Scanline decoding for gifs

include/codec/SkScanlineDecoder.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 SkScanlineDecoder_DEFINED
 #define SkScanlineDecoder_DEFINED
 
@@ skipping 184 matching lines @@
      *  the scanline decoder.
      */
     SkScanlineOrder getScanlineOrder() const { return this->onGetScanlineOrder(); }
 
     /**
      *  Returns the y-coordinate of the next row to be returned by the scanline
      *  decoder.  This will be overridden in the case of
      *  kOutOfOrder_SkScanlineOrder and should be unnecessary in the case of
      *  kNone_SkScanlineOrder.
      */
-    int getY() const {
+    int getY() {
         SkASSERT(kNone_SkScanlineOrder != this->getScanlineOrder());
         return this->onGetY();
     }
 
 protected:
     SkScanlineDecoder(const SkImageInfo& srcInfo)
         : fSrcInfo(srcInfo)
         , fDstInfo()
         , fOptions()
         , fCurrScanline(0) {}
 
     virtual SkISize onGetScaledDimensions(float /* desiredScale */) {
         // By default, scaling is not supported.
         return this->getInfo().dimensions();
     }
 
     virtual SkEncodedFormat onGetEncodedFormat() const = 0;
 
     virtual bool onReallyHasAlpha() const { return false; }
 
     /**
      *  Most images types will be kTopDown and will not need to override this function.
      */
     virtual SkScanlineOrder onGetScanlineOrder() const { return kTopDown_SkScanlineOrder; }
 
     /**
      *  Most images will be kTopDown and will not need to override this function.
      */
-    virtual int onGetY() const { return fCurrScanline; }
+    virtual int onGetY() { return fCurrScanline; }

[msarett, 2015/08/24 23:20:12]

The current implementation of SkGifScanlineDecoder calls the gif iterator, so
this cannot be const.

A possible alternate solution would be to, instead of using an iterator, create
a function that takes a fCurrScanline and fHeight, and returns Y.  I'm not sure
of the difficulty/performance impact of this vs using the iterator.

[scroggo, 2015/08/27 16:35:49]

FWIW, it does seem a little odd that it is not const, but getScanlines is not
const, so the typical user will not have a const SkScanlineDecoder anyway.

[msarett, 2015/09/01 17:50:15]

Acknowledged.

 
     const SkImageInfo& dstInfo() const { return fDstInfo; }
 
     const SkCodec::Options& options() const { return fOptions; }
 
 private:
     const SkImageInfo   fSrcInfo;
     SkImageInfo         fDstInfo;
     SkCodec::Options    fOptions;
     int                 fCurrScanline;
@@ skipping 11 matching lines @@
         // Calling the virtual method also means we do not double count
         // countLines.
         return this->onGetScanlines(storage.get(), countLines, 0);
     }
 
     virtual SkCodec::Result onGetScanlines(void* dst, int countLines,
                                                     size_t rowBytes) = 0;
 
 };
 #endif // SkScanlineDecoder_DEFINED