change API contract: disallow zero-sized images or surfaces

include/core/SkImage.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImage_DEFINED
 #define SkImage_DEFINED
 
@@ skipping 14 matching lines @@
 
 /**
  *  SkImage is an abstraction for drawing a rectagle of pixels, though the
  *  particular type of image could be actually storing its data on the GPU, or
  *  as drawing commands (picture or PDF or otherwise), ready to be played back
  *  into another canvas.
  *
  *  The content of SkImage is always immutable, though the actual storage may
  *  change, if for example that image can be re-created via encoded data or
  *  other means.
+ *
+ *  SkImage always has a non-zero dimensions. If there is a request to create a new image, either
+ *  directly or via SkSurface, and either of the requested dimensions are zero, then NULL will be
+ *  returned.
  */
 class SK_API SkImage : public SkRefCnt {
 public:
     SK_DECLARE_INST_COUNT(SkImage)
 
     typedef SkImageInfo Info;
 
     static SkImage* NewRasterCopy(const Info&, const void* pixels, size_t rowBytes);
     static SkImage* NewRasterData(const Info&, SkData* pixels, size_t rowBytes);
 
@@ skipping 78 matching lines @@
      *  was created (if it came from a surface).
      */
     SkSurface* newSurface(const SkImageInfo&, const SkSurfaceProps* = NULL) const;
 
 protected:
     SkImage(int width, int height) :
         fWidth(width),
         fHeight(height),
         fUniqueID(NextUniqueID()) {
 
-        SkASSERT(width >= 0);
-        SkASSERT(height >= 0);
+        SkASSERT(width > 0);
+        SkASSERT(height > 0);
     }
 
 private:
     const int       fWidth;
     const int       fHeight;
     const uint32_t  fUniqueID;
 
     static uint32_t NextUniqueID();
 
     typedef SkRefCnt INHERITED;
 
     friend class SkCanvas;
 
     void draw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) const;
 
     /**
      *  Draw the image, cropped to the src rect, to the dst rect of a canvas.
      *  If src is larger than the bounds of the image, the rest of the image is
      *  filled with transparent black pixels.
      *
      *  See SkCanvas::drawBitmapRectToRect for similar behavior.
      */
     void drawRect(SkCanvas*, const SkRect* src, const SkRect& dst, const SkPaint*) const;
 };
 
 #endif