add SkCanvas::NewRaster factory -- a very common use-case in chrome

BUG=skia:
R=scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/183533004

git-svn-id: http://skia.googlecode.com/svn/trunk@13617 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 29 insertions, 0 deletions

diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 8cc8bc9eb7..b73a1e48ca 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -49,6 +49,35 @@ public:
     SK_DECLARE_INST_COUNT(SkCanvas)
 
     /**
+     *  Attempt to allocate an offscreen raster canvas, matching the ImageInfo.
+     *  On success, return a new canvas that will draw into that offscreen.
+     *
+     *  The caller can access the pixels after drawing into this canvas by
+     *  calling readPixels() or peekPixels().
+     *
+     *  If the requested ImageInfo is opaque (either the colortype is
+     *  intrinsically opaque like RGB_565, or the info's alphatype is kOpaque)
+     *  then the pixel memory may be uninitialized. Otherwise, the pixel memory
+     *  will be initialized to 0, which is interpreted as transparent.
+     *
+     *  On failure, return NULL. This can fail for several reasons:
+     *  1. the memory allocation failed (e.g. request is too large)
+     *  2. invalid ImageInfo (e.g. negative dimensions)
+     *  3. unsupported ImageInfo for a canvas
+     *      - kUnknown_SkColorType, kIndex_8_SkColorType
+     *      - kIgnore_SkAlphaType
+     *      - this list is not complete, so others may also be unsupported
+     *
+     *  Note: it is valid to request a supported ImageInfo, but with zero
+     *  dimensions.
+     */
+    static SkCanvas* NewRaster(const SkImageInfo&);
+
+    static SkCanvas* NewRasterN32(int width, int height) {
+        return NewRaster(SkImageInfo::MakeN32Premul(width, height));
+    }
+
+    /**
      *  Creates an empty canvas with no backing device/pixels, and zero
      *  dimensions.
      */