Optionally clear PlatformCanvas instances.

ui/gfx/canvas_skia.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/canvas_skia.h"
 
 #include <limits>
 
 #include "base/i18n/rtl.h"
 #include "base/logging.h"
@@ skipping 36 matching lines @@
 
 namespace gfx {
 
 ////////////////////////////////////////////////////////////////////////////////
 // CanvasSkia, public:
 
 CanvasSkia::CanvasSkia(const gfx::Size& size, bool is_opaque)
     : owned_canvas_(new skia::PlatformCanvas(size.width(), size.height(),
                                              is_opaque)),
       canvas_(owned_canvas_.get()) {
+#if defined(OS_WIN) || defined(OS_MACOSX)
+  // skia::PlatformCanvas instances are initialized to 0 by Cairo on Linux, but

[msw, 2012/03/15 21:53:07]

Should this be done in Skia / SkCanvas?

[Jeff Timanus, 2012/03/15 22:26:53]

SkCanvas used to construct initialized 0, but that has since changed.  The
assumption is that SkCanvas instances are not initialized.  This change carries
those semantics to skia/ext.

+  // uninitialized on Win and Mac.
+  owned_canvas_->clear(SkColorSetARGB(0, 0, 0, 0));

[msw, 2012/03/15 21:53:07]

Can this be skipped for is_opaque == true or false?
What's the perf impact here?
I presume it's better than the status quo, given the bug.

[Jeff Timanus, 2012/03/15 22:26:53]

You're right, but the old behaviour was a little confusing.

On Mac, a PlatformCanvas would always be constructed with a cleared canvas,
regardless of the is_opaque argument.

On Win, the clear would only happen if !is_opaque. So there is a potential for a
perf hit here on Win.

The hit is on the order of ~2-3 ms (from observed results).

+#endif
 }
 
 CanvasSkia::CanvasSkia(const SkBitmap& bitmap, bool is_opaque)
     : owned_canvas_(new skia::PlatformCanvas(bitmap.width(), bitmap.height(),
                                              is_opaque)),
       canvas_(owned_canvas_.get()) {
   DrawBitmapInt(bitmap, 0, 0);
 }
 
 CanvasSkia::CanvasSkia()
@@ skipping 307 matching lines @@
 // CanvasSkia, private:
 
 bool CanvasSkia::IntersectsClipRectInt(int x, int y, int w, int h) {
   SkRect clip;
   return canvas_->getClipBounds(&clip) &&
       clip.intersect(SkIntToScalar(x), SkIntToScalar(y), SkIntToScalar(x + w),
                      SkIntToScalar(y + h));
 }
 
 }  // namespace gfx