Scanline decoding for bmp

include/codec/SkScaledCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #ifndef SkScaledCodec_DEFINED
 #define SkScaledCodec_DEFINED
 
 #include "SkCodec.h"
@@ skipping 22 matching lines @@
         if (dstInfo.height() != srcInfo.height()) {
             return false;
         }
         // only support down sampling, dstWidth cannot be larger that srcWidth
         if(dstInfo.width() > srcInfo.width()) {
             return false;
         }
         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);
 
 protected:
     /**
      * Recommend a set of destination dimensions given a requested scale
      */
     SkISize onGetScaledDimensions(float desiredScale) const override;
 
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override {
         return fScanlineDecoder->getEncodedFormat();
     }
 
     bool onReallyHasAlpha() const override {
         return fScanlineDecoder->reallyHasAlpha();
     }
 
 private:
 
     SkAutoTDelete<SkScanlineDecoder> fScanlineDecoder;
 
     explicit SkScaledCodec(SkScanlineDecoder*);
 
     typedef SkCodec INHERITED;
 };
 #endif // SkScaledCodec_DEFINED