Remove opaque alpha channel special case

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

Index: third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp b/third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp
index 734a7b2527bed95be6dc11f960bf2a1024f337ac..7a4bd735feb7ecb21dedf3e8409dad0357fb7601 100644
--- a/third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp
@@ -813,9 +813,6 @@ BMPImageReader::ProcessingResult BMPImageReader::processNonRLEData(
         // 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;
@@ -825,8 +822,7 @@ BMPImageReader::ProcessingResult BMPImageReader::processNonRLEData(
           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),