Reland of r19131, this time with real Math:...

base/gfx/png_decoder.cc

Index: base/gfx/png_decoder.cc
===================================================================
--- base/gfx/png_decoder.cc	(revision 19579)
+++ base/gfx/png_decoder.cc	(working copy)
@@ -261,8 +261,8 @@
 
 // static
 bool PNGDecoder::Decode(const unsigned char* input, size_t input_size,
-                      ColorFormat format, std::vector<unsigned char>* output,
-                      int* w, int* h) {
+                        ColorFormat format, std::vector<unsigned char>* output,
+                        int* w, int* h) {
   if (input_size < 8)
     return false;  // Input data too small to be a png
 
@@ -324,7 +324,22 @@
                          &decoded_data, &width, &height)) {
     bitmap->setConfig(SkBitmap::kARGB_8888_Config, width, height);
     bitmap->allocPixels();
-    memcpy(bitmap->getPixels(), &decoded_data.front(), width * height * 4);
+    unsigned char* bitmap_data =
+        reinterpret_cast<unsigned char*>(bitmap->getAddr32(0, 0));
+    for (int i = width * height * 4 - 4; i >= 0; i -= 4) {
+      unsigned char alpha = decoded_data[i + 3];
+      if (alpha != 0 && alpha != 255) {
+        bitmap_data[i + 3] = alpha;
+        bitmap_data[i] = (decoded_data[i] * alpha) >> 8;
+        bitmap_data[i + 1] = (decoded_data[i + 1] * alpha) >> 8;
+        bitmap_data[i + 2] = (decoded_data[i + 2] * alpha) >> 8;
+      } else {
+        bitmap_data[i + 3] = alpha;
+        bitmap_data[i] = decoded_data[i];
+        bitmap_data[i + 1] = decoded_data[i + 1];
+        bitmap_data[i + 2] = decoded_data[i + 2];
+      }
+    }
     return true;
   }
   return false;