Make H264BitReader ignore trailing zero bytes and stop bit.

content/common/gpu/media/h264_bit_reader.h

Index: content/common/gpu/media/h264_bit_reader.h
diff --git a/content/common/gpu/media/h264_bit_reader.h b/content/common/gpu/media/h264_bit_reader.h
index c43b6f774ecee53d1df9f2f7bd37a29e538ff566..96f28ca0d3e1a57d7b20d10de34bf9c4b603ef72 100644
--- a/content/common/gpu/media/h264_bit_reader.h
+++ b/content/common/gpu/media/h264_bit_reader.h
@@ -16,8 +16,13 @@ namespace content {
 
 // A class to provide bit-granularity reading of H.264 streams.
 // This is not a generic bit reader class, as it takes into account
-// H.264 stream-specific constraints, such as skipping emulation-prevention
-// bytes and stop bits. See spec for more details.
+// H.264 stream-specific constraints:
+// - Skipping emulation-prevention bytes, i.e. any byte aligned sequence
+//     0x00 0x00 0x03 will be replaced by 0x00 0x00.
+// - Ignore trailing zero bytes/bits, i.e. any consecutive zero bytes/bits at
+//     the end of the stream will be ignored.
+// - Ignore stop bit, i.e. the last non-zero bit will be ignored.
+// See ISO 14496 Part 10 - 7.3 for more details.
 class CONTENT_EXPORT H264BitReader {
  public:
   H264BitReader();
@@ -30,24 +35,31 @@ class CONTENT_EXPORT H264BitReader {
   // heap-allocating and creating bit readers on demand instead.
   bool Initialize(const uint8* data, off_t size);
 
-  // Read |num_bits| next bits from stream and return in |*out|, first bit
-  // from the stream starting at |num_bits| position in |*out|.
-  // |num_bits| may be 1-32, inclusive.
+  // Read |num_bits| (1 to 31 inclusive) from the stream and return them
+  // in |*out|, with first bit in the stream as MSB in |*out| at position
+  // (|num_bits| - 1).
   // Return false if the given number of bits cannot be read (not enough
   // bits in the stream), true otherwise.
   bool ReadBits(int num_bits, int *out);
 
-  // Return the number of bits left in the stream.
-  off_t NumBitsLeft();
+  // Return the number of bits left in the stream. This includes the
+  // stop bit and any trailing zero byte.

[Ami GONE FROM CHROMIUM, 2012/08/01 23:04:00]

Why is this API reasonable, given that these bits can never be returned in
ReadBits?

[xiaomings, 2012/08/01 23:39:54]

This is actually used in h264_parser.cc:1081 for calculating the size of the
head. It can be replaced by a Tell() implementation. Let me know if we should
add Tell().
On 2012/08/01 23:04:00, Ami Fischman wrote:

+  off_t NumBitsLeft() const;
 
   // See the definition of more_rbsp_data() in spec.
-  bool HasMoreRBSPData();
+  bool HasMoreRBSPData() const;

[Ami GONE FROM CHROMIUM, 2012/08/01 23:04:00]

This name no longer makes as much sense since you're making ReadBits really be
ReadRBSPBits.

[xiaomings, 2012/08/01 23:39:54]

Removed.

On 2012/08/01 23:04:00, Ami Fischman wrote:

 
  private:
   // Advance to the next byte, loading it into curr_byte_.
   // Return false on end of stream.
   bool UpdateCurrByte();
 
+  // Remove the trailing zero bits if there are any. The trailing zero bits
+  // can spread among more than byte. Then remove the stop bit. So the bit
+  // stream 11001100 00000000 will become 11001.
+  // It returns true if there are still data bits left.
+  bool RemoveTrailingZeroAndStopBit();
+
   // Pointer to the next unread (not in curr_byte_) byte in the stream.
   const uint8* data_;
 
@@ -65,6 +77,13 @@ class CONTENT_EXPORT H264BitReader {
   // Initially set to 0xffff to accept all initial two-byte sequences.
   int prev_two_bytes_;
 
+  // Save the number of trailing zero bytes to be used in NumBitsLeft.
+  int trailing_zero_bytes_;
+
+  // The valid data bits in the last byte, this doesn't take stop bit and
+  // trailing zero bits into account. It can never be 0.

[Ami GONE FROM CHROMIUM, 2012/08/01 23:04:00]

"this doesn't take ... into account"
is pretty confusing.  I think you just want "excluding stop bit & trailing zero
bits".

[xiaomings, 2012/08/01 23:39:54]

Modified.

On 2012/08/01 23:04:00, Ami Fischman wrote:

+  int data_bits_in_last_byte_;
+
   DISALLOW_COPY_AND_ASSIGN(H264BitReader);
 };