Fix some problems with scaled icons. This allows the TabIconView to automati...

chrome/common/gfx/chrome_canvas.cc

Index: chrome/common/gfx/chrome_canvas.cc
===================================================================
--- chrome/common/gfx/chrome_canvas.cc	(revision 8058)
+++ chrome/common/gfx/chrome_canvas.cc	(working copy)
@@ -189,11 +189,10 @@
                                                   SkShader::kRepeat_TileMode,
                                                   SkShader::kRepeat_TileMode);
   SkMatrix shader_scale;
-  shader_scale.setScale(
-      SkFloatToScalar(static_cast<float>(dest_w) / src_w),
-      SkFloatToScalar(static_cast<float>(dest_h) / src_h));
-  shader_scale.postTranslate(SkIntToScalar(dest_x - src_x),
-                             SkIntToScalar(dest_y - src_y));
+  shader_scale.setScale(SkFloatToScalar(static_cast<float>(dest_w) / src_w),
+                        SkFloatToScalar(static_cast<float>(dest_h) / src_h));
+  shader_scale.preTranslate(SkIntToScalar(-src_x), SkIntToScalar(-src_y));
+  shader_scale.postTranslate(SkIntToScalar(dest_x), SkIntToScalar(dest_y));

[sky, 2009/01/21 00:53:22]

I've no idea on this code, I'm assuming you know what you're donig.

[Peter Kasting, 2009/01/21 01:00:36]

Pre-translate means "do this before the existing matrix takes effect",
post-translate means "do after".  If we have a source offset, dest offset, and
scaling factor, what we want is:
* Get desired subimage from source image at source offset
* Scale subimage to desired size
* Display scaled subimage at appropriate dest offset

The new code does this.  The old code scaled first:
* Scale source image up
* Get subimage from within source image (at unscaled offset!)
* Display scaled subimage at appropriate dest offset

As a result we would pick the wrong pixels out of the subimage.  I could have
manually scaled the |src_x| and |src_y| factors in the single postTranslate()
call, but I think that is less clear and may also be less precise.

   shader->setLocalMatrix(shader_scale);
 
   // Set up our paint to use the shader & release our reference (now just owned