Remove opaque alpha channel special case

third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp

 /*
  * Copyright (c) 2008, 2009, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 795 matching lines @@
       // RGB data.  Decode pixels one at a time, left to right.
       while (m_coord.x() < endX) {
         const uint32_t pixel = readCurrentPixel(bytesPerPixel);
 
         // Some BMPs specify an alpha channel but don't actually use it
         // (it contains all 0s).  To avoid displaying these images as
         // fully-transparent, decode as if images are fully opaque
         // until we actually see a non-zero alpha value; at that point,
         // reset any previously-decoded pixels to fully transparent and
         // continue decoding based on the real alpha channel values.
-        // As an optimization, avoid setting "hasAlpha" to true for
-        // images where all alpha values are 255; opaque images are
-        // faster to draw.
         int alpha = getAlpha(pixel);
         if (!m_seenNonZeroAlphaPixel && !alpha) {
           m_seenZeroAlphaPixel = true;
           alpha = 255;
         } else {
           m_seenNonZeroAlphaPixel = true;
           if (m_seenZeroAlphaPixel) {
             m_buffer->zeroFillPixelData();
             m_seenZeroAlphaPixel = false;
-          } else if (alpha != 255)
-            m_buffer->setHasAlpha(true);

[Peter Kasting, 2017/03/16 04:37:14]

Removing this is not correct.  It will mean any BMP that does not begin with
zero-alpha pixels will be marked fully-opaque.

Consider a one-pixel RGB BMP, where the one pixel has alpha 128.  decodeBMP()
will call zeroFillPixelData(), which will call setHasAlpha(true).  It will then
immediately call setHasAlpha(false).  Then we'll come here.  Since alpha != 0
and m_seenZeroAlphaPixel == false, we would have taken this branch and marked
this image as having alpha.  Now we won't.  Then decoding will finish.

[cblume, 2017/03/16 09:36:47]

You are right that the call to zeroFillPixelData() is inside if
(m_seenZeroAlphaPixel);.
So that means the scenario you described -- a 1x1 BMP with 128 alpha will not
get m_buffer->setHasAlpha(true); called.

I meant to change this to:
} else    m_buffer->setHasAlpha(true);
which would have also covered case (3) you mentioned.

+          }
         }
 
         setRGBA(getComponent(pixel, 0), getComponent(pixel, 1),
                 getComponent(pixel, 2), alpha);
       }
     }
 
     // Success, keep going.
     m_decodedOffset += paddedNumBytes;
     if (inRLE)
       return Success;
     moveBufferToNextRow();
   }
 
   // Finished decoding whole image.
   return Success;
 }
 
 void BMPImageReader::moveBufferToNextRow() {
   m_coord.move(-m_coord.x(), m_isTopDown ? 1 : -1);
 }
 
 }  // namespace blink