* stands for
+Alpha, the opacity channel (used for transparency effects, but higher
+values are more opaque, so it makes sense to call it opacity).
+
+A note on formats
+-----------------
+
+When getting pixel data out of this module (reading) and presenting
+data to this module (writing) there are a number of ways the data could
+be represented as a Python value.  Generally this module uses one of
+three formats called "flat row flat pixel", "boxed row flat pixel", and
+"boxed row boxed pixel".  Basically the concern is whether each pixel
+and each row comes in its own little tuple (box), or not.
+
+Consider an image that is 3 pixels wide by 2 pixels high, and each pixel
+has RGB components:
+
+Boxed row flat pixel::
+
+  list([R,G,B, R,G,B, R,G,B],
+       [R,G,B, R,G,B, R,G,B])
+
+Each row appears as its own list, but the pixels are flattened so that
+three values for one pixel simply follow the three values for the previous
+pixel.  This is the most common format used, because it provides a good
+compromise between space and convenience.  PyPNG regards itself as
+at liberty to replace any sequence type with any sufficiently compatible
+other sequence type; in practice each row is an array (from the array
+module), and the outer list is sometimes an iterator rather than an
+explicit list (so that streaming is possible).
+
+Flat row flat pixel::
+
+  [R,G,B, R,G,B, R,G,B,
+   R,G,B, R,G,B, R,G,B]
+
+The entire image is one single giant sequence of colour values.
+Generally an array will be used (to save space), not a list.
+
+Boxed row boxed pixel::
+
+  list([ (R,G,B), (R,G,B), (R,G,B) ],
+       [ (R,G,B), (R,G,B), (R,G,B) ])
+
+Each row appears in its own list, but each pixel also appears in its own
+tuple.  A serious memory burn in Python.
+
+In all cases the top row comes first, and for each row the pixels are
+ordered from left-to-right.  Within a pixel the values appear in the
+order, R-G-B-A (or L-A for greyscale--alpha).
+
+There is a fourth format, mentioned because it is used internally,
+is close to what lies inside a PNG file itself, and has some support
+from the public API.  This format is called packed.  When packed,
+each row is a sequence of bytes (integers from 0 to 255), just as
+it is before PNG scanline filtering is applied.  When the bit depth
+is 8 this is essentially the same as boxed row flat pixel; when the
+bit depth is less than 8, several pixels are packed into each byte;
+when the bit depth is 16 (the only value more than 8 that is supported
+by the PNG image format) each pixel value is decomposed into 2 bytes
+(and `packed` is a misnomer).  This format is used by the
+:meth:`Writer.write_packed` method.  It isn't usually a convenient
+format, but may be just right if the source data for the PNG image
+comes from something that uses a similar format (for example, 1-bit
+BMPs, or another PNG file).
+
+And now, my famous members
+--------------------------
+"""
+
+# http://www.python.org/doc/2.2.3/whatsnew/node5.html
+from __future__ import generators
+
+__version__ = "$URL$ $Rev$"
+
+from array import array
+try: # See :pyver:old
+    import itertools
+except:
+    pass
+import math
+# http://www.python.org/doc/2.4.4/lib/module-operator.html
+import operator
+import struct
+import sys
+import zlib
+# http://www.python.org/doc/2.4.4/lib/module-warnings.html
+import warnings
+
+
+__all__ = ['Image', 'Reader', 'Writer', 'write_chunks', 'from_array']
+
+
+# The PNG signature.
+# http://www.w3.org/TR/PNG/#5PNG-file-signature
+_signature = struct.pack('8B', 137, 80, 78, 71, 13, 10, 26, 10)
+
+_adam7 = ((0, 0, 8, 8),
+          (4, 0, 8, 8),
+          (0, 4, 4, 8),
+          (2, 0, 4, 4),
+          (0, 2, 2, 4),
+          (1, 0, 2, 2),
+          (0, 1, 1, 2))
+
+def group(s, n):
+    # See
+    # http://www.python.org/doc/2.6/library/functions.html#zip
+    return zip(*[iter(s)]*n)
+
+def isarray(x):
+    """Same as ``isinstance(x, array)`` except on Python 2.2, where it
+    always returns ``False``.  This helps PyPNG work on Python 2.2.
+    """
+
+    try:
+        return isinstance(x, array)
+    except:
+        return False
+
+try:  # see :pyver:old
+    array.tostring
+except:
+    def tostring(row):
+        l = len(row)
+        return struct.pack('%dB' % l, *row)
+else:
+    def tostring(row):
+        """Convert row of bytes to string.  Expects `row` to be an
+        ``array``.
+        """
+        return row.tostring()
+
+# Conditionally convert to bytes.  Works on Python 2 and Python 3.
+try:
+    bytes('', 'ascii')
+    def strtobytes(x): return bytes(x, 'iso8859-1')
+    def bytestostr(x): return str(x, 'iso8859-1')
+except:
+    strtobytes = str
+    bytestostr = str
+
+def interleave_planes(ipixels, apixels, ipsize, apsize):
+    """
+    Interleave (colour) planes, e.g. RGB + A = RGBA.
+
+    Return an array of pixels consisting of the `ipsize` elements of data
+    from each pixel in `ipixels` followed by the `apsize` elements of data
+    from each pixel in `apixels`.  Conventionally `ipixels` and
+    `apixels` are byte arrays so the sizes are bytes, but it actually
+    works with any arrays of the same type.  The returned array is the
+    same type as the input arrays which should be the same type as each other.
+    """
+
+    itotal = len(ipixels)
+    atotal = len(apixels)
+    newtotal = itotal + atotal
+    newpsize = ipsize + apsize
+    # Set up the output buffer
+    # See http://www.python.org/doc/2.4.4/lib/module-array.html#l2h-1356
+    out = array(ipixels.typecode)
+    # It's annoying that there is no cheap way to set the array size :-(
+    out.extend(ipixels)
+    out.extend(apixels)
+    # Interleave in the pixel data
+    for i in range(ipsize):
+        out[i:newtotal:newpsize] = ipixels[i:itotal:ipsize]
+    for i in range(apsize):
+        out[i+ipsize:newtotal:newpsize] = apixels[i:atotal:apsize]
+    return out
+
+def check_palette(palette):
+    """Check a palette argument (to the :class:`Writer` class) for validity.
+    Returns the palette as a list if okay; raises an exception otherwise.
+    """
+
+    # None is the default and is allowed.
+    if palette is None:
+        return None
+
+    p = list(palette)
+    if not (0 < len(p) <= 256):
+        raise ValueError("a palette must have between 1 and 256 entries")
+    seen_triple = False
+    for i,t in enumerate(p):
+        if len(t) not in (3,4):
+            raise ValueError(
+              "palette entry %d: entries must be 3- or 4-tuples." % i)
+        if len(t) == 3:
+            seen_triple = True
+        if seen_triple and len(t) == 4:
+            raise ValueError(
+              "palette entry %d: all 4-tuples must precede all 3-tuples" % i)
+        for x in t:
+            if int(x) != x or not(0 <= x <= 255):
+                raise ValueError(
+                  "palette entry %d: values must be integer: 0 <= x <= 255" % i)
+    return p
+
+class Error(Exception):
+    prefix = 'Error'
+    def __str__(self):
+        return self.prefix + ': ' + ' '.join(self.args)
+
+class FormatError(Error):
+    """Problem with input file format.  In other words, PNG file does
+    not conform to the specification in some way and is invalid.
+    """
+
+    prefix = 'FormatError'
+
+class ChunkError(FormatError):
+    prefix = 'ChunkError'
+
+
+class Writer:
+    """
+    PNG encoder in pure Python.
+    """
+
+    def __init__(self, width=None, height=None,
+                 size=None,
+                 greyscale=False,
+                 alpha=False,
+                 bitdepth=8,
+                 palette=None,
+                 transparent=None,
+                 background=None,
+                 gamma=None,
+                 compression=None,
+                 interlace=False,
+                 bytes_per_sample=None, # deprecated
+                 planes=None,
+                 colormap=None,
+                 maxval=None,
+                 chunk_limit=2**20):
+        """
+        Create a PNG encoder object.
+
+        Arguments:
+
+        width, height
+          Image size in pixels, as two separate arguments.
+        size
+          Image size (w,h) in pixels, as single argument.
+        greyscale
+          Input data is greyscale, not RGB.
+        alpha
+          Input data has alpha channel (RGBA or LA).
+        bitdepth
+          Bit depth: from 1 to 16.
+        palette
+          Create a palette for a colour mapped image (colour type 3).
+        transparent
+          Specify a transparent colour (create a ``tRNS`` chunk).
+        background
+          Specify a default background colour (create a ``bKGD`` chunk).
+        gamma
+          Specify a gamma value (create a ``gAMA`` chunk).
+        compression
+          zlib compression level (1-9).
+        interlace
+          Create an interlaced image.
+        chunk_limit
+          Write multiple ``IDAT`` chunks to save memory.
+
+        The image size (in pixels) can be specified either by using the
+        `width` and `height` arguments, or with the single `size`
+        argument.  If `size` is used it should be a pair (*width*,
+        *height*).
+
+        `greyscale` and `alpha` are booleans that specify whether
+        an image is greyscale (or colour), and whether it has an
+        alpha channel (or not).
+
+        `bitdepth` specifies the bit depth of the source pixel values.
+        Each source pixel value must be an integer between 0 and
+        ``2**bitdepth-1``.  For example, 8-bit images have values
+        between 0 and 255.  PNG only stores images with bit depths of
+        1,2,4,8, or 16.  When `bitdepth` is not one of these values,
+        the next highest valid bit depth is selected, and an ``sBIT``
+        (significant bits) chunk is generated that specifies the original
+        precision of the source image.  In this case the supplied pixel
+        values will be rescaled to fit the range of the selected bit depth.
+
+        The details of which bit depth / colour model combinations the
+        PNG file format supports directly, are somewhat arcane
+        (refer to the PNG specification for full details).  Briefly:
+        "small" bit depths (1,2,4) are only allowed with greyscale and
+        colour mapped images; colour mapped images cannot have bit depth
+        16.
+
+        For colour mapped images (in other words, when the `palette`
+        argument is specified) the `bitdepth` argument must match one of
+        the valid PNG bit depths: 1, 2, 4, or 8.  (It is valid to have a
+        PNG image with a palette and an ``sBIT`` chunk, but the meaning
+        is slightly different; it would be awkward to press the
+        `bitdepth` argument into service for this.)
+
+        The `palette` option, when specified, causes a colour mapped image
+        to be created: the PNG colour type is set to 3; greyscale
+        must not be set; alpha must not be set; transparent must
+        not be set; the bit depth must be 1,2,4, or 8.  When a colour
+        mapped image is created, the pixel values are palette indexes
+        and the `bitdepth` argument specifies the size of these indexes
+        (not the size of the colour values in the palette).
+
+        The palette argument value should be a sequence of 3- or
+        4-tuples.  3-tuples specify RGB palette entries; 4-tuples
+        specify RGBA palette entries.  If both 4-tuples and 3-tuples
+        appear in the sequence then all the 4-tuples must come
+        before all the 3-tuples.  A ``PLTE`` chunk is created; if there
+        are 4-tuples then a ``tRNS`` chunk is created as well.  The
+        ``PLTE`` chunk will contain all the RGB triples in the same
+        sequence; the ``tRNS`` chunk will contain the alpha channel for
+        all the 4-tuples, in the same sequence.  Palette entries
+        are always 8-bit.
+
+        If specified, the `transparent` and `background` parameters must
+        be a tuple with three integer values for red, green, blue, or
+        a simple integer (or singleton tuple) for a greyscale image.
+
+        If specified, the `gamma` parameter must be a positive number
+        (generally, a float).  A ``gAMA`` chunk will be created.  Note that
+        this will not change the values of the pixels as they appear in
+        the PNG file, they are assumed to have already been converted
+        appropriately for the gamma specified.
+
+        The `compression` argument specifies the compression level
+        to be used by the ``zlib`` module.  Higher values are likely
+        to compress better, but will be slower to compress.  The
+        default for this argument is ``None``; this does not mean
+        no compression, rather it means that the default from the
+        ``zlib`` module is used (which is generally acceptable).
+
+        If `interlace` is true then an interlaced image is created
+        (using PNG's so far only interace method, *Adam7*).  This does not
+        affect how the pixels should be presented to the encoder, rather
+        it changes how they are arranged into the PNG file.  On slow
+        connexions interlaced images can be partially decoded by the
+        browser to give a rough view of the image that is successively
+        refined as more image data appears.
+        
+        .. note ::
+        
+          Enabling the `interlace` option requires the entire image
+          to be processed in working memory.
+
+        `chunk_limit` is used to limit the amount of memory used whilst
+        compressing the image.  In order to avoid using large amounts of
+        memory, multiple ``IDAT`` chunks may be created.
+        """
+
+        # At the moment the `planes` argument is ignored;
+        # its purpose is to act as a dummy so that
+        # ``Writer(x, y, **info)`` works, where `info` is a dictionary
+        # returned by Reader.read and friends.
+        # Ditto for `colormap`.
+
+        # A couple of helper functions come first.  Best skipped if you
+        # are reading through.
+
+        def isinteger(x):
+            try:
+                return int(x) == x
+            except:
+                return False
+
+        def check_color(c, which):
+            """Checks that a colour argument for transparent or
+            background options is the right form.  Also "corrects" bare
+            integers to 1-tuples.
+            """
+
+            if c is None:
+                return c
+            if greyscale:
+                try:
+                    l = len(c)
+                except TypeError:
+                    c = (c,)
+                if len(c) != 1:
+                    raise ValueError("%s for greyscale must be 1-tuple" %
+                        which)
+                if not isinteger(c[0]):
+                    raise ValueError(
+                        "%s colour for greyscale must be integer" %
+                        which)
+            else:
+                if not (len(c) == 3 and
+                        isinteger(c[0]) and
+                        isinteger(c[1]) and
+                        isinteger(c[2])):
+                    raise ValueError(
+                        "%s colour must be a triple of integers" %
+                        which)
+            return c
+
+        if size:
+            if len(size) != 2:
+                raise ValueError(
+                  "size argument should be a pair (width, height)")
+            if width is not None and width != size[0]:
+                raise ValueError(
+                  "size[0] (%r) and width (%r) should match when both are used."
+                    % (size[0], width))
+            if height is not None and height != size[1]:
+                raise ValueError(
+                  "size[1] (%r) and height (%r) should match when both are used."
+                    % (size[1], height))
+            width,height = size
+        del size
+
+        if width <= 0 or height <= 0:
+            raise ValueError("width and height must be greater than zero")
+        if not isinteger(width) or not isinteger(height):
+            raise ValueError("width and height must be integers")
+        # http://www.w3.org/TR/PNG/#7Integers-and-byte-order
+        if width > 2**32-1 or height > 2**32-1:
+            raise ValueError("width and height cannot exceed 2**32-1")
+
+        if alpha and transparent is not None:
+            raise ValueError(
+                "transparent colour not allowed with alpha channel")
+
+        if bytes_per_sample is not None:
+            warnings.warn('please use bitdepth instead of bytes_per_sample',
+                          DeprecationWarning)
+            if bytes_per_sample not in (0.125, 0.25, 0.5, 1, 2):
+                raise ValueError(
+                    "bytes per sample must be .125, .25, .5, 1, or 2")
+            bitdepth = int(8*bytes_per_sample)
+        del bytes_per_sample
+        if not isinteger(bitdepth) or bitdepth < 1 or 16 < bitdepth:
+            raise ValueError("bitdepth (%r) must be a postive integer <= 16" %
+              bitdepth)
+
+        self.rescale = None
+        if palette:
+            if bitdepth not in (1,2,4,8):
+                raise ValueError("with palette, bitdepth must be 1, 2, 4, or 8")
+            if transparent is not None:
+                raise ValueError("transparent and palette not compatible")
+            if alpha:
+                raise ValueError("alpha and palette not compatible")
+            if greyscale:
+                raise ValueError("greyscale and palette not compatible")
+        else:
+            # No palette, check for sBIT chunk generation.
+            if alpha or not greyscale:
+                if bitdepth not in (8,16):
+                    targetbitdepth = (8,16)[bitdepth > 8]
+                    self.rescale = (bitdepth, targetbitdepth)
+                    bitdepth = targetbitdepth
+                    del targetbitdepth
+            else:
+                assert greyscale
+                assert not alpha
+                if bitdepth not in (1,2,4,8,16):
+                    if bitdepth > 8:
+                        targetbitdepth = 16
+                    elif bitdepth == 3:
+                        targetbitdepth = 4
+                    else:
+                        assert bitdepth in (5,6,7)
+                        targetbitdepth = 8
+                    self.rescale = (bitdepth, targetbitdepth)
+                    bitdepth = targetbitdepth
+                    del targetbitdepth
+
+        if bitdepth < 8 and (alpha or not greyscale and not palette):
+            raise ValueError(
+              "bitdepth < 8 only permitted with greyscale or palette")
+        if bitdepth > 8 and palette:
+            raise ValueError(
+                "bit depth must be 8 or less for images with palette")
+
+        transparent = check_color(transparent, 'transparent')
+        background = check_color(background, 'background')
+
+        # It's important that the true boolean values (greyscale, alpha,
+        # colormap, interlace) are converted to bool because Iverson's
+        # convention is relied upon later on.
+        self.width = width
+        self.height = height
+        self.transparent = transparent
+        self.background = background
+        self.gamma = gamma
+        self.greyscale = bool(greyscale)
+        self.alpha = bool(alpha)
+        self.colormap = bool(palette)
+        self.bitdepth = int(bitdepth)
+        self.compression = compression
+        self.chunk_limit = chunk_limit
+        self.interlace = bool(interlace)
+        self.palette = check_palette(palette)
+
+        self.color_type = 4*self.alpha + 2*(not greyscale) + 1*self.colormap
+        assert self.color_type in (0,2,3,4,6)
+
+        self.color_planes = (3,1)[self.greyscale or self.colormap]
+        self.planes = self.color_planes + self.alpha
+        # :todo: fix for bitdepth < 8
+        self.psize = (self.bitdepth/8) * self.planes
+
+    def make_palette(self):
+        """Create the byte sequences for a ``PLTE`` and if necessary a
+        ``tRNS`` chunk.  Returned as a pair (*p*, *t*).  *t* will be
+        ``None`` if no ``tRNS`` chunk is necessary.
+        """
+
+        p = array('B')
+        t = array('B')
+
+        for x in self.palette:
+            p.extend(x[0:3])
+            if len(x) > 3:
+                t.append(x[3])
+        p = tostring(p)
+        t = tostring(t)
+        if t:
+            return p,t
+        return p,None
+
+    def write(self, outfile, rows):
+        """Write a PNG image to the output file.  `rows` should be
+        an iterable that yields each row in boxed row flat pixel format.
+        The rows should be the rows of the original image, so there
+        should be ``self.height`` rows of ``self.width * self.planes`` values.
+        If `interlace` is specified (when creating the instance), then
+        an interlaced PNG file will be written.  Supply the rows in the
+        normal image order; the interlacing is carried out internally.
+        
+        .. note ::
+
+          Interlacing will require the entire image to be in working memory.
+        """
+
+        if self.interlace:
+            fmt = 'BH'[self.bitdepth > 8]
+            a = array(fmt, itertools.chain(*rows))
+            return self.write_array(outfile, a)
+        else:
+            nrows = self.write_passes(outfile, rows)
+            if nrows != self.height:
+                raise ValueError(
+                  "rows supplied (%d) does not match height (%d)" %
+                  (nrows, self.height))
+
+    def write_passes(self, outfile, rows, packed=False):
+        """
+        Write a PNG image to the output file.
+
+        Most users are expected to find the :meth:`write` or
+        :meth:`write_array` method more convenient.
+        
+        The rows should be given to this method in the order that
+        they appear in the output file.  For straightlaced images,
+        this is the usual top to bottom ordering, but for interlaced
+        images the rows should have already been interlaced before
+        passing them to this function.
+
+        `rows` should be an iterable that yields each row.  When
+        `packed` is ``False`` the rows should be in boxed row flat pixel
+        format; when `packed` is ``True`` each row should be a packed
+        sequence of bytes.
+
+        """
+
+        # http://www.w3.org/TR/PNG/#5PNG-file-signature
+        outfile.write(_signature)
+
+        # http://www.w3.org/TR/PNG/#11IHDR
+        write_chunk(outfile, 'IHDR',
+                    struct.pack("!2I5B", self.width, self.height,
+                                self.bitdepth, self.color_type,
+                                0, 0, self.interlace))
+
+        # See :chunk:order
+        # http://www.w3.org/TR/PNG/#11gAMA
+        if self.gamma is not None:
+            write_chunk(outfile, 'gAMA',
+                        struct.pack("!L", int(round(self.gamma*1e5))))
+
+        # See :chunk:order
+        # http://www.w3.org/TR/PNG/#11sBIT
+        if self.rescale:
+            write_chunk(outfile, 'sBIT',
+                struct.pack('%dB' % self.planes,
+                            *[self.rescale[0]]*self.planes))
+        
+        # :chunk:order: Without a palette (PLTE chunk), ordering is
+        # relatively relaxed.  With one, gAMA chunk must precede PLTE
+        # chunk which must precede tRNS and bKGD.
+        # See http://www.w3.org/TR/PNG/#5ChunkOrdering
+        if self.palette:
+            p,t = self.make_palette()
+            write_chunk(outfile, 'PLTE', p)
+            if t:
+                # tRNS chunk is optional.  Only needed if palette entries
+                # have alpha.
+                write_chunk(outfile, 'tRNS', t)
+
+        # http://www.w3.org/TR/PNG/#11tRNS
+        if self.transparent is not None:
+            if self.greyscale:
+                write_chunk(outfile, 'tRNS',
+                            struct.pack("!1H", *self.transparent))
+            else:
+                write_chunk(outfile, 'tRNS',
+                            struct.pack("!3H", *self.transparent))
+
+        # http://www.w3.org/TR/PNG/#11bKGD
+        if self.background is not None:
+            if self.greyscale:
+                write_chunk(outfile, 'bKGD',
+                            struct.pack("!1H", *self.background))
+            else:
+                write_chunk(outfile, 'bKGD',
+                            struct.pack("!3H", *self.background))
+
+        # http://www.w3.org/TR/PNG/#11IDAT
+        if self.compression is not None:
+            compressor = zlib.compressobj(self.compression)
+        else:
+            compressor = zlib.compressobj()
+
+        # Choose an extend function based on the bitdepth.  The extend
+        # function packs/decomposes the pixel values into bytes and
+        # stuffs them onto the data array.
+        data = array('B')
+        if self.bitdepth == 8 or packed:
+            extend = data.extend
+        elif self.bitdepth == 16:
+            # Decompose into bytes
+            def extend(sl):
+                fmt = '!%dH' % len(sl)
+                data.extend(array('B', struct.pack(fmt, *sl)))
+        else:
+            # Pack into bytes
+            assert self.bitdepth < 8
+            # samples per byte
+            spb = int(8/self.bitdepth)
+            def extend(sl):
+                a = array('B', sl)
+                # Adding padding bytes so we can group into a whole
+                # number of spb-tuples.
+                l = float(len(a))
+                extra = math.ceil(l / float(spb))*spb - l
+                a.extend([0]*int(extra))
+                # Pack into bytes
+                l = group(a, spb)
+                l = map(lambda e: reduce(lambda x,y:
+                                           (x << self.bitdepth) + y, e), l)
+                data.extend(l)
+        if self.rescale:
+            oldextend = extend
+            factor = \
+              float(2**self.rescale[1]-1) / float(2**self.rescale[0]-1)
+            def extend(sl):
+                oldextend(map(lambda x: int(round(factor*x)), sl))
+
+        # Build the first row, testing mostly to see if we need to
+        # changed the extend function to cope with NumPy integer types
+        # (they cause our ordinary definition of extend to fail, so we
+        # wrap it).  See
+        # http://code.google.com/p/pypng/issues/detail?id=44
+        enumrows = enumerate(rows)
+        del rows
+
+        # First row's filter type.
+        data.append(0)
+        # :todo: Certain exceptions in the call to ``.next()`` or the
+        # following try would indicate no row data supplied.
+        # Should catch.
+        i,row = enumrows.next()
+        try:
+            # If this fails...
+            extend(row)
+        except:
+            # ... try a version that converts the values to int first.
+            # Not only does this work for the (slightly broken) NumPy
+            # types, there are probably lots of other, unknown, "nearly"
+            # int types it works for.
+            def wrapmapint(f):
+                return lambda sl: f(map(int, sl))
+            extend = wrapmapint(extend)
+            del wrapmapint
+            extend(row)
+
+        for i,row in enumrows:
+            # Add "None" filter type.  Currently, it's essential that
+            # this filter type be used for every scanline as we do not
+            # mark the first row of a reduced pass image; that means we
+            # could accidentally compute the wrong filtered scanline if
+            # we used "up", "average", or "paeth" on such a line.
+            data.append(0)
+            extend(row)
+            if len(data) > self.chunk_limit:
+                compressed = compressor.compress(tostring(data))
+                if len(compressed):
+                    # print >> sys.stderr, len(data), len(compressed)
+                    write_chunk(outfile, 'IDAT', compressed)
+                # Because of our very witty definition of ``extend``,
+                # above, we must re-use the same ``data`` object.  Hence
+                # we use ``del`` to empty this one, rather than create a
+                # fresh one (which would be my natural FP instinct).
+                del data[:]
+        if len(data):
+            compressed = compressor.compress(tostring(data))
+        else:
+            compressed = ''
+        flushed = compressor.flush()
+        if len(compressed) or len(flushed):
+            # print >> sys.stderr, len(data), len(compressed), len(flushed)
+            write_chunk(outfile, 'IDAT', compressed + flushed)
+        # http://www.w3.org/TR/PNG/#11IEND
+        write_chunk(outfile, 'IEND')
+        return i+1
+
+    def write_array(self, outfile, pixels):
+        """
+        Write an array in flat row flat pixel format as a PNG file on
+        the output file.  See also :meth:`write` method.
+        """
+
+        if self.interlace:
+            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
+        else:
+            self.write_passes(outfile, self.array_scanlines(pixels))
+
+    def write_packed(self, outfile, rows):
+        """
+        Write PNG file to `outfile`.  The pixel data comes from `rows`
+        which should be in boxed row packed format.  Each row should be
+        a sequence of packed bytes.
+
+        Technically, this method does work for interlaced images but it
+        is best avoided.  For interlaced images, the rows should be
+        presented in the order that they appear in the file.
+
+        This method should not be used when the source image bit depth
+        is not one naturally supported by PNG; the bit depth should be
+        1, 2, 4, 8, or 16.
+        """
+
+        if self.rescale:
+            raise Error("write_packed method not suitable for bit depth %d" %
+              self.rescale[0])
+        return self.write_passes(outfile, rows, packed=True)
+
+    def convert_pnm(self, infile, outfile):
+        """
+        Convert a PNM file containing raw pixel data into a PNG file
+        with the parameters set in the writer object.  Works for
+        (binary) PGM, PPM, and PAM formats.
+        """
+
+        if self.interlace:
+            pixels = array('B')
+            pixels.fromfile(infile,
+                            (self.bitdepth/8) * self.color_planes *
+                            self.width * self.height)
+            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
+        else:
+            self.write_passes(outfile, self.file_scanlines(infile))
+
+    def convert_ppm_and_pgm(self, ppmfile, pgmfile, outfile):
+        """
+        Convert a PPM and PGM file containing raw pixel data into a
+        PNG outfile with the parameters set in the writer object.
+        """
+        pixels = array('B')
+        pixels.fromfile(ppmfile,
+                        (self.bitdepth/8) * self.color_planes *
+                        self.width * self.height)
+        apixels = array('B')
+        apixels.fromfile(pgmfile,
+                         (self.bitdepth/8) *
+                         self.width * self.height)
+        pixels = interleave_planes(pixels, apixels,
+                                   (self.bitdepth/8) * self.color_planes,
+                                   (self.bitdepth/8))
+        if self.interlace:
+            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
+        else:
+            self.write_passes(outfile, self.array_scanlines(pixels))
+
+    def file_scanlines(self, infile):
+        """
+        Generates boxed rows in flat pixel format, from the input file
+        `infile`.  It assumes that the input file is in a "Netpbm-like"
+        binary format, and is positioned at the beginning of the first
+        pixel.  The number of pixels to read is taken from the image
+        dimensions (`width`, `height`, `planes`) and the number of bytes
+        per value is implied by the image `bitdepth`.
+        """
+
+        # Values per row
+        vpr = self.width * self.planes
+        row_bytes = vpr
+        if self.bitdepth > 8:
+            assert self.bitdepth == 16
+            row_bytes *= 2
+            fmt = '>%dH' % vpr
+            def line():
+                return array('H', struct.unpack(fmt, infile.read(row_bytes)))
+        else:
+            def line():
+                scanline = array('B', infile.read(row_bytes))
+                return scanline
+        for y in range(self.height):
+            yield line()
+
+    def array_scanlines(self, pixels):
+        """
+        Generates boxed rows (flat pixels) from flat rows (flat pixels)
+        in an array.
+        """
+
+        # Values per row
+        vpr = self.width * self.planes
+        stop = 0
+        for y in range(self.height):
+            start = stop
+            stop = start + vpr
+            yield pixels[start:stop]
+
+    def array_scanlines_interlace(self, pixels):
+        """
+        Generator for interlaced scanlines from an array.  `pixels` is
+        the full source image in flat row flat pixel format.  The
+        generator yields each scanline of the reduced passes in turn, in
+        boxed row flat pixel format.
+        """
+
+        # http://www.w3.org/TR/PNG/#8InterlaceMethods
+        # Array type.
+        fmt = 'BH'[self.bitdepth > 8]
+        # Value per row
+        vpr = self.width * self.planes
+        for xstart, ystart, xstep, ystep in _adam7:
+            if xstart >= self.width:
+                continue
+            # Pixels per row (of reduced image)
+            ppr = int(math.ceil((self.width-xstart)/float(xstep)))
+            # number of values in reduced image row.
+            row_len = ppr*self.planes
+            for y in range(ystart, self.height, ystep):
+                if xstep == 1:
+                    offset = y * vpr
+                    yield pixels[offset:offset+vpr]
+                else:
+                    row = array(fmt)
+                    # There's no easier way to set the length of an array
+                    row.extend(pixels[0:row_len])
+                    offset = y * vpr + xstart * self.planes
+                    end_offset = (y+1) * vpr
+                    skip = self.planes * xstep
+                    for i in range(self.planes):
+                        row[i::self.planes] = \
+                            pixels[offset+i:end_offset:skip]
+                    yield row
+
+def write_chunk(outfile, tag, data=strtobytes('')):
+    """
+    Write a PNG chunk to the output file, including length and
+    checksum.
+    """
+
+    # http://www.w3.org/TR/PNG/#5Chunk-layout
+    outfile.write(struct.pack("!I", len(data)))
+    tag = strtobytes(tag)
+    outfile.write(tag)
+    outfile.write(data)
+    checksum = zlib.crc32(tag)
+    checksum = zlib.crc32(data, checksum)
+    checksum &= 2**32-1
+    outfile.write(struct.pack("!I", checksum))
+
+def write_chunks(out, chunks):
+    """Create a PNG file by writing out the chunks."""
+
+    out.write(_signature)
+    for chunk in chunks:
+        write_chunk(out, *chunk)
+
+def filter_scanline(type, line, fo, prev=None):
+    """Apply a scanline filter to a scanline.  `type` specifies the
+    filter type (0 to 4); `line` specifies the current (unfiltered)
+    scanline as a sequence of bytes; `prev` specifies the previous
+    (unfiltered) scanline as a sequence of bytes. `fo` specifies the
+    filter offset; normally this is size of a pixel in bytes (the number
+    of bytes per sample times the number of channels), but when this is
+    < 1 (for bit depths < 8) then the filter offset is 1.
+    """
+
+    assert 0 <= type < 5
+
+    # The output array.  Which, pathetically, we extend one-byte at a
+    # time (fortunately this is linear).
+    out = array('B', [type])
+
+    def sub():
+        ai = -fo
+        for x in line:
+            if ai >= 0:
+                x = (x - line[ai]) & 0xff
+            out.append(x)
+            ai += 1
+    def up():
+        for i,x in enumerate(line):
+            x = (x - prev[i]) & 0xff
+            out.append(x)
+    def average():
+        ai = -fo
+        for i,x in enumerate(line):
+            if ai >= 0:
+                x = (x - ((line[ai] + prev[i]) >> 1)) & 0xff
+            else:
+                x = (x - (prev[i] >> 1)) & 0xff
+            out.append(x)
+            ai += 1
+    def paeth():
+        # http://www.w3.org/TR/PNG/#9Filter-type-4-Paeth
+        ai = -fo # also used for ci
+        for i,x in enumerate(line):
+            a = 0
+            b = prev[i]
+            c = 0
+
+            if ai >= 0:
+                a = line[ai]
+                c = prev[ai]
+            p = a + b - c
+            pa = abs(p - a)
+            pb = abs(p - b)
+            pc = abs(p - c)
+            if pa <= pb and pa <= pc: Pr = a
+            elif pb <= pc: Pr = b
+            else: Pr = c
+
+            x = (x - Pr) & 0xff
+            out.append(x)
+            ai += 1
+
+    if not prev:
+        # We're on the first line.  Some of the filters can be reduced
+        # to simpler cases which makes handling the line "off the top"
+        # of the image simpler.  "up" becomes "none"; "paeth" becomes
+        # "left" (non-trivial, but true). "average" needs to be handled
+        # specially.
+        if type == 2: # "up"
+            return line # type = 0
+        elif type == 3:
+            prev = [0]*len(line)
+        elif type == 4: # "paeth"
+            type = 1
+    if type == 0:
+        out.extend(line)
+    elif type == 1:
+        sub()
+    elif type == 2:
+        up()
+    elif type == 3:
+        average()
+    else: # type == 4
+        paeth()
+    return out
+
+
+def from_array(a, mode=None, info={}):
+    """Create a PNG :class:`Image` object from a 2- or 3-dimensional array.
+    One application of this function is easy PIL-style saving:
+    ``png.from_array(pixels, 'L').save('foo.png')``.
+
+    .. note :
+
+      The use of the term *3-dimensional* is for marketing purposes
+      only.  It doesn't actually work.  Please bear with us.  Meanwhile
+      enjoy the complimentary snacks (on request) and please use a
+      2-dimensional array.
+    
+    Unless they are specified using the *info* parameter, the PNG's
+    height and width are taken from the array size.  For a 3 dimensional
+    array the first axis is the height; the second axis is the width;
+    and the third axis is the channel number.  Thus an RGB image that is
+    16 pixels high and 8 wide will use an array that is 16x8x3.  For 2
+    dimensional arrays the first axis is the height, but the second axis
+    is ``width*channels``, so an RGB image that is 16 pixels high and 8
+    wide will use a 2-dimensional array that is 16x24 (each row will be
+    8*3==24 sample values).
+
+    *mode* is a string that specifies the image colour format in a
+    PIL-style mode.  It can be:
+
+    ``'L'``
+      greyscale (1 channel)
+    ``'LA'``
+      greyscale with alpha (2 channel)
+    ``'RGB'``
+      colour image (3 channel)
+    ``'RGBA'``
+      colour image with alpha (4 channel)
+
+    The mode string can also specify the bit depth (overriding how this
+    function normally derives the bit depth, see below).  Appending
+    ``';16'`` to the mode will cause the PNG to be 16 bits per channel;
+    any decimal from 1 to 16 can be used to specify the bit depth.
+
+    When a 2-dimensional array is used *mode* determines how many
+    channels the image has, and so allows the width to be derived from
+    the second array dimension.
+
+    The array is expected to be a ``numpy`` array, but it can be any
+    suitable Python sequence.  For example, a list of lists can be used:
+    ``png.from_array([[0, 255, 0], [255, 0, 255]], 'L')``.  The exact
+    rules are: ``len(a)`` gives the first dimension, height;
+    ``len(a[0])`` gives the second dimension; ``len(a[0][0])`` gives the
+    third dimension, unless an exception is raised in which case a
+    2-dimensional array is assumed.  It's slightly more complicated than
+    that because an iterator of rows can be used, and it all still
+    works.  Using an iterator allows data to be streamed efficiently.
+
+    The bit depth of the PNG is normally taken from the array element's
+    datatype (but if *mode* specifies a bitdepth then that is used
+    instead).  The array element's datatype is determined in a way which
+    is supposed to work both for ``numpy`` arrays and for Python
+    ``array.array`` objects.  A 1 byte datatype will give a bit depth of
+    8, a 2 byte datatype will give a bit depth of 16.  If the datatype
+    does not have an implicit size, for example it is a plain Python
+    list of lists, as above, then a default of 8 is used.
+
+    The *info* parameter is a dictionary that can be used to specify
+    metadata (in the same style as the arguments to the
+    :class:``png.Writer`` class).  For this function the keys that are
+    useful are:
+    
+    height
+      overrides the height derived from the array dimensions and allows
+      *a* to be an iterable.
+    width
+      overrides the width derived from the array dimensions.
+    bitdepth
+      overrides the bit depth derived from the element datatype (but
+      must match *mode* if that also specifies a bit depth).
+
+    Generally anything specified in the
+    *info* dictionary will override any implicit choices that this
+    function would otherwise make, but must match any explicit ones.
+    For example, if the *info* dictionary has a ``greyscale`` key then
+    this must be true when mode is ``'L'`` or ``'LA'`` and false when
+    mode is ``'RGB'`` or ``'RGBA'``.
+    """
+
+    # We abuse the *info* parameter by modifying it.  Take a copy here.
+    # (Also typechecks *info* to some extent).
+    info = dict(info)
+
+    # Syntax check mode string.
+    bitdepth = None
+    try:
+        mode = mode.split(';')
+        if len(mode) not in (1,2):
+            raise Error()
+        if mode[0] not in ('L', 'LA', 'RGB', 'RGBA'):
+            raise Error()
+        if len(mode) == 2:
+            try:
+                bitdepth = int(mode[1])
+            except:
+                raise Error()
+    except Error:
+        raise Error("mode string should be 'RGB' or 'L;16' or similar.")
+    mode = mode[0]
+
+    # Get bitdepth from *mode* if possible.
+    if bitdepth:
+        if info.get('bitdepth') and bitdepth != info['bitdepth']:
+            raise Error("mode bitdepth (%d) should match info bitdepth (%d)." %
+              (bitdepth, info['bitdepth']))
+        info['bitdepth'] = bitdepth
+
+    # Fill in and/or check entries in *info*.
+    # Dimensions.
+    if 'size' in info:
+        # Check width, height, size all match where used.
+        for dimension,axis in [('width', 0), ('height', 1)]:
+            if dimension in info:
+                if info[dimension] != info['size'][axis]:
+                    raise Error(
+                      "info[%r] shhould match info['size'][%r]." %
+                      (dimension, axis))
+        info['width'],info['height'] = info['size']
+    if 'height' not in info:
+        try:
+            l = len(a)
+        except:
+            raise Error(
+              "len(a) does not work, supply info['height'] instead.")
+        info['height'] = l
+    # Colour format.
+    if 'greyscale' in info:
+        if bool(info['greyscale']) != ('L' in mode):
+            raise Error("info['greyscale'] should match mode.")
+    info['greyscale'] = 'L' in mode
+    if 'alpha' in info:
+        if bool(info['alpha']) != ('A' in mode):
+            raise Error("info['alpha'] should match mode.")
+    info['alpha'] = 'A' in mode
+
+    planes = len(mode)
+    if 'planes' in info:
+        if info['planes'] != planes:
+            raise Error("info['planes'] should match mode.")
+
+    # In order to work out whether we the array is 2D or 3D we need its
+    # first row, which requires that we take a copy of its iterator.
+    # We may also need the first row to derive width and bitdepth.
+    a,t = itertools.tee(a)
+    row = t.next()
+    del t
+    try:
+        row[0][0]
+        threed = True
+        testelement = row[0]
+    except:
+        threed = False
+        testelement = row
+    if 'width' not in info:
+        if threed:
+            width = len(row)
+        else:
+            width = len(row) // planes
+        info['width'] = width
+
+    # Not implemented yet
+    assert not threed
+
+    if 'bitdepth' not in info:
+        try:
+            dtype = testelement.dtype
+            # goto the "else:" clause.  Sorry.
+        except:
+            try:
+                # Try a Python array.array.
+                bitdepth = 8 * testelement.itemsize
+            except:
+                # We can't determine it from the array element's
+                # datatype, use a default of 8.
+                bitdepth = 8
+        else:
+            # If we got here without exception, we now assume that
+            # the array is a numpy array.
+            if dtype.kind == 'b':
+                bitdepth = 1
+            else:
+                bitdepth = 8 * dtype.itemsize
+        info['bitdepth'] = bitdepth
+
+    for thing in 'width height bitdepth greyscale alpha'.split():
+        assert thing in info
+    return Image(a, info)
+
+# So that refugee's from PIL feel more at home.  Not documented.
+fromarray = from_array
+
+class Image:
+    """A PNG image.
+    You can create an :class:`Image` object from an array of pixels by calling
+    :meth:`png.from_array`.  It can be saved to disk with the
+    :meth:`save` method."""
+    def __init__(self, rows, info):
+        """
+        .. note ::
+        
+          The constructor is not public.  Please do not call it.
+        """
+        
+        self.rows = rows
+        self.info = info
+
+    def save(self, file):
+        """Save the image to *file*.  If *file* looks like an open file
+        descriptor then it is used, otherwise it is treated as a
+        filename and a fresh file is opened.
+
+        In general, you can only call this method once; after it has
+        been called the first time and the PNG image has been saved, the
+        source data will have been streamed, and cannot be streamed
+        again.
+        """
+
+        w = Writer(**self.info)
+
+        try:
+            file.write
+            def close(): pass
+        except:
+            file = open(file, 'wb')
+            def close(): file.close()
+
+        try:
+            w.write(file, self.rows)
+        finally:
+            close()
+
+class _readable:
+    """
+    A simple file-like interface for strings and arrays.
+    """
+
+    def __init__(self, buf):
+        self.buf = buf
+        self.offset = 0
+
+    def read(self, n):
+        r = self.buf[self.offset:self.offset+n]
+        if isarray(r):
+            r = r.tostring()
+        self.offset += n
+        return r
+
+
+class Reader:
+    """
+    PNG decoder in pure Python.
+    """
+
+    def __init__(self, _guess=None, **kw):
+        """
+        Create a PNG decoder object.
+
+        The constructor expects exactly one keyword argument. If you
+        supply a positional argument instead, it will guess the input
+        type. You can choose among the following keyword arguments:
+
+        filename
+          Name of input file (a PNG file).
+        file
+          A file-like object (object with a read() method).
+        bytes
+          ``array`` or ``string`` with PNG data.
+
+        """
+        if ((_guess is not None and len(kw) != 0) or
+            (_guess is None and len(kw) != 1)):
+            raise TypeError("Reader() takes exactly 1 argument")
+
+        # Will be the first 8 bytes, later on.  See validate_signature.
+        self.signature = None
+        self.transparent = None
+        # A pair of (len,type) if a chunk has been read but its data and
+        # checksum have not (in other words the file position is just
+        # past the 4 bytes that specify the chunk type).  See preamble
+        # method for how this is used.
+        self.atchunk = None
+
+        if _guess is not None:
+            if isarray(_guess):
+                kw["bytes"] = _guess
+            elif isinstance(_guess, str):
+                kw["filename"] = _guess
+            elif isinstance(_guess, file):
+                kw["file"] = _guess
+
+        if "filename" in kw:
+            self.file = open(kw["filename"], "rb")
+        elif "file" in kw:
+            self.file = kw["file"]
+        elif "bytes" in kw:
+            self.file = _readable(kw["bytes"])
+        else:
+            raise TypeError("expecting filename, file or bytes array")
+
+
+    def chunk(self, seek=None, lenient=False):
+        """
+        Read the next PNG chunk from the input file; returns a
+        (*type*,*data*) tuple.  *type* is the chunk's type as a string
+        (all PNG chunk types are 4 characters long).  *data* is the
+        chunk's data content, as a string.
+
+        If the optional `seek` argument is
+        specified then it will keep reading chunks until it either runs
+        out of file or finds the type specified by the argument.  Note
+        that in general the order of chunks in PNGs is unspecified, so
+        using `seek` can cause you to miss chunks.
+
+        If the optional `lenient` argument evaluates to True,
+        checksum failures will raise warnings rather than exceptions.
+        """
+
+        self.validate_signature()
+
+        while True:
+            # http://www.w3.org/TR/PNG/#5Chunk-layout
+            if not self.atchunk:
+                self.atchunk = self.chunklentype()
+            length,type = self.atchunk
+            self.atchunk = None
+            data = self.file.read(length)
+            if len(data) != length:
+                raise ChunkError('Chunk %s too short for required %i octets.'
+                  % (type, length))
+            checksum = self.file.read(4)
+            if len(checksum) != 4:
+                raise ValueError('Chunk %s too short for checksum.', tag)
+            if seek and type != seek:
+                continue
+            verify = zlib.crc32(strtobytes(type))
+            verify = zlib.crc32(data, verify)
+            # Whether the output from zlib.crc32 is signed or not varies
+            # according to hideous implementation details, see
+            # http://bugs.python.org/issue1202 .
+            # We coerce it to be positive here (in a way which works on
+            # Python 2.3 and older).
+            verify &= 2**32 - 1
+            verify = struct.pack('!I', verify)
+            if checksum != verify:
+                # print repr(checksum)
+                (a, ) = struct.unpack('!I', checksum)
+                (b, ) = struct.unpack('!I', verify)
+                message = "Checksum error in %s chunk: 0x%08X != 0x%08X." % (type, a, b)
+                if lenient:
+                    warnings.warn(message, RuntimeWarning)
+                else:
+                    raise ChunkError(message)
+            return type, data
+
+    def chunks(self):
+        """Return an iterator that will yield each chunk as a
+        (*chunktype*, *content*) pair.
+        """
+
+        while True:
+            t,v = self.chunk()
+            yield t,v
+            if t == 'IEND':
+                break
+
+    def undo_filter(self, filter_type, scanline, previous):
+        """Undo the filter for a scanline.  `scanline` is a sequence of
+        bytes that does not include the initial filter type byte.
+        `previous` is decoded previous scanline (for straightlaced
+        images this is the previous pixel row, but for interlaced
+        images, it is the previous scanline in the reduced image, which
+        in general is not the previous pixel row in the final image).
+        When there is no previous scanline (the first row of a
+        straightlaced image, or the first row in one of the passes in an
+        interlaced image), then this argument should be ``None``.
+
+        The scanline will have the effects of filtering removed, and the
+        result will be returned as a fresh sequence of bytes.
+        """
+
+        # :todo: Would it be better to update scanline in place?
+
+        # Create the result byte array.  It seems that the best way to
+        # create the array to be the right size is to copy from an
+        # existing sequence.  *sigh*
+        # If we fill the result with scanline, then this allows a
+        # micro-optimisation in the "null" and "sub" cases.
+        result = array('B', scanline)
+
+        if filter_type == 0:
+            # And here, we _rely_ on filling the result with scanline,
+            # above.
+            return result
+
+        if filter_type not in (1,2,3,4):
+            raise FormatError('Invalid PNG Filter Type.'
+              '  See http://www.w3.org/TR/2003/REC-PNG-20031110/#9Filters .')
+
+        # Filter unit.  The stride from one pixel to the corresponding
+        # byte from the previous previous.  Normally this is the pixel
+        # size in bytes, but when this is smaller than 1, the previous
+        # byte is used instead.
+        fu = max(1, self.psize)
+
+        # For the first line of a pass, synthesize a dummy previous
+        # line.  An alternative approach would be to observe that on the
+        # first line 'up' is the same as 'null', 'paeth' is the same
+        # as 'sub', with only 'average' requiring any special case.
+        if not previous:
+            previous = array('B', [0]*len(scanline))
+
+        def sub():
+            """Undo sub filter."""
+
+            ai = 0
+            # Loops starts at index fu.  Observe that the initial part
+            # of the result is already filled in correctly with
+            # scanline.
+            for i in range(fu, len(result)):
+                x = scanline[i]
+                a = result[ai]
+                result[i] = (x + a) & 0xff
+                ai += 1
+
+        def up():
+            """Undo up filter."""
+
+            for i in range(len(result)):
+                x = scanline[i]
+                b = previous[i]
+                result[i] = (x + b) & 0xff
+
+        def average():
+            """Undo average filter."""
+
+            ai = -fu
+            for i in range(len(result)):
+                x = scanline[i]
+                if ai < 0:
+                    a = 0
+                else:
+                    a = result[ai]
+                b = previous[i]
+                result[i] = (x + ((a + b) >> 1)) & 0xff
+                ai += 1
+
+        def paeth():
+            """Undo Paeth filter."""
+
+            # Also used for ci.
+            ai = -fu
+            for i in range(len(result)):
+                x = scanline[i]
+                if ai < 0:
+                    a = c = 0
+                else:
+                    a = result[ai]
+                    c = previous[ai]
+                b = previous[i]
+                p = a + b - c
+                pa = abs(p - a)
+                pb = abs(p - b)
+                pc = abs(p - c)
+                if pa <= pb and pa <= pc:
+                    pr = a
+                elif pb <= pc:
+                    pr = b
+                else:
+                    pr = c
+                result[i] = (x + pr) & 0xff
+                ai += 1
+
+        # Call appropriate filter algorithm.  Note that 0 has already
+        # been dealt with.
+        (None, sub, up, average, paeth)[filter_type]()
+        return result
+
+    def deinterlace(self, raw):
+        """
+        Read raw pixel data, undo filters, deinterlace, and flatten.
+        Return in flat row flat pixel format.
+        """
+
+        # print >> sys.stderr, ("Reading interlaced, w=%s, r=%s, planes=%s," +
+        #     " bpp=%s") % (self.width, self.height, self.planes, self.bps)
+        # Values per row (of the target image)
+        vpr = self.width * self.planes
+
+        # Make a result array, and make it big enough.  Interleaving
+        # writes to the output array randomly (well, not quite), so the
+        # entire output array must be in memory.
+        fmt = 'BH'[self.bitdepth > 8]
+        a = array(fmt, [0]*vpr*self.height)
+        source_offset = 0
+
+        for xstart, ystart, xstep, ystep in _adam7:
+            # print >> sys.stderr, "Adam7: start=%s,%s step=%s,%s" % (
+            #     xstart, ystart, xstep, ystep)
+            if xstart >= self.width:
+                continue
+            # The previous (reconstructed) scanline.  None at the
+            # beginning of a pass to indicate that there is no previous
+            # line.
+            recon = None
+            # Pixels per row (reduced pass image)
+            ppr = int(math.ceil((self.width-xstart)/float(xstep)))
+            # Row size in bytes for this pass.
+            row_size = int(math.ceil(self.psize * ppr))
+            for y in range(ystart, self.height, ystep):
+                filter_type = raw[source_offset]
+                source_offset += 1
+                scanline = raw[source_offset:source_offset+row_size]
+                source_offset += row_size
+                recon = self.undo_filter(filter_type, scanline, recon)
+                # Convert so that there is one element per pixel value
+                flat = self.serialtoflat(recon, ppr)
+                if xstep == 1:
+                    assert xstart == 0
+                    offset = y * vpr
+                    a[offset:offset+vpr] = flat
+                else:
+                    offset = y * vpr + xstart * self.planes
+                    end_offset = (y+1) * vpr
+                    skip = self.planes * xstep
+                    for i in range(self.planes):
+                        a[offset+i:end_offset:skip] = \
+                            flat[i::self.planes]
+        return a
+
+    def iterboxed(self, rows):
+        """Iterator that yields each scanline in boxed row flat pixel
+        format.  `rows` should be an iterator that yields the bytes of
+        each row in turn.
+        """
+
+        def asvalues(raw):
+            """Convert a row of raw bytes into a flat row.  Result may
+            or may not share with argument"""
+
+            if self.bitdepth == 8:
+                return raw
+            if self.bitdepth == 16:
+                raw = tostring(raw)
+                return array('H', struct.unpack('!%dH' % (len(raw)//2), raw))
+            assert self.bitdepth < 8
+            width = self.width
+            # Samples per byte
+            spb = 8//self.bitdepth
+            out = array('B')
+            mask = 2**self.bitdepth - 1
+            shifts = map(self.bitdepth.__mul__, reversed(range(spb)))
+            for o in raw:
+                out.extend(map(lambda i: mask&(o>>i), shifts))
+            return out[:width]
+
+        return itertools.imap(asvalues, rows)
+
+    def serialtoflat(self, bytes, width=None):
+        """Convert serial format (byte stream) pixel data to flat row
+        flat pixel.
+        """
+
+        if self.bitdepth == 8:
+            return bytes
+        if self.bitdepth == 16:
+            bytes = tostring(bytes)
+            return array('H',
+              struct.unpack('!%dH' % (len(bytes)//2), bytes))
+        assert self.bitdepth < 8
+        if width is None:
+            width = self.width
+        # Samples per byte
+        spb = 8//self.bitdepth
+        out = array('B')
+        mask = 2**self.bitdepth - 1
+        shifts = map(self.bitdepth.__mul__, reversed(range(spb)))
+        l = width
+        for o in bytes:
+            out.extend([(mask&(o>>s)) for s in shifts][:l])
+            l -= spb
+            if l <= 0:
+                l = width
+        return out
+
+    def iterstraight(self, raw):
+        """Iterator that undoes the effect of filtering, and yields each
+        row in serialised format (as a sequence of bytes).  Assumes input
+        is straightlaced.  `raw` should be an iterable that yields the
+        raw bytes in chunks of arbitrary size."""
+
+        # length of row, in bytes
+        rb = self.row_bytes
+        a = array('B')
+        # The previous (reconstructed) scanline.  None indicates first
+        # line of image.
+        recon = None
+        for some in raw:
+            a.extend(some)
+            while len(a) >= rb + 1:
+                filter_type = a[0]
+                scanline = a[1:rb+1]
+                del a[:rb+1]
+                recon = self.undo_filter(filter_type, scanline, recon)
+                yield recon
+        if len(a) != 0:
+            # :file:format We get here with a file format error: when the
+            # available bytes (after decompressing) do not pack into exact
+            # rows.
+            raise FormatError(
+              'Wrong size for decompressed IDAT chunk.')
+        assert len(a) == 0
+
+    def validate_signature(self):
+        """If signature (header) has not been read then read and
+        validate it; otherwise do nothing.
+        """
+
+        if self.signature:
+            return
+        self.signature = self.file.read(8)
+        if self.signature != _signature:
+            raise FormatError("PNG file has invalid signature.")
+
+    def preamble(self, lenient=False):
+        """
+        Extract the image metadata by reading the initial part of the PNG
+        file up to the start of the ``IDAT`` chunk.  All the chunks that
+        precede the ``IDAT`` chunk are read and either processed for
+        metadata or discarded.
+
+        If the optional `lenient` argument evaluates to True,
+        checksum failures will raise warnings rather than exceptions.
+        """
+
+        self.validate_signature()
+
+        while True:
+            if not self.atchunk:
+                self.atchunk = self.chunklentype()
+                if self.atchunk is None:
+                    raise FormatError(
+                      'This PNG file has no IDAT chunks.')
+            if self.atchunk[1] == 'IDAT':
+                return
+            self.process_chunk(lenient=lenient)
+
+    def chunklentype(self):
+        """Reads just enough of the input to determine the next
+        chunk's length and type, returned as a (*length*, *type*) pair
+        where *type* is a string.  If there are no more chunks, ``None``
+        is returned.
+        """
+
+        x = self.file.read(8)
+        if not x:
+            return None
+        if len(x) != 8:
+            raise FormatError(
+              'End of file whilst reading chunk length and type.')
+        length,type = struct.unpack('!I4s', x)
+        type = bytestostr(type)
+        if length > 2**31-1:
+            raise FormatError('Chunk %s is too large: %d.' % (type,length))
+        return length,type
+
+    def process_chunk(self, lenient=False):
+        """Process the next chunk and its data.  This only processes the
+        following chunk types, all others are ignored: ``IHDR``,
+        ``PLTE``, ``bKGD``, ``tRNS``, ``gAMA``, ``sBIT``.
+
+        If the optional `lenient` argument evaluates to True,
+        checksum failures will raise warnings rather than exceptions.
+        """
+
+        type, data = self.chunk(lenient=lenient)
+        if type == 'IHDR':
+            # http://www.w3.org/TR/PNG/#11IHDR
+            if len(data) != 13:
+                raise FormatError('IHDR chunk has incorrect length.')
+            (self.width, self.height, self.bitdepth, self.color_type,
+             self.compression, self.filter,
+             self.interlace) = struct.unpack("!2I5B", data)
+
+            # Check that the header specifies only valid combinations.
+            if self.bitdepth not in (1,2,4,8,16):
+                raise Error("invalid bit depth %d" % self.bitdepth)
+            if self.color_type not in (0,2,3,4,6):
+                raise Error("invalid colour type %d" % self.color_type)
+            # Check indexed (palettized) images have 8 or fewer bits
+            # per pixel; check only indexed or greyscale images have
+            # fewer than 8 bits per pixel.
+            if ((self.color_type & 1 and self.bitdepth > 8) or
+                (self.bitdepth < 8 and self.color_type not in (0,3))):
+                raise FormatError("Illegal combination of bit depth (%d)"
+                  " and colour type (%d)."
+                  " See http://www.w3.org/TR/2003/REC-PNG-20031110/#table111 ."
+                  % (self.bitdepth, self.color_type))
+            if self.compression != 0:
+                raise Error("unknown compression method %d" % self.compression)
+            if self.filter != 0:
+                raise FormatError("Unknown filter method %d,"
+                  " see http://www.w3.org/TR/2003/REC-PNG-20031110/#9Filters ."
+                  % self.filter)
+            if self.interlace not in (0,1):
+                raise FormatError("Unknown interlace method %d,"
+                  " see http://www.w3.org/TR/2003/REC-PNG-20031110/#8InterlaceMethods ."
+                  % self.interlace)
+
+            # Derived values
+            # http://www.w3.org/TR/PNG/#6Colour-values
+            colormap =  bool(self.color_type & 1)
+            greyscale = not (self.color_type & 2)
+            alpha = bool(self.color_type & 4)
+            color_planes = (3,1)[greyscale or colormap]
+            planes = color_planes + alpha
+
+            self.colormap = colormap
+            self.greyscale = greyscale
+            self.alpha = alpha
+            self.color_planes = color_planes
+            self.planes = planes
+            self.psize = float(self.bitdepth)/float(8) * planes
+            if int(self.psize) == self.psize:
+                self.psize = int(self.psize)
+            self.row_bytes = int(math.ceil(self.width * self.psize))
+            # Stores PLTE chunk if present, and is used to check
+            # chunk ordering constraints.
+            self.plte = None
+            # Stores tRNS chunk if present, and is used to check chunk
+            # ordering constraints.
+            self.trns = None
+            # Stores sbit chunk if present.
+            self.sbit = None
+        elif type == 'PLTE':
+            # http://www.w3.org/TR/PNG/#11PLTE
+            if self.plte:
+                warnings.warn("Multiple PLTE chunks present.")
+            self.plte = data
+            if len(data) % 3 != 0:
+                raise FormatError(
+                  "PLTE chunk's length should be a multiple of 3.")
+            if len(data) > (2**self.bitdepth)*3:
+                raise FormatError("PLTE chunk is too long.")
+            if len(data) == 0:
+                raise FormatError("Empty PLTE is not allowed.")
+        elif type == 'bKGD':
+            try:
+                if self.colormap:
+                    if not self.plte:
+                        warnings.warn(
+                          "PLTE chunk is required before bKGD chunk.")
+                    self.background = struct.unpack('B', data)
+                else:
+                    self.background = struct.unpack("!%dH" % self.color_planes,
+                      data)
+            except struct.error:
+                raise FormatError("bKGD chunk has incorrect length.")
+        elif type == 'tRNS':
+            # http://www.w3.org/TR/PNG/#11tRNS
+            self.trns = data
+            if self.colormap:
+                if not self.plte:
+                    warnings.warn("PLTE chunk is required before tRNS chunk.")
+                else:
+                    if len(data) > len(self.plte)/3:
+                        # Was warning, but promoted to Error as it
+                        # would otherwise cause pain later on.
+                        raise FormatError("tRNS chunk is too long.")
+            else:
+                if self.alpha:
+                    raise FormatError(
+                      "tRNS chunk is not valid with colour type %d." %
+                      self.color_type)
+                try:
+                    self.transparent = \
+                        struct.unpack("!%dH" % self.color_planes, data)
+                except struct.error:
+                    raise FormatError("tRNS chunk has incorrect length.")
+        elif type == 'gAMA':
+            try:
+                self.gamma = struct.unpack("!L", data)[0] / 100000.0
+            except struct.error:
+                raise FormatError("gAMA chunk has incorrect length.")
+        elif type == 'sBIT':
+            self.sbit = data
+            if (self.colormap and len(data) != 3 or
+                not self.colormap and len(data) != self.planes):
+                raise FormatError("sBIT chunk has incorrect length.")
+
+    def read(self, lenient=False):
+        """
+        Read the PNG file and decode it.  Returns (`width`, `height`,
+        `pixels`, `metadata`).
+
+        May use excessive memory.
+
+        `pixels` are returned in boxed row flat pixel format.
+
+        If the optional `lenient` argument evaluates to True,
+        checksum failures will raise warnings rather than exceptions.
+        """
+
+        def iteridat():
+            """Iterator that yields all the ``IDAT`` chunks as strings."""
+            while True:
+                try:
+                    type, data = self.chunk(lenient=lenient)
+                except ValueError, e:
+                    raise ChunkError(e.args[0])
+                if type == 'IEND':
+                    # http://www.w3.org/TR/PNG/#11IEND
+                    break
+                if type != 'IDAT':
+                    continue
+                # type == 'IDAT'
+                # http://www.w3.org/TR/PNG/#11IDAT
+                if self.colormap and not self.plte:
+                    warnings.warn("PLTE chunk is required before IDAT chunk")
+                yield data
+
+        def iterdecomp(idat):
+            """Iterator that yields decompressed strings.  `idat` should
+            be an iterator that yields the ``IDAT`` chunk data.
+            """
+
+            # Currently, with no max_length paramter to decompress, this
+            # routine will do one yield per IDAT chunk.  So not very
+            # incremental.
+            d = zlib.decompressobj()
+            # Each IDAT chunk is passed to the decompressor, then any
+            # remaining state is decompressed out.
+            for data in idat:
+                # :todo: add a max_length argument here to limit output
+                # size.
+                yield array('B', d.decompress(data))
+            yield array('B', d.flush())
+
+        self.preamble(lenient=lenient)
+        raw = iterdecomp(iteridat())
+
+        if self.interlace:
+            raw = array('B', itertools.chain(*raw))
+            arraycode = 'BH'[self.bitdepth>8]
+            # Like :meth:`group` but producing an array.array object for
+            # each row.
+            pixels = itertools.imap(lambda *row: array(arraycode, row),
+                       *[iter(self.deinterlace(raw))]*self.width*self.planes)
+        else:
+            pixels = self.iterboxed(self.iterstraight(raw))
+        meta = dict()
+        for attr in 'greyscale alpha planes bitdepth interlace'.split():
+            meta[attr] = getattr(self, attr)
+        meta['size'] = (self.width, self.height)
+        for attr in 'gamma transparent background'.split():
+            a = getattr(self, attr, None)
+            if a is not None:
+                meta[attr] = a
+        return self.width, self.height, pixels, meta
+
+
+    def read_flat(self):
+        """
+        Read a PNG file and decode it into flat row flat pixel format.
+        Returns (*width*, *height*, *pixels*, *metadata*).
+
+        May use excessive memory.
+
+        `pixels` are returned in flat row flat pixel format.
+
+        See also the :meth:`read` method which returns pixels in the
+        more stream-friendly boxed row flat pixel format.
+        """
+
+        x, y, pixel, meta = self.read()
+        arraycode = 'BH'[meta['bitdepth']>8]
+        pixel = array(arraycode, itertools.chain(*pixel))
+        return x, y, pixel, meta
+
+    def palette(self, alpha='natural'):
+        """Returns a palette that is a sequence of 3-tuples or 4-tuples,
+        synthesizing it from the ``PLTE`` and ``tRNS`` chunks.  These
+        chunks should have already been processed (for example, by
+        calling the :meth:`preamble` method).  All the tuples are the
+        same size: 3-tuples if there is no ``tRNS`` chunk, 4-tuples when
+        there is a ``tRNS`` chunk.  Assumes that the image is colour type
+        3 and therefore a ``PLTE`` chunk is required.
+
+        If the `alpha` argument is ``'force'`` then an alpha channel is
+        always added, forcing the result to be a sequence of 4-tuples.
+        """
+
+        if not self.plte:
+            raise FormatError(
+                "Required PLTE chunk is missing in colour type 3 image.")
+        plte = group(array('B', self.plte), 3)
+        if self.trns or alpha == 'force':
+            trns = array('B', self.trns or '')
+            trns.extend([255]*(len(plte)-len(trns)))
+            plte = map(operator.add, plte, group(trns, 1))
+        return plte
+
+    def asDirect(self):
+        """Returns the image data as a direct representation of an
+        ``x * y * planes`` array.  This method is intended to remove the
+        need for callers to deal with palettes and transparency
+        themselves.  Images with a palette (colour type 3)
+        are converted to RGB or RGBA; images with transparency (a
+        ``tRNS`` chunk) are converted to LA or RGBA as appropriate.
+        When returned in this format the pixel values represent the
+        colour value directly without needing to refer to palettes or
+        transparency information.
+
+        Like the :meth:`read` method this method returns a 4-tuple:
+
+        (*width*, *height*, *pixels*, *meta*)
+
+        This method normally returns pixel values with the bit depth
+        they have in the source image, but when the source PNG has an
+        ``sBIT`` chunk it is inspected and can reduce the bit depth of
+        the result pixels; pixel values will be reduced according to
+        the bit depth specified in the ``sBIT`` chunk (PNG nerds should
+        note a single result bit depth is used for all channels; the
+        maximum of the ones specified in the ``sBIT`` chunk.  An RGB565
+        image will be rescaled to 6-bit RGB666).
+
+        The *meta* dictionary that is returned reflects the `direct`
+        format and not the original source image.  For example, an RGB
+        source image with a ``tRNS`` chunk to represent a transparent
+        colour, will have ``planes=3`` and ``alpha=False`` for the
+        source image, but the *meta* dictionary returned by this method
+        will have ``planes=4`` and ``alpha=True`` because an alpha
+        channel is synthesized and added.
+
+        *pixels* is the pixel data in boxed row flat pixel format (just
+        like the :meth:`read` method).
+
+        All the other aspects of the image data are not changed.
+        """
+
+        self.preamble()
+
+        # Simple case, no conversion necessary.
+        if not self.colormap and not self.trns and not self.sbit:
+            return self.read()
+
+        x,y,pixels,meta = self.read()
+
+        if self.colormap:
+            meta['colormap'] = False
+            meta['alpha'] = bool(self.trns)
+            meta['bitdepth'] = 8
+            meta['planes'] = 3 + bool(self.trns)
+            plte = self.palette()
+            def iterpal(pixels):
+                for row in pixels:
+                    row = map(plte.__getitem__, row)
+                    yield array('B', itertools.chain(*row))
+            pixels = iterpal(pixels)
+        elif self.trns:
+            # It would be nice if there was some reasonable way of doing
+            # this without generating a whole load of intermediate tuples.
+            # But tuples does seem like the easiest way, with no other way
+            # clearly much simpler or much faster.  (Actually, the L to LA
+            # conversion could perhaps go faster (all those 1-tuples!), but
+            # I still wonder whether the code proliferation is worth it)
+            it = self.transparent
+            maxval = 2**meta['bitdepth']-1
+            planes = meta['planes']
+            meta['alpha'] = True
+            meta['planes'] += 1
+            typecode = 'BH'[meta['bitdepth']>8]
+            def itertrns(pixels):
+                for row in pixels:
+                    # For each row we group it into pixels, then form a
+                    # characterisation vector that says whether each pixel
+                    # is opaque or not.  Then we convert True/False to
+                    # 0/maxval (by multiplication), and add it as the extra
+                    # channel.
+                    row = group(row, planes)
+                    opa = map(it.__ne__, row)
+                    opa = map(maxval.__mul__, opa)
+                    opa = zip(opa) # convert to 1-tuples
+                    yield array(typecode,
+                      itertools.chain(*map(operator.add, row, opa)))
+            pixels = itertrns(pixels)
+        targetbitdepth = None
+        if self.sbit:
+            sbit = struct.unpack('%dB' % len(self.sbit), self.sbit)
+            targetbitdepth = max(sbit)
+            if targetbitdepth > meta['bitdepth']:
+                raise Error('sBIT chunk %r exceeds bitdepth %d' %
+                    (sbit,self.bitdepth))
+            if min(sbit) <= 0:
+                raise Error('sBIT chunk %r has a 0-entry' % sbit)
+            if targetbitdepth == meta['bitdepth']:
+                targetbitdepth = None
+        if targetbitdepth:
+            shift = meta['bitdepth'] - targetbitdepth
+            meta['bitdepth'] = targetbitdepth
+            def itershift(pixels):
+                for row in pixels:
+                    yield map(shift.__rrshift__, row)
+            pixels = itershift(pixels)
+        return x,y,pixels,meta
+
+    def asFloat(self, maxval=1.0):
+        """Return image pixels as per :meth:`asDirect` method, but scale
+        all pixel values to be floating point values between 0.0 and
+        *maxval*.
+        """
+
+        x,y,pixels,info = self.asDirect()
+        sourcemaxval = 2**info['bitdepth']-1
+        del info['bitdepth']
+        info['maxval'] = float(maxval)
+        factor = float(maxval)/float(sourcemaxval)
+        def iterfloat():
+            for row in pixels:
+                yield map(factor.__mul__, row)
+        return x,y,iterfloat(),info
+
+    def _as_rescale(self, get, targetbitdepth):
+        """Helper used by :meth:`asRGB8` and :meth:`asRGBA8`."""
+
+        width,height,pixels,meta = get()
+        maxval = 2**meta['bitdepth'] - 1
+        targetmaxval = 2**targetbitdepth - 1
+        factor = float(targetmaxval) / float(maxval)
+        meta['bitdepth'] = targetbitdepth
+        def iterscale():
+            for row in pixels:
+                yield map(lambda x: int(round(x*factor)), row)
+        return width, height, iterscale(), meta
+
+    def asRGB8(self):
+        """Return the image data as an RGB pixels with 8-bits per
+        sample.  This is like the :meth:`asRGB` method except that
+        this method additionally rescales the values so that they
+        are all between 0 and 255 (8-bit).  In the case where the
+        source image has a bit depth < 8 the transformation preserves
+        all the information; where the source image has bit depth
+        > 8, then rescaling to 8-bit values loses precision.  No
+        dithering is performed.  Like :meth:`asRGB`, an alpha channel
+        in the source image will raise an exception.
+
+        This function returns a 4-tuple:
+        (*width*, *height*, *pixels*, *metadata*).
+        *width*, *height*, *metadata* are as per the :meth:`read` method.
+        
+        *pixels* is the pixel data in boxed row flat pixel format.
+        """
+
+        return self._as_rescale(self.asRGB, 8)
+
+    def asRGBA8(self):
+        """Return the image data as RGBA pixels with 8-bits per
+        sample.  This method is similar to :meth:`asRGB8` and
+        :meth:`asRGBA`:  The result pixels have an alpha channel, *and*
+        values are rescaled to the range 0 to 255.  The alpha channel is
+        synthesized if necessary (with a small speed penalty).
+        """
+
+        return self._as_rescale(self.asRGBA, 8)
+
+    def asRGB(self):
+        """Return image as RGB pixels.  RGB colour images are passed
+        through unchanged; greyscales are expanded into RGB
+        triplets (there is a small speed overhead for doing this).
+
+        An alpha channel in the source image will raise an
+        exception.
+
+        The return values are as for the :meth:`read` method
+        except that the *metadata* reflect the returned pixels, not the
+        source image.  In particular, for this method
+        ``metadata['greyscale']`` will be ``False``.
+        """
+
+        width,height,pixels,meta = self.asDirect()
+        if meta['alpha']:
+            raise Error("will not convert image with alpha channel to RGB")
+        if not meta['greyscale']:
+            return width,height,pixels,meta
+        meta['greyscale'] = False
+        typecode = 'BH'[meta['bitdepth'] > 8]
+        def iterrgb():
+            for row in pixels:
+                a = array(typecode, [0]) * 3 * width
+                for i in range(3):
+                    a[i::3] = row
+                yield a
+        return width,height,iterrgb(),meta
+
+    def asRGBA(self):
+        """Return image as RGBA pixels.  Greyscales are expanded into
+        RGB triplets; an alpha channel is synthesized if necessary.
+        The return values are as for the :meth:`read` method
+        except that the *metadata* reflect the returned pixels, not the
+        source image.  In particular, for this method
+        ``metadata['greyscale']`` will be ``False``, and
+        ``metadata['alpha']`` will be ``True``.
+        """
+
+        width,height,pixels,meta = self.asDirect()
+        if meta['alpha'] and not meta['greyscale']:
+            return width,height,pixels,meta
+        typecode = 'BH'[meta['bitdepth'] > 8]
+        maxval = 2**meta['bitdepth'] - 1
+        def newarray():
+            return array(typecode, [0]) * 4 * width
+        if meta['alpha'] and meta['greyscale']:
+            # LA to RGBA
+            def convert():
+                for row in pixels:
+                    # Create a fresh target row, then copy L channel
+                    # into first three target channels, and A channel
+                    # into fourth channel.
+                    a = newarray()
+                    for i in range(3):
+                        a[i::4] = row[0::2]
+                    a[3::4] = row[1::2]
+                    yield a
+        elif meta['greyscale']:
+            # L to RGBA
+            def convert():
+                for row in pixels:
+                    a = newarray()
+                    for i in range(3):
+                        a[i::4] = row
+                    a[3::4] = array(typecode, [maxval]) * width
+                    yield a
+        else:
+            assert not meta['alpha'] and not meta['greyscale']
+            # RGB to RGBA
+            def convert():
+                for row in pixels:
+                    a = newarray()
+                    for i in range(3):
+                        a[i::4] = row[i::3]
+                    a[3::4] = array(typecode, [maxval]) * width
+                    yield a
+        meta['alpha'] = True
+        meta['greyscale'] = False
+        return width,height,convert(),meta
+
+
+# === Legacy Version Support ===
+
+# :pyver:old:  PyPNG works on Python versions 2.3 and 2.2, but not
+# without some awkward problems.  Really PyPNG works on Python 2.4 (and
+# above); it works on Pythons 2.3 and 2.2 by virtue of fixing up
+# problems here.  It's a bit ugly (which is why it's hidden down here).
+#
+# Generally the strategy is one of pretending that we're running on
+# Python 2.4 (or above), and patching up the library support on earlier
+# versions so that it looks enough like Python 2.4.  When it comes to
+# Python 2.2 there is one thing we cannot patch: extended slices
+# http://www.python.org/doc/2.3/whatsnew/section-slices.html.
+# Instead we simply declare that features that are implemented using
+# extended slices will not work on Python 2.2.
+#
+# In order to work on Python 2.3 we fix up a recurring annoyance involving
+# the array type.  In Python 2.3 an array cannot be initialised with an
+# array, and it cannot be extended with a list (or other sequence).
+# Both of those are repeated issues in the code.  Whilst I would not
+# normally tolerate this sort of behaviour, here we "shim" a replacement
+# for array into place (and hope no-ones notices).  You never read this.
+#
+# In an amusing case of warty hacks on top of warty hacks... the array
+# shimming we try and do only works on Python 2.3 and above (you can't
+# subclass array.array in Python 2.2).  So to get it working on Python
+# 2.2 we go for something much simpler and (probably) way slower.
+try:
+    array('B').extend([])
+    array('B', array('B'))
+except:
+    # Expect to get here on Python 2.3
+    try:
+        class _array_shim(array):
+            true_array = array
+            def __new__(cls, typecode, init=None):
+                super_new = super(_array_shim, cls).__new__
+                it = super_new(cls, typecode)
+                if init is None:
+                    return it
+                it.extend(init)
+                return it
+            def extend(self, extension):
+                super_extend = super(_array_shim, self).extend
+                if isinstance(extension, self.true_array):
+                    return super_extend(extension)
+                if not isinstance(extension, (list, str)):
+                    # Convert to list.  Allows iterators to work.
+                    extension = list(extension)
+                return super_extend(self.true_array(self.typecode, extension))
+        array = _array_shim
+    except:
+        # Expect to get here on Python 2.2
+        def array(typecode, init=()):
+            if type(init) == str:
+                return map(ord, init)
+            return list(init)
+
+# Further hacks to get it limping along on Python 2.2
+try:
+    enumerate
+except:
+    def enumerate(seq):
+        i=0
+        for x in seq:
+            yield i,x
+            i += 1
+
+try:
+    reversed
+except:
+    def reversed(l):
+        l = list(l)
+        l.reverse()
+        for x in l:
+            yield x
+
+try:
+    itertools
+except:
+    class _dummy_itertools:
+        pass
+    itertools = _dummy_itertools()
+    def _itertools_imap(f, seq):
+        for x in seq:
+            yield f(x)
+    itertools.imap = _itertools_imap
+    def _itertools_chain(*iterables):
+        for it in iterables:
+            for element in it:
+                yield element
+    itertools.chain = _itertools_chain
+
+
+
+# === Internal Test Support ===
+
+# This section comprises the tests that are internally validated (as
+# opposed to tests which produce output files that are externally
+# validated).  Primarily they are unittests.
+
+# Note that it is difficult to internally validate the results of
+# writing a PNG file.  The only thing we can do is read it back in
+# again, which merely checks consistency, not that the PNG file we
+# produce is valid.
+
+# Run the tests from the command line:
+# python -c 'import png;png.test()'
+
+# (For an in-memory binary file IO object) We use BytesIO where
+# available, otherwise we use StringIO, but name it BytesIO.
+try:
+    from io import BytesIO
+except:
+    from StringIO import StringIO as BytesIO
+import tempfile
+# http://www.python.org/doc/2.4.4/lib/module-unittest.html
+import unittest
+
+
+def test():
+    unittest.main(__name__)
+
+def topngbytes(name, rows, x, y, **k):
+    """Convenience function for creating a PNG file "in memory" as a
+    string.  Creates a :class:`Writer` instance using the keyword arguments,
+    then passes `rows` to its :meth:`Writer.write` method.  The resulting
+    PNG file is returned as a string.  `name` is used to identify the file for
+    debugging.
+    """
+
+    import os
+
+    print name
+    f = BytesIO()
+    w = Writer(x, y, **k)
+    w.write(f, rows)
+    if os.environ.get('PYPNG_TEST_TMP'):
+        w = open(name, 'wb')
+        w.write(f.getvalue())
+        w.close()
+    return f.getvalue()
+
+def testWithIO(inp, out, f):
+    """Calls the function `f` with ``sys.stdin`` changed to `inp`
+    and ``sys.stdout`` changed to `out`.  They are restored when `f`
+    returns.  This function returns whatever `f` returns.
+    """
+
+    import os
+
+    try:
+        oldin,sys.stdin = sys.stdin,inp
+        oldout,sys.stdout = sys.stdout,out
+        x = f()
+    finally:
+        sys.stdin = oldin
+        sys.stdout = oldout
+    if os.environ.get('PYPNG_TEST_TMP') and hasattr(out,'getvalue'):
+        name = mycallersname()
+        if name:
+            w = open(name+'.png', 'wb')
+            w.write(out.getvalue())
+            w.close()
+    return x
+
+def mycallersname():
+    """Returns the name of the caller of the caller of this function
+    (hence the name of the caller of the function in which
+    "mycallersname()" textually appears).  Returns None if this cannot
+    be determined."""
+
+    # http://docs.python.org/library/inspect.html#the-interpreter-stack
+    import inspect
+
+    frame = inspect.currentframe()
+    if not frame:
+        return None
+    frame_,filename_,lineno_,funname,linelist_,listi_ = (
+      inspect.getouterframes(frame)[2])
+    return funname
+
+def seqtobytes(s):
+    """Convert a sequence of integers to a *bytes* instance.  Good for
+    plastering over Python 2 / Python 3 cracks.
+    """
+
+    return strtobytes(''.join(chr(x) for x in s))
+
+class Test(unittest.TestCase):
+    # This member is used by the superclass.  If we don't define a new
+    # class here then when we use self.assertRaises() and the PyPNG code
+    # raises an assertion then we get no proper traceback.  I can't work
+    # out why, but defining a new class here means we get a proper
+    # traceback.
+    class failureException(Exception):
+        pass
+
+    def helperLN(self, n):
+        mask = (1 << n) - 1
+        # Use small chunk_limit so that multiple chunk writing is
+        # tested.  Making it a test for Issue 20.
+        w = Writer(15, 17, greyscale=True, bitdepth=n, chunk_limit=99)
+        f = BytesIO()
+        w.write_array(f, array('B', map(mask.__and__, range(1, 256))))
+        r = Reader(bytes=f.getvalue())
+        x,y,pixels,meta = r.read()
+        self.assertEqual(x, 15)
+        self.assertEqual(y, 17)
+        self.assertEqual(list(itertools.chain(*pixels)),
+                         map(mask.__and__, range(1,256)))
+    def testL8(self):
+        return self.helperLN(8)
+    def testL4(self):
+        return self.helperLN(4)
+    def testL2(self):
+        "Also tests asRGB8."
+        w = Writer(1, 4, greyscale=True, bitdepth=2)
+        f = BytesIO()
+        w.write_array(f, array('B', range(4)))
+        r = Reader(bytes=f.getvalue())
+        x,y,pixels,meta = r.asRGB8()
+        self.assertEqual(x, 1)
+        self.assertEqual(y, 4)
+        for i,row in enumerate(pixels):
+            self.assertEqual(len(row), 3)
+            self.assertEqual(list(row), [0x55*i]*3)
+    def testP2(self):
+        "2-bit palette."
+        a = (255,255,255)
+        b = (200,120,120)
+        c = (50,99,50)
+        w = Writer(1, 4, bitdepth=2, palette=[a,b,c])
+        f = BytesIO()
+        w.write_array(f, array('B', (0,1,1,2)))
+        r = Reader(bytes=f.getvalue())
+        x,y,pixels,meta = r.asRGB8()
+        self.assertEqual(x, 1)
+        self.assertEqual(y, 4)
+        self.assertEqual(list(pixels), map(list, [a, b, b, c]))
+    def testPtrns(self):
+        "Test colour type 3 and tRNS chunk (and 4-bit palette)."
+        a = (50,99,50,50)
+        b = (200,120,120,80)
+        c = (255,255,255)
+        d = (200,120,120)
+        e = (50,99,50)
+        w = Writer(3, 3, bitdepth=4, palette=[a,b,c,d,e])
+        f = BytesIO()
+        w.write_array(f, array('B', (4, 3, 2, 3, 2, 0, 2, 0, 1)))
+        r = Reader(bytes=f.getvalue())
+        x,y,pixels,meta = r.asRGBA8()
+        self.assertEqual(x, 3)
+        self.assertEqual(y, 3)
+        c = c+(255,)
+        d = d+(255,)
+        e = e+(255,)
+        boxed = [(e,d,c),(d,c,a),(c,a,b)]
+        flat = map(lambda row: itertools.chain(*row), boxed)
+        self.assertEqual(map(list, pixels), map(list, flat))
+    def testRGBtoRGBA(self):
+        "asRGBA8() on colour type 2 source."""
+        # Test for Issue 26
+        r = Reader(bytes=_pngsuite['basn2c08'])
+        x,y,pixels,meta = r.asRGBA8()
+        # Test the pixels at row 9 columns 0 and 1.
+        row9 = list(pixels)[9]
+        self.assertEqual(row9[0:8],
+                         [0xff, 0xdf, 0xff, 0xff, 0xff, 0xde, 0xff, 0xff])
+    def testLtoRGBA(self):
+        "asRGBA() on grey source."""
+        # Test for Issue 60
+        r = Reader(bytes=_pngsuite['basi0g08'])
+        x,y,pixels,meta = r.asRGBA()
+        row9 = list(list(pixels)[9])
+        self.assertEqual(row9[0:8],
+          [222, 222, 222, 255, 221, 221, 221, 255])
+    def testCtrns(self):
+        "Test colour type 2 and tRNS chunk."
+        # Test for Issue 25
+        r = Reader(bytes=_pngsuite['tbrn2c08'])
+        x,y,pixels,meta = r.asRGBA8()
+        # I just happen to know that the first pixel is transparent.
+        # In particular it should be #7f7f7f00
+        row0 = list(pixels)[0]
+        self.assertEqual(tuple(row0[0:4]), (0x7f, 0x7f, 0x7f, 0x00))
+    def testAdam7read(self):
+        """Adam7 interlace reading.
+        Specifically, test that for images in the PngSuite that
+        have both an interlaced and straightlaced pair that both
+        images from the pair produce the same array of pixels."""
+        for candidate in _pngsuite:
+            if not candidate.startswith('basn'):
+                continue
+            candi = candidate.replace('n', 'i')
+            if candi not in _pngsuite:
+                continue
+            print 'adam7 read', candidate
+            straight = Reader(bytes=_pngsuite[candidate])
+            adam7 = Reader(bytes=_pngsuite[candi])
+            # Just compare the pixels.  Ignore x,y (because they're
+            # likely to be correct?); metadata is ignored because the
+            # "interlace" member differs.  Lame.
+            straight = straight.read()[2]
+            adam7 = adam7.read()[2]
+            self.assertEqual(map(list, straight), map(list, adam7))
+    def testAdam7write(self):
+        """Adam7 interlace writing.
+        For each test image in the PngSuite, write an interlaced
+        and a straightlaced version.  Decode both, and compare results.
+        """
+        # Not such a great test, because the only way we can check what
+        # we have written is to read it back again.
+
+        for name,bytes in _pngsuite.items():
+            # Only certain colour types supported for this test.
+            if name[3:5] not in ['n0', 'n2', 'n4', 'n6']:
+                continue
+            it = Reader(bytes=bytes)
+            x,y,pixels,meta = it.read()
+            pngi = topngbytes('adam7wn'+name+'.png', pixels,
+              x=x, y=y, bitdepth=it.bitdepth,
+              greyscale=it.greyscale, alpha=it.alpha,
+              transparent=it.transparent,
+              interlace=False)
+            x,y,ps,meta = Reader(bytes=pngi).read()
+            it = Reader(bytes=bytes)
+            x,y,pixels,meta = it.read()
+            pngs = topngbytes('adam7wi'+name+'.png', pixels,
+              x=x, y=y, bitdepth=it.bitdepth,
+              greyscale=it.greyscale, alpha=it.alpha,
+              transparent=it.transparent,
+              interlace=True)
+            x,y,pi,meta = Reader(bytes=pngs).read()
+            self.assertEqual(map(list, ps), map(list, pi))
+    def testPGMin(self):
+        """Test that the command line tool can read PGM files."""
+        def do():
+            return _main(['testPGMin'])
+        s = BytesIO()
+        s.write(strtobytes('P5 2 2 3\n'))
+        s.write(strtobytes('\x00\x01\x02\x03'))
+        s.flush()
+        s.seek(0)
+        o = BytesIO()
+        testWithIO(s, o, do)
+        r = Reader(bytes=o.getvalue())
+        x,y,pixels,meta = r.read()
+        self.assertTrue(r.greyscale)
+        self.assertEqual(r.bitdepth, 2)
+    def testPAMin(self):
+        """Test that the command line tool can read PAM file."""
+        def do():
+            return _main(['testPAMin'])
+        s = BytesIO()
+        s.write(strtobytes('P7\nWIDTH 3\nHEIGHT 1\nDEPTH 4\nMAXVAL 255\n'
+                'TUPLTYPE RGB_ALPHA\nENDHDR\n'))
+        # The pixels in flat row flat pixel format
+        flat =  [255,0,0,255, 0,255,0,120, 0,0,255,30]
+        asbytes = seqtobytes(flat)
+        s.write(asbytes)
+        s.flush()
+        s.seek(0)
+        o = BytesIO()
+        testWithIO(s, o, do)
+        r = Reader(bytes=o.getvalue())
+        x,y,pixels,meta = r.read()
+        self.assertTrue(r.alpha)
+        self.assertTrue(not r.greyscale)
+        self.assertEqual(list(itertools.chain(*pixels)), flat)
+    def testLA4(self):
+        """Create an LA image with bitdepth 4."""
+        bytes = topngbytes('la4.png', [[5, 12]], 1, 1,
+          greyscale=True, alpha=True, bitdepth=4)
+        sbit = Reader(bytes=bytes).chunk('sBIT')[1]
+        self.assertEqual(sbit, strtobytes('\x04\x04'))
+    def testPNMsbit(self):
+        """Test that PNM files can generates sBIT chunk."""
+        def do():
+            return _main(['testPNMsbit'])
+        s = BytesIO()
+        s.write(strtobytes('P6 8 1 1\n'))
+        for pixel in range(8):
+            s.write(struct.pack('<I', (0x4081*pixel)&0x10101)[:3])
+        s.flush()
+        s.seek(0)
+        o = BytesIO()
+        testWithIO(s, o, do)
+        r = Reader(bytes=o.getvalue())
+        sbit = r.chunk('sBIT')[1]
+        self.assertEqual(sbit, strtobytes('\x01\x01\x01'))
+    def testLtrns0(self):
+        """Create greyscale image with tRNS chunk."""
+        return self.helperLtrns(0)
+    def testLtrns1(self):
+        """Using 1-tuple for transparent arg."""
+        return self.helperLtrns((0,))
+    def helperLtrns(self, transparent):
+        """Helper used by :meth:`testLtrns*`."""
+        pixels = zip([0x00, 0x38, 0x4c, 0x54, 0x5c, 0x40, 0x38, 0x00])
+        o = BytesIO()
+        w = Writer(8, 8, greyscale=True, bitdepth=1, transparent=transparent)
+        w.write_packed(o, pixels)
+        r = Reader(bytes=o.getvalue())
+        x,y,pixels,meta = r.asDirect()
+        self.assertTrue(meta['alpha'])
+        self.assertTrue(meta['greyscale'])
+        self.assertEqual(meta['bitdepth'], 1)
+    def testWinfo(self):
+        """Test the dictionary returned by a `read` method can be used
+        as args for :meth:`Writer`.
+        """
+        r = Reader(bytes=_pngsuite['basn2c16'])
+        info = r.read()[3]
+        w = Writer(**info)
+    def testPackedIter(self):
+        """Test iterator for row when using write_packed.
+
+        Indicative for Issue 47.
+        """
+        w = Writer(16, 2, greyscale=True, alpha=False, bitdepth=1)
+        o = BytesIO()
+        w.write_packed(o, [itertools.chain([0x0a], [0xaa]),
+                           itertools.chain([0x0f], [0xff])])
+        r = Reader(bytes=o.getvalue())
+        x,y,pixels,info = r.asDirect()
+        pixels = list(pixels)
+        self.assertEqual(len(pixels), 2)
+        self.assertEqual(len(pixels[0]), 16)
+    def testInterlacedArray(self):
+        """Test that reading an interlaced PNG yields each row as an
+        array."""
+        r = Reader(bytes=_pngsuite['basi0g08'])
+        list(r.read()[2])[0].tostring
+    def testTrnsArray(self):
+        """Test that reading a type 2 PNG with tRNS chunk yields each
+        row as an array (using asDirect)."""
+        r = Reader(bytes=_pngsuite['tbrn2c08'])
+        list(r.asDirect()[2])[0].tostring
+
+    # Invalid file format tests.  These construct various badly
+    # formatted PNG files, then feed them into a Reader.  When
+    # everything is working properly, we should get FormatError
+    # exceptions raised.
+    def testEmpty(self):
+        """Test empty file."""
+
+        r = Reader(bytes='')
+        self.assertRaises(FormatError, r.asDirect)
+    def testSigOnly(self):
+        """Test file containing just signature bytes."""
+
+        r = Reader(bytes=_signature)
+        self.assertRaises(FormatError, r.asDirect)
+    def testExtraPixels(self):
+        """Test file that contains too many pixels."""
+
+        def eachchunk(chunk):
+            if chunk[0] != 'IDAT':
+                return chunk
+            data = zlib.decompress(chunk[1])
+            data += strtobytes('\x00garbage')
+            data = zlib.compress(data)
+            chunk = (chunk[0], data)
+            return chunk
+        self.assertRaises(FormatError, self.helperFormat, eachchunk)
+    def testNotEnoughPixels(self):
+        def eachchunk(chunk):
+            if chunk[0] != 'IDAT':
+                return chunk
+            # Remove last byte.
+            data = zlib.decompress(chunk[1])
+            data = data[:-1]
+            data = zlib.compress(data)
+            return (chunk[0], data)
+        self.assertRaises(FormatError, self.helperFormat, eachchunk)
+    def helperFormat(self, f):
+        r = Reader(bytes=_pngsuite['basn0g01'])
+        o = BytesIO()
+        def newchunks():
+            for chunk in r.chunks():
+                yield f(chunk)
+        write_chunks(o, newchunks())
+        r = Reader(bytes=o.getvalue())
+        return list(r.asDirect()[2])
+    def testBadFilter(self):
+        def eachchunk(chunk):
+            if chunk[0] != 'IDAT':
+                return chunk
+            data = zlib.decompress(chunk[1])
+            # Corrupt the first filter byte
+            data = strtobytes('\x99') + data[1:]
+            data = zlib.compress(data)
+            return (chunk[0], data)
+        self.assertRaises(FormatError, self.helperFormat, eachchunk)
+    def testFlat(self):
+        """Test read_flat."""
+        import hashlib
+
+        r = Reader(bytes=_pngsuite['basn0g02'])
+        x,y,pixel,meta = r.read_flat()
+        d = hashlib.md5(seqtobytes(pixel)).digest()
+        self.assertEqual(_enhex(d), '255cd971ab8cd9e7275ff906e5041aa0')
+    def testfromarray(self):
+        img = from_array([[0, 0x33, 0x66], [0xff, 0xcc, 0x99]], 'L')
+        img.save('testfromarray.png')
+    def testfromarrayL16(self):
+        img = from_array(group(range(2**16), 256), 'L;16')
+        img.save('testL16.png')
+    def testfromarrayRGB(self):
+        img = from_array([[0,0,0, 0,0,1, 0,1,0, 0,1,1],
+                          [1,0,0, 1,0,1, 1,1,0, 1,1,1]], 'RGB;1')
+        o = BytesIO()
+        img.save(o)
+    def testfromarrayIter(self):
+        import itertools
+
+        i = itertools.islice(itertools.count(10), 20)
+        i = itertools.imap(lambda x: [x, x, x], i)
+        img = from_array(i, 'RGB;5', dict(height=20))
+        f = open('testiter.png', 'wb')
+        img.save(f)
+        f.close()
+
+    # numpy dependent tests.  These are skipped (with a message to
+    # sys.stderr) if numpy cannot be imported.
+    def testNumpyuint16(self):
+        """numpy uint16."""
+
+        try:
+            import numpy
+        except ImportError:
+            print >>sys.stderr, "skipping numpy test"
+            return
+
+        rows = [map(numpy.uint16, range(0,0x10000,0x5555))]
+        b = topngbytes('numpyuint16.png', rows, 4, 1,
+            greyscale=True, alpha=False, bitdepth=16)
+    def testNumpyuint8(self):
+        """numpy uint8."""
+
+        try:
+            import numpy
+        except ImportError:
+            print >>sys.stderr, "skipping numpy test"
+            return
+
+        rows = [map(numpy.uint8, range(0,0x100,0x55))]
+        b = topngbytes('numpyuint8.png', rows, 4, 1,
+            greyscale=True, alpha=False, bitdepth=8)
+    def testNumpybool(self):
+        """numpy bool."""
+
+        try:
+            import numpy
+        except ImportError:
+            print >>sys.stderr, "skipping numpy test"
+            return
+
+        rows = [map(numpy.bool, [0,1])]
+        b = topngbytes('numpybool.png', rows, 2, 1,
+            greyscale=True, alpha=False, bitdepth=1)
+    def testNumpyarray(self):
+        """numpy array."""
+        try:
+            import numpy
+        except ImportError:
+            print >>sys.stderr, "skipping numpy test"
+            return
+
+        pixels = numpy.array([[0,0x5555],[0x5555,0xaaaa]], numpy.uint16)
+        img = from_array(pixels, 'L')
+        img.save('testnumpyL16.png')
+
+# === Command Line Support ===
+
+def _dehex(s):
+    """Liberally convert from hex string to binary string."""
+    import re
+    import binascii
+
+    # Remove all non-hexadecimal digits
+    s = re.sub(r'[^a-fA-F\d]', '', s)
+    # binscii.unhexlify works in Python 2 and Python 3 (unlike
+    # thing.decode('hex')).
+    return binascii.unhexlify(strtobytes(s))
+def _enhex(s):
+    """Convert from binary string (bytes) to hex string (str)."""
+
+    import binascii
+
+    return bytestostr(binascii.hexlify(s))
+
+# Copies of PngSuite test files taken
+# from http://www.schaik.com/pngsuite/pngsuite_bas_png.html
+# on 2009-02-19 by drj and converted to hex.
+# Some of these are not actually in PngSuite (but maybe they should
+# be?), they use the same naming scheme, but start with a capital
+# letter.
+_pngsuite = {
+  'basi0g01': _dehex("""
+89504e470d0a1a0a0000000d49484452000000200000002001000000012c0677
+cf0000000467414d41000186a031e8965f0000009049444154789c2d8d310ec2
+300c45dfc682c415187a00a42e197ab81e83b127e00c5639001363a580d8582c
+65c910357c4b78b0bfbfdf4f70168c19e7acb970a3f2d1ded9695ce5bf5963df
+d92aaf4c9fd927ea449e6487df5b9c36e799b91bdf082b4d4bd4014fe4014b01
+ab7a17aee694d28d328a2d63837a70451e1648702d9a9ff4a11d2f7a51aa21e5
+a18c7ffd0094e3511d661822f20000000049454e44ae426082
+"""),
+  'basi0g02': _dehex("""
+89504e470d0a1a0a0000000d49484452000000200000002002000000016ba60d
+1f0000000467414d41000186a031e8965f0000005149444154789c635062e860
+00e17286bb609c93c370ec189494960631366e4467b3ae675dcf10f521ea0303
+90c1ca006444e11643482064114a4852c710baea3f18c31918020c30410403a6
+0ac1a09239009c52804d85b6d97d0000000049454e44ae426082
+"""),
+  'basi0g04': _dehex("""
+89504e470d0a1a0a0000000d4948445200000020000000200400000001e4e6f8
+bf0000000467414d41000186a031e8965f000000ae49444154789c658e5111c2
+301044171c141c141c041c843a287510ea20d441c041c141c141c04191102454
+03994998cecd7edcecedbb9bdbc3b2c2b6457545fbc4bac1be437347f7c66a77
+3c23d60db15e88f5c5627338a5416c2e691a9b475a89cd27eda12895ae8dfdab
+43d61e590764f5c83a226b40d669bec307f93247701687723abf31ff83a2284b
+a5b4ae6b63ac6520ad730ca4ed7b06d20e030369bd6720ed383290360406d24e
+13811f2781eba9d34d07160000000049454e44ae426082
+"""),
+  'basi0g08': _dehex("""
+89504e470d0a1a0a0000000d4948445200000020000000200800000001211615
+be0000000467414d41000186a031e8965f000000b549444154789cb5905d0ac2
+3010849dbac81c42c47bf843cf253e8878b0aa17110f214bdca6be240f5d21a5
+94ced3e49bcd322c1624115515154998aa424822a82a5624a1aa8a8b24c58f99
+999908130989a04a00d76c2c09e76cf21adcb209393a6553577da17140a2c59e
+70ecbfa388dff1f03b82fb82bd07f05f7cb13f80bb07ad2fd60c011c3c588eef
+f1f4e03bbec7ce832dca927aea005e431b625796345307b019c845e6bfc3bb98
+769d84f9efb02ea6c00f9bb9ff45e81f9f280000000049454e44ae426082
+"""),
+  'basi0g16': _dehex("""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+"""),
+  'basi2c08': _dehex("""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+"""),
+  'basi2c16': _dehex("""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+"""),
+  'basi3p08': _dehex("""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+3c0b0ba1303abaae8ecdeeed950d6669a9a7a7a141d4de9e9d5d5cdcd2229b94
+c572716132f97cb1d8db9bc3110864a39795d9db6b6a26267a7a9a98d4d6a6a7
+cb76090ef6f030354d4d75766e686030545464cb393a1a1ac6c68686eae8f8f9
+a9aa4644c8b66d6e1689dcdd2512a994cb35330b0991ad9f9b6b659596a6addd
+d8282fafae5e5323fb8f41d01f76c22fd8061be01bfc041a0323e1002c81cd30
+0b9ec027a0c930014ec035580fc3e112bc069a0b53e11c0c8095f00176c163a0
+e5301baec06a580677600ddc05ba0f13e120bc81a770133ec355a017300d4ec2
+0c7800bbe1219c02fa08f3e13c1c85dbb00a2ec05ea0dff00a6ec15a98027360
+070c047a06d7e1085c84f1b014f6c03fa0b33018b6c0211801ebe018fc00da0a
+6f61113c877eb01d4ec317a085700f26c130f80efbe132bc039a0733e106fc81
+f7f017f6c10aa0d1300a0ec374780943e1382c06fa0a9b60238c83473016cec0
+02f80f73fefe1072afc1e50000000049454e44ae426082
+"""),
+  'basi6a08': _dehex("""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+200314cf801faab200ea752803a8d7a90c503a039f824a53f4694e7342000000
+0049454e44ae426082
+"""),
+  'basn0g01': _dehex("""
+89504e470d0a1a0a0000000d49484452000000200000002001000000005b0147
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+6aca30692e9d29616ee96f3065f0bf1f1087492fd02f14c90000000049454e44
+ae426082
+"""),
+  'basn0g02': _dehex("""
+89504e470d0a1a0a0000000d49484452000000200000002002000000001ca13d
+890000000467414d41000186a031e8965f0000001f49444154789c6360085df5
+1f8cf1308850c20053868f0133091f6390b90700bd497f818b0989a900000000
+49454e44ae426082
+"""),
+  # A version of basn0g04 dithered down to 3 bits.
+  'Basn0g03': _dehex("""
+89504e470d0a1a0a0000000d494844520000002000000020040000000093e1c8
+2900000001734249540371d88211000000fd49444154789c6d90d18906210c84
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+b5880aa27744fc5c4aecb5e7bcef5fe528ec6293a930690000000049454e44ae
+426082
+"""),
+  'basn0g04': _dehex("""
+89504e470d0a1a0a0000000d494844520000002000000020040000000093e1c8
+290000000467414d41000186a031e8965f0000004849444154789c6360601014
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+912380caddbd9b1c0154ee9933e408a072efde25470095fbee1d1902001f14ee
+01eaff41fa0000000049454e44ae426082
+"""),
+  'basn0g08': _dehex("""
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+8c8ff0cafeffcff887e67131181430cae0956564040050e5fe7135e2d8590000
+000049454e44ae426082
+"""),
+  'basn0g16': _dehex("""
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+d02d680fa44c603f6f07ec4ff41938cf7f0016d84bd85fae2b9fd70000000049
+454e44ae426082
+"""),
+  'basn2c08': _dehex("""
+89504e470d0a1a0a0000000d4948445200000020000000200802000000fc18ed
+a30000000467414d41000186a031e8965f0000004849444154789cedd5c10900
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+7d4c784081020f4a871fc284071428f0a0743823a94081bb7077a3c00182b1f9
+5e0f40cf4b0000000049454e44ae426082
+"""),
+  'basn2c16': _dehex("""
+89504e470d0a1a0a0000000d4948445200000020000000201002000000ac8831
+e00000000467414d41000186a031e8965f000000e549444154789cd596c10a83
+301044a7e0417fcb7eb7fdadf6961e06039286266693cc7a188645e43dd6a08f
+1042003e2fe09aef6472737e183d27335fcee2f35a77b702ebce742870a23397
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+fb8d3630039dbd59601e7ab3c06cf428507f0634d039afdc80123a7bb1801e7a
+b1802a7a14c89f016d74ce331bf080ce9e08f8414f04bca133bfe642fe5e07bb
+c4ec0000000049454e44ae426082
+"""),
+  'basn6a08': _dehex("""
+89504e470d0a1a0a0000000d4948445200000020000000200806000000737a7a
+f40000000467414d41000186a031e8965f0000006f49444154789cedd6310a80
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+011bf00052201a9c160fb84c0000000049454e44ae426082
+"""),
+  'cs3n3p08': _dehex("""
+89504e470d0a1a0a0000000d494844520000002000000020080300000044a48a
+c60000000467414d41000186a031e8965f0000000373424954030303a392a042
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+6ed0121cd6d004bda0013a421ff803224033e177f4ae260000000049454e44ae
+426082
+"""),
+  's09n3p02': _dehex("""
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+789c63600002fbff0c0c56ab19182ca381581a4283f82071200000696505c36a
+437f230000000049454e44ae426082
+"""),
+  'tbgn3p08': _dehex("""
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+"""),
+  'Tp2n3p08': _dehex("""
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+"""),
+  'tbbn1g04': _dehex("""
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+"""),
+  'tbrn2c08': _dehex("""
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+"""),
+  'basn6a16': _dehex("""
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+"""),
+}
+
+def read_pam_header(infile):
+    """
+    Read (the rest of a) PAM header.  `infile` should be positioned
+    immediately after the initial 'P7' line (at the beginning of the
+    second line).  Returns are as for `read_pnm_header`.
+    """
+    
+    # Unlike PBM, PGM, and PPM, we can read the header a line at a time.
+    header = dict()
+    while True:
+        l = infile.readline().strip()
+        if l == strtobytes('ENDHDR'):
+            break
+        if not l:
+            raise EOFError('PAM ended prematurely')
+        if l[0] == strtobytes('#'):
+            continue
+        l = l.split(None, 1)
+        if l[0] not in header:
+            header[l[0]] = l[1]
+        else:
+            header[l[0]] += strtobytes(' ') + l[1]
+
+    required = ['WIDTH', 'HEIGHT', 'DEPTH', 'MAXVAL']
+    required = [strtobytes(x) for x in required]
+    WIDTH,HEIGHT,DEPTH,MAXVAL = required
+    present = [x for x in required if x in header]
+    if len(present) != len(required):
+        raise Error('PAM file must specify WIDTH, HEIGHT, DEPTH, and MAXVAL')
+    width = int(header[WIDTH])
+    height = int(header[HEIGHT])
+    depth = int(header[DEPTH])
+    maxval = int(header[MAXVAL])
+    if (width <= 0 or
+        height <= 0 or
+        depth <= 0 or
+        maxval <= 0):
+        raise Error(
+          'WIDTH, HEIGHT, DEPTH, MAXVAL must all be positive integers')
+    return 'P7', width, height, depth, maxval
+
+def read_pnm_header(infile, supported=('P5','P6')):
+    """
+    Read a PNM header, returning (format,width,height,depth,maxval).
+    `width` and `height` are in pixels.  `depth` is the number of
+    channels in the image; for PBM and PGM it is synthesized as 1, for
+    PPM as 3; for PAM images it is read from the header.  `maxval` is
+    synthesized (as 1) for PBM images.
+    """
+
+    # Generally, see http://netpbm.sourceforge.net/doc/ppm.html
+    # and http://netpbm.sourceforge.net/doc/pam.html
+
+    supported = [strtobytes(x) for x in supported]
+
+    # Technically 'P7' must be followed by a newline, so by using
+    # rstrip() we are being liberal in what we accept.  I think this
+    # is acceptable.
+    type = infile.read(3).rstrip()
+    if type not in supported:
+        raise NotImplementedError('file format %s not supported' % type)
+    if type == strtobytes('P7'):
+        # PAM header parsing is completely different.
+        return read_pam_header(infile)
+    # Expected number of tokens in header (3 for P4, 4 for P6)
+    expected = 4
+    pbm = ('P1', 'P4')
+    if type in pbm:
+        expected = 3
+    header = [type]
+
+    # We have to read the rest of the header byte by byte because the
+    # final whitespace character (immediately following the MAXVAL in
+    # the case of P6) may not be a newline.  Of course all PNM files in
+    # the wild use a newline at this point, so it's tempting to use
+    # readline; but it would be wrong.
+    def getc():
+        c = infile.read(1)
+        if not c:
+            raise Error('premature EOF reading PNM header')
+        return c
+
+    c = getc()
+    while True:
+        # Skip whitespace that precedes a token.
+        while c.isspace():
+            c = getc()
+        # Skip comments.
+        while c == '#':
+            while c not in '\n\r':
+                c = getc()
+        if not c.isdigit():
+            raise Error('unexpected character %s found in header' % c)
+        # According to the specification it is legal to have comments
+        # that appear in the middle of a token.
+        # This is bonkers; I've never seen it; and it's a bit awkward to
+        # code good lexers in Python (no goto).  So we break on such
+        # cases.
+        token = strtobytes('')
+        while c.isdigit():
+            token += c
+            c = getc()
+        # Slight hack.  All "tokens" are decimal integers, so convert
+        # them here.
+        header.append(int(token))
+        if len(header) == expected:
+            break
+    # Skip comments (again)
+    while c == '#':
+        while c not in '\n\r':
+            c = getc()
+    if not c.isspace():
+        raise Error('expected header to end with whitespace, not %s' % c)
+
+    if type in pbm:
+        # synthesize a MAXVAL
+        header.append(1)
+    depth = (1,3)[type == strtobytes('P6')]
+    return header[0], header[1], header[2], depth, header[3]
+
+def write_pnm(file, width, height, pixels, meta):
+    """Write a Netpbm PNM/PAM file."""
+
+    bitdepth = meta['bitdepth']
+    maxval = 2**bitdepth - 1
+    # Rudely, the number of image planes can be used to determine
+    # whether we are L (PGM), LA (PAM), RGB (PPM), or RGBA (PAM).
+    planes = meta['planes']
+    # Can be an assert as long as we assume that pixels and meta came
+    # from a PNG file.
+    assert planes in (1,2,3,4)
+    if planes in (1,3):
+        if 1 == planes:
+            # PGM
+            # Could generate PBM if maxval is 1, but we don't (for one
+            # thing, we'd have to convert the data, not just blat it
+            # out).
+            fmt = 'P5'
+        else:
+            # PPM
+            fmt = 'P6'
+        file.write('%s %d %d %d\n' % (fmt, width, height, maxval))
+    if planes in (2,4):
+        # PAM
+        # See http://netpbm.sourceforge.net/doc/pam.html
+        if 2 == planes:
+            tupltype = 'GRAYSCALE_ALPHA'
+        else:
+            tupltype = 'RGB_ALPHA'
+        file.write('P7\nWIDTH %d\nHEIGHT %d\nDEPTH %d\nMAXVAL %d\n'
+                   'TUPLTYPE %s\nENDHDR\n' %
+                   (width, height, planes, maxval, tupltype))
+    # Values per row
+    vpr = planes * width
+    # struct format
+    fmt = '>%d' % vpr
+    if maxval > 0xff:
+        fmt = fmt + 'H'
+    else:
+        fmt = fmt + 'B'
+    for row in pixels:
+        file.write(struct.pack(fmt, *row))
+    file.flush()
+
+def color_triple(color):
+    """
+    Convert a command line colour value to a RGB triple of integers.
+    FIXME: Somewhere we need support for greyscale backgrounds etc.
+    """
+    if color.startswith('#') and len(color) == 4:
+        return (int(color[1], 16),
+                int(color[2], 16),
+                int(color[3], 16))
+    if color.startswith('#') and len(color) == 7:
+        return (int(color[1:3], 16),
+                int(color[3:5], 16),
+                int(color[5:7], 16))
+    elif color.startswith('#') and len(color) == 13:
+        return (int(color[1:5], 16),
+                int(color[5:9], 16),
+                int(color[9:13], 16))
+
+def _add_common_options(parser):
+    """Call *parser.add_option* for each of the options that are
+    common between this PNG--PNM conversion tool and the gen
+    tool.
+    """
+    parser.add_option("-i", "--interlace",
+                      default=False, action="store_true",
+                      help="create an interlaced PNG file (Adam7)")
+    parser.add_option("-t", "--transparent",
+                      action="store", type="string", metavar="#RRGGBB",
+                      help="mark the specified colour as transparent")
+    parser.add_option("-b", "--background",
+                      action="store", type="string", metavar="#RRGGBB",
+                      help="save the specified background colour")
+    parser.add_option("-g", "--gamma",
+                      action="store", type="float", metavar="value",
+                      help="save the specified gamma value")
+    parser.add_option("-c", "--compression",
+                      action="store", type="int", metavar="level",
+                      help="zlib compression level (0-9)")
+    return parser
+
+def _main(argv):
+    """
+    Run the PNG encoder with options from the command line.
+    """
+
+    # Parse command line arguments
+    from optparse import OptionParser
+    import re
+    version = '%prog ' + re.sub(r'( ?\$|URL: |Rev:)', '', __version__)
+    parser = OptionParser(version=version)
+    parser.set_usage("%prog [options] [imagefile]")
+    parser.add_option('-r', '--read-png', default=False,
+                      action='store_true',
+                      help='Read PNG, write PNM')
+    parser.add_option("-a", "--alpha",
+                      action="store", type="string", metavar="pgmfile",
+                      help="alpha channel transparency (RGBA)")
+    _add_common_options(parser)
+
+    (options, args) = parser.parse_args(args=argv[1:])
+
+    # Convert options
+    if options.transparent is not None:
+        options.transparent = color_triple(options.transparent)
+    if options.background is not None:
+        options.background = color_triple(options.background)
+
+    # Prepare input and output files
+    if len(args) == 0:
+        infilename = '-'
+        infile = sys.stdin
+    elif len(args) == 1:
+        infilename = args[0]
+        infile = open(infilename, 'rb')
+    else:
+        parser.error("more than one input file")
+    outfile = sys.stdout
+    if sys.platform == "win32":
+        import msvcrt, os
+        msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
+
+    if options.read_png:
+        # Encode PNG to PPM
+        png = Reader(file=infile)
+        width,height,pixels,meta = png.asDirect()
+        write_pnm(outfile, width, height, pixels, meta) 
+    else:
+        # Encode PNM to PNG
+        format, width, height, depth, maxval = \
+          read_pnm_header(infile, ('P5','P6','P7'))
+        # When it comes to the variety of input formats, we do something
+        # rather rude.  Observe that L, LA, RGB, RGBA are the 4 colour
+        # types supported by PNG and that they correspond to 1, 2, 3, 4
+        # channels respectively.  So we use the number of channels in
+        # the source image to determine which one we have.  We do not
+        # care about TUPLTYPE.
+        greyscale = depth <= 2
+        pamalpha = depth in (2,4)
+        supported = map(lambda x: 2**x-1, range(1,17))
+        try:
+            mi = supported.index(maxval)
+        except ValueError:
+            raise NotImplementedError(
+              'your maxval (%s) not in supported list %s' %
+              (maxval, str(supported)))
+        bitdepth = mi+1
+        writer = Writer(width, height,
+                        greyscale=greyscale,
+                        bitdepth=bitdepth,
+                        interlace=options.interlace,
+                        transparent=options.transparent,
+                        background=options.background,
+                        alpha=bool(pamalpha or options.alpha),
+                        gamma=options.gamma,
+                        compression=options.compression)
+        if options.alpha:
+            pgmfile = open(options.alpha, 'rb')
+            format, awidth, aheight, adepth, amaxval = \
+              read_pnm_header(pgmfile, 'P5')
+            if amaxval != '255':
+                raise NotImplementedError(
+                  'maxval %s not supported for alpha channel' % amaxval)
+            if (awidth, aheight) != (width, height):
+                raise ValueError("alpha channel image size mismatch"
+                                 " (%s has %sx%s but %s has %sx%s)"
+                                 % (infilename, width, height,
+                                    options.alpha, awidth, aheight))
+            writer.convert_ppm_and_pgm(infile, pgmfile, outfile)
+        else:
+            writer.convert_pnm(infile, outfile)
+
+
+if __name__ == '__main__':
+    try:
+        _main(sys.argv)
+    except Error, e:
+        print >>sys.stderr, e