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

chrome/common/gfx/chrome_canvas.cc

 // Copyright (c) 2006-2008 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 "chrome/common/gfx/chrome_canvas.h"
 
 #include <limits>
 
 #include "base/gfx/rect.h"
 #include "base/logging.h"
@@ skipping 171 matching lines @@
   }
 
   // Make a bitmap shader that contains the bitmap we want to draw. This is
   // basically what SkCanvas.drawBitmap does internally, but it gives us
   // more control over quality and will use the mipmap in the source image if
   // it has one, whereas drawBitmap won't.
   SkShader* shader = SkShader::CreateBitmapShader(bitmap,
                                                   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
   // by the paint).
   SkPaint p(paint);
   p.setFilterBitmap(filter);
   p.setShader(shader);
   shader->unref();
 
   // The rect will be filled by the bitmap.
@@ skipping 32 matching lines @@
 SkBitmap ChromeCanvas::ExtractBitmap() {
   const SkBitmap& device_bitmap = getDevice()->accessBitmap(false);
 
   // Make a bitmap to return, and a canvas to draw into it. We don't just want
   // to call extractSubset or the copy constuctor, since we want an actual copy
   // of the bitmap.
   SkBitmap result;
   device_bitmap.copyTo(&result, SkBitmap::kARGB_8888_Config);
   return result;
 }