Use alpha channel instead of red in drawCachedGlyphBitmap

the correct value to copy in bitmap is alpha component from
the cachebuffer. currently it is offset to red channel if format is
RGBA which is wrong.

this is followup fix for https://android-review.googlesource.com/#/c/93943/

Change-Id: I96b5ba35df5ccaef44caf2542d35d6585ba1df80
Signed-off-by: Digish Pandya <digishp@codeaurora.org>

2 files changed, 21 insertions, 1 deletions

diff --git a/libs/hwui/PixelBuffer.h b/libs/hwui/PixelBuffer.h
index 9725a6162a2c..04225a20cfc2 100644
--- a/libs/hwui/PixelBuffer.h
+++ b/libs/hwui/PixelBuffer.h
@@ -175,6 +175,25 @@ public:
         return 0;
     }
 
+    /**
+     * Returns the alpha channel offset in the specified format.
+     *
+     * Supported formats:
+     *      GL_ALPHA
+     *      GL_RGBA
+     */
+    static uint32_t formatAlphaOffset(GLenum format) {
+        switch (format) {
+            case GL_ALPHA:
+                return 0;
+            case GL_RGBA:
+                return 3;
+        }
+
+        ALOGE("unsupported format: %d",format);
+        return 0;
+    }
+
 protected:
     /**
      * Creates a new render buffer in the specified format and dimensions.
diff --git a/libs/hwui/font/Font.cpp b/libs/hwui/font/Font.cpp
index b49329834760..eb33cf1f3fdc 100644
--- a/libs/hwui/font/Font.cpp
+++ b/libs/hwui/font/Font.cpp
@@ -215,6 +215,7 @@ void Font::drawCachedGlyphBitmap(CachedGlyphInfo* glyph, int x, int y, uint8_t*
     PixelBuffer* pixelBuffer = cacheTexture->getPixelBuffer();
 
     uint32_t formatSize = PixelBuffer::formatSize(pixelBuffer->getFormat());
+    uint32_t alpha_channel_offset = PixelBuffer::formatAlphaOffset(pixelBuffer->getFormat());
     uint32_t cacheWidth = cacheTexture->getWidth();
     uint32_t srcStride = formatSize * cacheWidth;
     uint32_t startY = glyph->mStartY * srcStride;
@@ -229,7 +230,7 @@ void Font::drawCachedGlyphBitmap(CachedGlyphInfo* glyph, int x, int y, uint8_t*
             memcpy(&bitmap[bitmapY + dstX], &cacheBuffer[cacheY + glyph->mStartX], glyph->mBitmapWidth);
         } else {
             for (uint32_t i = 0; i < glyph->mBitmapWidth; ++i) {
-                bitmap[bitmapY + dstX + i] = cacheBuffer[cacheY + (glyph->mStartX + i)*formatSize];
+                bitmap[bitmapY + dstX + i] = cacheBuffer[cacheY + (glyph->mStartX + i)*formatSize + alpha_channel_offset];
             }
         }
     }