Scanline decoding for bmp

include/codec/SkScaledCodec.h

Index: include/codec/SkScaledCodec.h
diff --git a/include/codec/SkScaledCodec.h b/include/codec/SkScaledCodec.h
index 1bcdf085b213f7eaab1cb9d9eca51a491138c58f..14b7f65f5c2600ae819b193826f7076fb37c356f 100644
--- a/include/codec/SkScaledCodec.h
+++ b/include/codec/SkScaledCodec.h
@@ -40,6 +40,34 @@ public:
         return true;   
     }
 
+    /*
+     * Returns the first coordinate that we will keep during sampling.
+     * The output can be interpreted as an x-coordinate or a y-coordinate.
+     *
+     * This does not need to be called and is not called when sampleFactor == 1.
+     */
+    static int GetStartCoord(int sampleFactor) { return sampleFactor / 2 };

[msarett, 2015/08/28 15:09:18]

It occurs to me that maybe we don't want these implementations visible in the .h
file?  If that is the case, I can move them back to the .cpp file.  And later on
we can investigate if it's actually worth it to move them to SkCodecPriv.h and
try to inline.

+
+    /*
+     * Given a coordinate in the original image, this returns the corresponding
+     * coordinate in the scaled image.  This function is meaningless if
+     * IsCoordNecessary returns false.
+     * The output can be interpreted as an x-coordinate or a y-coordinate.
+     *
+     * This does not need to be called and is not called when sampleFactor == 1.
+     */
+    static int GetDstCoord(int srcCoord, int sampleFactor) { return srcCoord / sampleFactor; }
+
+    /*
+     * When sampling, we will discard certain y-coordinates (rows) and
+     * x-coordinates (columns).  This function returns true if we should keep the
+     * coordinate and false otherwise.
+     * The inputs may be x-coordinates or y-coordinates.
+     *
+     * This does not need to be called and is not called when sampleFactor == 1.
+     */
+    static bool IsCoordNecessary(int srcCoord, int sampleFactor, int scaledDim);
+
     static void ComputeSampleSize(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo,
                                   int* sampleSizeX, int* sampleSizeY);