Add heuristic for fixing the alpha channel when reading clipboard images on Windows

ui/base/clipboard/clipboard_win.cc

Index: ui/base/clipboard/clipboard_win.cc
diff --git a/ui/base/clipboard/clipboard_win.cc b/ui/base/clipboard/clipboard_win.cc
index cec06012fbf50e9540fe27d69155a15eba6122e2..a736f638feeb37b53440f69c03fc0a1991e49354 100644
--- a/ui/base/clipboard/clipboard_win.cc
+++ b/ui/base/clipboard/clipboard_win.cc
@@ -476,18 +476,38 @@ SkBitmap Clipboard::ReadImage(Buffer buffer) const {
                         bitmap->bmiHeader.biHeight, bitmap_bits, bitmap,
                         DIB_RGB_COLORS);
   }
-  // SetDIBitsToDevice doesn't properly set alpha values for bitmaps with
-  // depth < 32bpp so manually fix it up.
-  if (bitmap->bmiHeader.biBitCount < 32) {
-    const SkBitmap& device_bitmap = canvas.getDevice()->accessBitmap(true);
+  // Windows doesn't really handle alpha channels well in many situations. We
+  // attempt to manually fix the alpha channel here. Since Windows uses
+  // premultiplied alpha, we scan for instances where (R, G, B) > A. If there
+  // are any invalid premultiplied colors in the image, we assume the alpha
+  // channel contains junk and reset the alpha value for all pixels to 255. This
+  // heuristic will fail on a transparent bitmap containing only black pixels...
+  const SkBitmap& device_bitmap = canvas.getDevice()->accessBitmap(true);
+  {
     SkAutoLockPixels lock(device_bitmap);
+    bool has_invalid_alpha_channel = false;
     for (int x = 0; x < device_bitmap.width(); ++x) {
       for (int y = 0; y < device_bitmap.height(); ++y) {
-        *device_bitmap.getAddr32(x, y) =
-            SkColorSetA(*device_bitmap.getAddr32(x, y), 0xFF);
+        uint32_t pixel = *device_bitmap.getAddr32(x, y);
+        if (SkColorGetR(pixel) > SkColorGetA(pixel) ||
+            SkColorGetG(pixel) > SkColorGetA(pixel) ||
+            SkColorGetB(pixel) > SkColorGetA(pixel)) {
+          has_invalid_alpha_channel = true;
+          break;

[tony, 2011/09/20 23:26:54]

This only breaks out of the inner for loop.  You probably want to break out of
the outer loop too (or just make a helper function and early return).

[dcheng, 2011/09/20 23:56:33]

Done. Thanks for the catch.

+        }
+      }
+    }
+
+    if (has_invalid_alpha_channel) {
+      for (int x = 0; x < device_bitmap.width(); ++x) {
+        for (int y = 0; y < device_bitmap.height(); ++y) {
+          *device_bitmap.getAddr32(x, y) =
+              SkColorSetA(*device_bitmap.getAddr32(x, y), 0xFF);
+        }
       }
     }
   }
+
   return canvas.ExtractBitmap();
 }