Add an option on SkImageDecoder to skip writing 0s.

include/core/SkImageDecoder.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkImageDecoder_DEFINED
@@ skipping 41 matching lines @@
     static Format GetStreamFormat(SkStream*);
 
     /** Return a readable string of the Format provided.
     */
     static const char* GetFormatName(Format);
 
     /** Return a readable string of the value returned by getFormat().
     */
     const char* getFormatName() const;
 
+    /** Whether the decoder should skip writing zeroes to output if possible.
+    */
+    bool getSkipWritingZeroes() const { return fSkipWritingZeroes; }
+
+    /** Set to true if the decoder should skip writing any zeroes when
+        creating the output image.
+        This is a hint that may not be respected by the decoder.
+        It should only be used if it is known that the memory to write
+        to has already been set to 0; otherwise the resulting image will
+        have garbage.
+        This is ideal for images that contain a lot of completely transparent
+        pixels, but may be a performance hit for an image that has only a
+        few transparent pixels.
+        The default is false.
+    */
+    void setSkipWritingZeroes(bool skip) { fSkipWritingZeroes = skip; }
+
     /** Returns true if the decoder should try to dither the resulting image.
         The default setting is true.
     */
     bool getDitherImage() const { return fDitherImage; }
 
     /** Set to true if the the decoder should try to dither the resulting image.
         The default setting is true.
     */
     void setDitherImage(bool dither) { fDitherImage = dither; }
 
@@ skipping 428 matching lines @@
 
 private:
     Peeker*                 fPeeker;
     Chooser*                fChooser;
     SkBitmap::Allocator*    fAllocator;
     int                     fSampleSize;
     SkBitmap::Config        fDefaultPref;   // use if fUsePrefTable is false
     PrefConfigTable         fPrefTable;     // use if fUsePrefTable is true
     bool                    fDitherImage;
     bool                    fUsePrefTable;
+    bool                    fSkipWritingZeroes;
     mutable bool            fShouldCancelDecode;
     bool                    fPreferQualityOverSpeed;
     bool                    fRequireUnpremultipliedColors;
 };
 
 /** Calling newDecoder with a stream returns a new matching imagedecoder
     instance, or NULL if none can be found. The caller must manage its ownership
     of the stream as usual, calling unref() when it is done, as the returned
     decoder may have called ref() (and if so, the decoder is responsible for
     balancing its ownership when it is destroyed).
@@ skipping 38 matching lines @@
 DECLARE_DECODER_CREATOR(WBMPImageDecoder);
 DECLARE_DECODER_CREATOR(WEBPImageDecoder);
 
 
 // Typedefs to make registering decoder and formatter callbacks easier.
 // These have to be defined outside SkImageDecoder. :(
 typedef SkTRegistry<SkImageDecoder*(*)(SkStream*)>        SkImageDecoder_DecodeReg;
 typedef SkTRegistry<SkImageDecoder::Format(*)(SkStream*)> SkImageDecoder_FormatReg;
 
 #endif