Fix SkBitmapOperations::CreateDropShadow drawing too large a shadow.

ui/gfx/skbitmap_operations.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/skbitmap_operations.h"
 
 #include <string.h>
 #include <algorithm>
 
 #include "base/logging.h"
@@ skipping 723 matching lines @@
   SkCanvas canvas(image_with_shadow);
   canvas.translate(SkIntToScalar(shadow_margin.left()),
                    SkIntToScalar(shadow_margin.top()));
 
   SkPaint paint;
   for (size_t i = 0; i < shadows.size(); ++i) {
     const gfx::ShadowValue& shadow = shadows[i];
     SkBitmap shadow_image = SkBitmapOperations::CreateColorMask(bitmap,
                                                                 shadow.color());
 
-    skia::RefPtr<SkBlurImageFilter> filter =
-        skia::AdoptRef(SkBlurImageFilter::Create(
-            SkDoubleToScalar(shadow.blur()), SkDoubleToScalar(shadow.blur())));
+    skia::RefPtr<SkBlurImageFilter> filter = skia::AdoptRef(
+        SkBlurImageFilter::Create(SkDoubleToScalar(shadow.blur() / 2),

[Alexei Svitkine (slow), 2015/05/19 15:12:04]

It's not clear to me how this ensures that the shadows stay within the margin?
Wouldn't it depend on the size of the margins?

Also, wouldn't this affect all the places using this - even ones which may not
have the problem? (i.e. they would now look different due to this change - which
may not be desired?)

[calamity, 2015/05/20 04:26:51]

Sorry, I should have explained better. At the beginning of CreateDropShadow(),
the result canvas is enlarged by ShadowValue::GetMargin() so that the painted
shadow will fit. The current code produces a shadow that's twice as large as
ShadowValue::GetMargin()'s insets.

I vetted the other callsites and there's only the app list and the ash shelf
buttons. The ash shelf buttons aren't affected by this because they use a blur
value of 1 and the inset size for that is 1px.

I can change the shadow value for the shelf to be 2 if you want pixel perfect
congruence with the old behavior.

[Alexei Svitkine (slow), 2015/05/20 20:47:42]

Thanks for the explanation, that makes sense. Please add a comment above the
division by 2 to also make it clear to the reader of the code.

[calamity, 2015/05/21 06:54:30]

Done.

+                                  SkDoubleToScalar(shadow.blur() / 2)));

[Alexei Svitkine (slow), 2015/05/20 20:47:42]

Nit: Since you use SkDoubleToScalar(shadow.blur() / 2) twice, please make a
local var above to make the code both more concise and more efficient.

[calamity, 2015/05/21 06:54:30]

Done.

     paint.setImageFilter(filter.get());
 
     canvas.saveLayer(0, &paint);
     canvas.drawBitmap(shadow_image,
                       SkIntToScalar(shadow.x()),
                       SkIntToScalar(shadow.y()));
     canvas.restore();
   }
 
   canvas.drawBitmap(bitmap, SkIntToScalar(0), SkIntToScalar(0));
@@ skipping 27 matching lines @@
   canvas.translate(SkFloatToScalar(result.width() * 0.5f),
                    SkFloatToScalar(result.height() * 0.5f));
   canvas.rotate(angle);
   canvas.translate(-SkFloatToScalar(source.width() * 0.5f),
                    -SkFloatToScalar(source.height() * 0.5f));
   canvas.drawBitmap(source, 0, 0);
   canvas.flush();
 
   return result;
 }