Don't decode AND mask for an icon that already has alpha information

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

 /*
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  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
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@@ skipping 120 matching lines @@
         if ((m_infoHeader.biCompression != RLE4) && (m_infoHeader.biCompression != RLE8) && (m_infoHeader.biCompression != RLE24)) {
             const ProcessingResult result = processNonRLEData(false, 0);
             if (result != Success)
                 return (result == Failure) ? m_parent->setFailed() : false;
         } else if (!processRLEData())
             return false;
     }
 
     // If the image has an AND mask and there was no alpha data, process the
     // mask.
-    if (m_isInICO && !m_decodingAndMask && !m_buffer->hasAlpha()) {
+    if (m_isInICO && !m_decodingAndMask && !m_buffer->hasAlpha() && !m_seenNonZeroAlphaPixel) {

[Peter Kasting, 2014/11/21 20:05:54]

You shouldn't need to check m_buffer->hasAlpha() anymore.

I wonder if you need to check m_bitMasks[3], though:

  ... && (!m_bitMasks[3] || !m_seenNonZeroAlphaPixel)

If m_bitMasks[3] is zero, the image has explicitly said it has no alpha channel.
 In this case, m_seenNonZeroAlphaPixel is always going to be true, since we
pretend the alpha value is always 255.  But I think you want to use the AND mask
anyway.  I don't know if Mozilla does this -- it looks to me like they may not
handle BI_BITFIELDS correctly for 32bpp images (but rather assume the image is
always in 8888 form).  But it might be worth creating a test image that is 32bpp
with a zero-width alpha channel and seeing whether e.g. Windows respects the AND
mask.  We could use that in a layout test too.

         // Reset decoding coordinates to start of image.
         m_coord.setX(0);
         m_coord.setY(m_isTopDown ? 0 : (m_parent->size().height() - 1));
 
         // The AND mask is stored as 1-bit data.
         m_infoHeader.biBitCount = 1;
 
         m_decodingAndMask = true;
     }
     if (m_decodingAndMask) {
@@ skipping 640 matching lines @@
     // Finished decoding whole image.
     return Success;
 }
 
 void BMPImageReader::moveBufferToNextRow()
 {
     m_coord.move(-m_coord.x(), m_isTopDown ? 1 : -1);
 }
 
 } // namespace blink