Optionally clear PlatformCanvas instances.

ui/gfx/canvas_skia_win.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.h"
 
 #include <limits>
 
 #include "base/i18n/rtl.h"
 #include "base/logging.h"
@@ skipping 533 matching lines @@
 
   // Grow |display_rect| by |offset_x|.
   gfx::Rect text_rect(gfx::Point(), display_rect.size());
   if (!is_rtl)
     text_rect.set_x(text_rect.x() - offset_x);
   text_rect.set_width(text_rect.width() + offset_x);
 
   // Create a temporary bitmap to draw the gradient to.
   scoped_ptr<SkCanvas> gradient_canvas(skia::CreateBitmapCanvas(
       display_rect.width(), display_rect.height(), false));
+  gradient_canvas->clear(SkColorSetARGB(0, 0, 0, 0));

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

Is this needed by other direct SkCanvas users too?
It seems like this ought to be in the SkCanvas impl.

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

I searched through all users of SkCanvas (and assoc.), and found that only the
sites in this change needed the clear.

Putting it in the impl forces the perf impact for all canvas users, even those
that don't need the clear.  An earlier revision of this change introduced a
construction argument to request the clear, but I found that since there are so
few sites requiring the clear, it was easier to simply assume that
PlatformCanvas instances are not initialized.

 
   {
     skia::ScopedPlatformPaint scoped_platform_paint(canvas_);
     HDC hdc = scoped_platform_paint.GetPlatformSurface();
     if (is_truncating_head)
       DrawTextGradientPart(hdc, gradient_canvas.get(), text, color,
                            font.GetNativeFont(), text_rect, head_part, is_rtl,
                            flags);
     if (is_truncating_tail)
       DrawTextGradientPart(hdc, gradient_canvas.get(), text, color,
                            font.GetNativeFont(), text_rect, tail_part, !is_rtl,
                            flags);
   }
 
   // Draw the solid part.
   canvas_->save(SkCanvas::kClip_SaveFlag);
   ClipRect(solid_part);
   DrawStringInt(text, font, color,
                 text_rect.x(), text_rect.y(),
                 text_rect.width(), text_rect.height(),
                 flags);
   canvas_->restore();
   canvas_->restore();
 }
 
 }  // namespace gfx