Proposal for SkImageGenerator::canDecodeAndScale API

include/core/SkImageGenerator.h

Index: include/core/SkImageGenerator.h
diff --git a/include/core/SkImageGenerator.h b/include/core/SkImageGenerator.h
index 0e2a45e145c5fb6403355b0ec86e63e4248be654..9108cc95757f97e2f580b0ab7842d9cf312e2c5c 100644
--- a/include/core/SkImageGenerator.h
+++ b/include/core/SkImageGenerator.h
@@ -108,6 +108,36 @@ public:
                    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.
      */
@@ -198,6 +228,9 @@ protected:
         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: