SkScaledCodec class

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
 
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      *  of the encoded data, but then never use any colors which have alpha
      *  less than 100%. This function can be called *after* decoding to
      *  determine if such an image truly had alpha. Calling it before decoding
      *  is undefined.
      *  FIXME: see skbug.com/3582.
      */
     bool reallyHasAlpha() const {
         return this->onReallyHasAlpha();
     }
 
+    /**
+     * returns true if the image must be scaled, in the y direction, after reading, not during.
+     * To scale afterwards, we first decode every line and then sample the lines we want afterwards.
+     * An example is interlaced pngs, where calling getScanlines once (regardless of the count
+     * used) needs to read the entire image, therefore it is inefficient to call
+     * getScanlines more than once. Instead, it should only ever be called with all the
+     * rows needed.
+     */
+    bool requiresPostYSampling() {
+        return this->onRequiresPostYSampling();
+    }
+
 protected:
     SkScanlineDecoder(const SkImageInfo& requested)
         : fDstInfo(requested)
         , fCurrScanline(0) {}
 
     virtual bool onReallyHasAlpha() const { return false; }
 
+    /**
+     * returns true if the image type is hard to sample and must be scaled after reading, not during
+     * An example is interlaced pngs, where the entire image must be read for each decode
+     */
+    virtual bool onRequiresPostYSampling() { return false; }
+
     const SkImageInfo& dstInfo() const { return fDstInfo; }
 
 private:
     const SkImageInfo   fDstInfo;
     int                 fCurrScanline;
 
     // Naive default version just calls onGetScanlines on temp memory.
     virtual SkCodec::Result onSkipScanlines(int countLines) {
         SkAutoMalloc storage(fDstInfo.minRowBytes());
         // Note that we pass 0 to rowBytes so we continue to use the same memory.
         // Also note that while getScanlines checks that rowBytes is big enough,
         // onGetScanlines bypasses that check.
         // 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