Add subsetting to SkScaledCodec

src/codec/SkSwizzler.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 SkSwizzler_DEFINED
 #define SkSwizzler_DEFINED
 
@@ skipping 106 matching lines @@
 
     /**
      *  Create a new SkSwizzler.
      *  @param SrcConfig Description of the format of the source.
      *  @param ctable Unowned pointer to an array of up to 256 colors for an
      *                index source.
      *  @param dstInfo Describes the destination.
      *  @param ZeroInitialized Whether dst is zero-initialized. The
      *                         implementation may choose to skip writing zeroes
      *                         if set to kYes_ZeroInitialized.
+     *  @param srcInfo is the info of the source. Used to calculate the width samplesize.
+     *                  Width sampling is supported by the swizzler, by skipping pixels when
+     *                  swizzling the row. Height sampling is not supported by the swizzler,
+     *                  but is implemented in SkScaledCodec.
+     *                  Sampling in Y can be done by a client with a scanline decoder,
+     *                  but sampling in X allows the swizzler to skip swizzling pixels and
+     *                  reading from and writing to memory.
+     *  @param subsetLeft the left offset on the src for each row swizzle.
+     *  @param subsetWidth the width of each swizzle in destination pixels.
      *  @return A new SkSwizzler or nullptr on failure.
      */
     static SkSwizzler* CreateSwizzler(SrcConfig, const SkPMColor* ctable,
-                                      const SkImageInfo& dstInfo, SkCodec::ZeroInitialized);
+                                      const SkImageInfo& dstInfo, SkCodec::ZeroInitialized, 
+                                      int subsetLeft, int subsetWidth);
 
     /**
      *  Swizzle a line. Generally this will be called height times, once
      *  for each row of source.
      *  By allowing the caller to pass in the dst pointer, we give the caller
      *  flexibility to use the swizzler even when the encoded data does not
      *  store the rows in order.  This also improves usability for scaled and
      *  subset decodes.
      *  @param dst Where we write the output.
      *  @param src The next row of the source data.
@@ skipping 16 matching lines @@
      *  @param offset The offset before the first pixel to sample.
                         Is in bytes or bits based on what deltaSrc is in.
      */
     typedef ResultAlpha (*RowProc)(void* SK_RESTRICT dstRow,
                                    const uint8_t* SK_RESTRICT src,
                                    int dstWidth, int bpp, int deltaSrc, int offset,
                                    const SkPMColor ctable[]);
 
     const RowProc       fRowProc;
     const SkPMColor*    fColorTable;      // Unowned pointer
-    const int           fDeltaSrc;        // if bitsPerPixel % 8 == 0
-                                          //     deltaSrc is bytesPerPixel
-                                          // else
-                                          //     deltaSrc is bitsPerPixel
+    const int           fSrcOffset;       // Offset of the src in pixels, allows for partial
+                                          // scanline decodes.
+    int                 fX0;              // Start coordinate for the src, may be different than
+                                          // fSrcOffset if we are sampling.
     const int           fSrcWidth;        // Width of the source - i.e. before any sampling.
     int                 fDstWidth;        // Width of dst, which may differ with sampling.
-    int                 fX0;              // first X coord to sample
     int                 fSampleX;         // step between X samples
+    const int           fBPP;             // if bitsPerPixel % 8 == 0
+                                          //     fBPP is bytesPerPixel
+                                          // else
+                                          //     fBPP is bitsPerPixel
 
-    SkSwizzler(RowProc proc, const SkPMColor* ctable, int deltaSrc, int srcWidth);
+    SkSwizzler(RowProc proc, const SkPMColor* ctable, int srcOffset, int srcWidth, int bpp);
 
     int onSetSampleX(int) override;
 
     int onGetScaledWidth() override {
         return fDstWidth;
     }
 };
 #endif // SkSwizzler_DEFINED