Option for SkCodec to treat dst as all zeroes.

include/codec/SkCodec.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 SkCodec_DEFINED
 #define SkCodec_DEFINED
 
@@ skipping 30 matching lines @@
 
     /**
      *  Return a size that approximately supports the desired scale factor.
      *  The codec may not be able to scale efficiently to the exact scale
      *  factor requested, so return a size that approximates that scale.
      *
      *  FIXME: Move to SkImageGenerator?
      */
     SkISize getScaledDimensions(float desiredScale) const;
 
+    /**
+     *  Whether or not the memory passed to getPixels is zero initialized.
+     */
+    enum ZeroInitialized {

[scroggo, 2015/03/05 16:12:23]

I decided to go with ZeroInitialized instead of SkipZeroes, since the decoder
may not skip zeroes. This seems more accurate as to what's going on.

That said, it may be that a caller happened to use zero initialized memory, but
there's very few zero pixels, so it's not worth skipping. In that case, they
could just not set it to kYes, even though it's technically not true.

+        /**
+         *  The memory passed to getPixels is zero initialized. The SkCodec
+         *  may take advantage of this by skipping writing zeroes.
+         */
+        kYes_ZeroInitialized,
+        /**
+         *  The memory passed to getPixels has not been initialized to zero,
+         *  so the SkCodec must write all zeroes to memory.
+         *
+         *  This is the default.
+         */
+        kNo_ZeroInitialized,
+    };
+
+    /**
+     *  Specify whether this SkCodec should treat the memory passed to
+     *  getPixels as zero initialized.
+     *
+     *  The default is No.
+     */
+    void setZeroInitialized(ZeroInitialized zero) { fZeroInitialized = zero; }

[scroggo, 2015/03/05 16:12:23]

It probably makes sense to include this as part of getPixels, although getPixels
already has a long parameter list, and I don't know that this is relevant for
other SkImageGenerators.

+
+    /**
+     *  Return whether this SkCodec considers memory passed to getPixels as
+     *  zero initialized.
+     */
+    ZeroInitialized getZeroInitialized() const { return fZeroInitialized; }
+
 protected:
     SkCodec(const SkImageInfo&, SkStream*);
 
     /**
      *  The SkAlphaType is a conservative answer. i.e. it is possible that it
      *  initially returns a non-opaque answer, but completing the decode
      *  reveals that the image is actually opaque.
      */
     bool onGetInfo(SkImageInfo* info) SK_OVERRIDE {
         *info = fInfo;
@@ skipping 17 matching lines @@
      *       - if the stream did not need to be rewound.
      *      false
      *       - if the stream needed to be rewound, and rewind failed.
      *  Subclasses MUST call this function before reading the stream (e.g. in
      *  onGetPixels). If it returns false, onGetPixels should return
      *  kCouldNotRewind.
      */
     bool SK_WARN_UNUSED_RESULT rewindIfNeeded();
 
 private:
-    const SkImageInfo fInfo;
+    const SkImageInfo       fInfo;
     SkAutoTDelete<SkStream> fStream;
-    bool fNeedsRewind;
+    bool                    fNeedsRewind;
+    ZeroInitialized         fZeroInitialized;
 };
 #endif // SkCodec_DEFINED