Proposal for SkImageGenerator::canDecodeAndScale API

include/core/SkImageGenerator.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageGenerator_DEFINED
 #define SkImageGenerator_DEFINED
 
@@ skipping 90 matching lines @@
      *
      *  If info is not kIndex8_SkColorType, then the last two parameters may be NULL. If ctableCount
      *  is not null, it will be set to 0.
      *
      *  @return true on success.
      */
     bool getPixels(const SkImageInfo& info, void* pixels, size_t rowBytes,
                    SkPMColor ctable[], int* ctableCount);
 
     /**
+     *  Some generators can efficiently scale their contents and provide multiple
+     *  different pixel formats while decoding.
+     *  If scaling is supported, the generator may only support certain scaled dimensions.
+     *  Call this with the desired scale and color type, and it will return true if scaling
+     *  and specified color type is supported.
+     *
+     *  @param colorType A description of the color type expected by the caller.
+     *         Returns true if it is the same as getInfo() or if decoder implementations
+     *         support this output.
+     *
+     *  @param scale Vale larger than 0 and less or equal to 1.
+     *
+     *  @param availableSize If specified colorType and scale on decode is supported,
+     *         this holds a value of equal or nearest larger possible decode output.
+     *         Can be NULL, in which case caller specifies no interest in getting the value.
+     *
+     *  @param lowerSize If specified colorType and scale on decode is supported,
+     *         this holds a value of nearest lower possible decode output.
+     *         Can be NULL, in which case caller specifies no interest in getting the value.
+     *
+     *  Disclaimer: This method was added later to this class and it is possible
+     *  that some of the SkImageGenerator implementations are not overriding it to
+     *  precisely declare supported decoding. Purpose of this call is an attempt to prevent
+     *  fruitless memory allocations (from within Skia) before calling getPixels - it is
+     *  still OK to call getPixels even this method is not overridden in generator implementation.
+     */
+    bool canDecodeAndScale(const SkColorType colorType, const SkScalar scale = SK_Scalar1,

[scroggo, 2015/10/15 16:26:32]

Should this also specify an alpha type? Or should we just assume e.g. that the
caller wants opaque for 565?

+            SkISize *availableSize = nullptr, SkISize *lowerSize = nullptr);

[scroggo, 2015/10/15 16:26:32]

I like that the sizes are named to distinguish what they mean (instead of using
an array of sizes in the other CL).

+
+    /**
      *  Simplified version of getPixels() that asserts that info is NOT kIndex8_SkColorType and
      *  uses the default Options.
      */
     bool getPixels(const SkImageInfo& info, void* pixels, size_t rowBytes);
 
     /**
      *  If planes or rowBytes is NULL or if any entry in planes is NULL or if any entry in rowBytes
      *  is 0, this imagegenerator should output the sizes and return true if it can efficiently
      *  return YUV planar data. If it cannot, it should return false. Note that either planes and
      *  rowBytes are both fully defined and non NULL/non 0 or they are both NULL or have NULL or 0
@@ skipping 70 matching lines @@
     virtual bool onGetPixels(const SkImageInfo& info, void* pixels, size_t rowBytes,
                              SkPMColor ctable[], int* ctableCount);
     virtual bool onGetYUV8Planes(SkISize sizes[3], void* planes[3], size_t rowBytes[3]);
     virtual bool onGetYUV8Planes(SkISize sizes[3], void* planes[3], size_t rowBytes[3],
                                  SkYUVColorSpace* colorSpace);
 
     virtual GrTexture* onGenerateTexture(GrContext*, SkImageUsageType, const SkIRect*) {
         return nullptr;
     }
 
+    virtual bool onCanDecodeAndScale(const SkColorType colorType, const SkScalar scale,
+            SkISize *availableSize, SkISize *lowerSize);
+
     bool tryGenerateBitmap(SkBitmap* bm, const SkImageInfo* optionalInfo, SkBitmap::Allocator*);
 
 private:
     const SkImageInfo fInfo;
     const uint32_t fUniqueID;
 
     // This is our default impl, which may be different on different platforms.
     // It is called from NewFromEncoded() after it has checked for any runtime factory.
     // The SkData will never be NULL, as that will have been checked by NewFromEncoded.
     static SkImageGenerator* NewFromEncodedImpl(SkData*);
 };
 
 #endif  // SkImageGenerator_DEFINED