Add an option to create unpremultiplied bitmaps.

src/core/SkColor.cpp

Index: src/core/SkColor.cpp
diff --git a/src/core/SkColor.cpp b/src/core/SkColor.cpp
index 6fa239ff539399eb0b061348ee0f0170f014cd97..1133e53f2538d2cac83b467fd2cd8a05709c837f 100644
--- a/src/core/SkColor.cpp
+++ b/src/core/SkColor.cpp
@@ -19,6 +19,18 @@ SkPMColor SkPreMultiplyColor(SkColor c) {
                                    SkColorGetG(c), SkColorGetB(c));
 }
 
+/**
+ *  Interprets c as an unpremultiplied color, and returns the
+ *  premultiplied equivalent.
+ */
+SkPMColor SkPreMultiplyUnPMColor(SkPMColor c) {

[bsalomon, 2013/06/13 12:24:59]

Should we have a new type, SkUPMColor? It wouldn't provide any safety, just
documentation. Maybe there is too much code that operates on SkPMColor that we
want to also use with UPM bitmaps to make this a viable suggestion.

[reed1, 2013/06/13 13:51:39]

Who calls this? Is it declared in a public header? Should it be?

[scroggo, 2013/06/13 19:04:49]

It is called by tests/ImageDecodingTest.cpp and samplecode/SampleUnpremul.cpp
(both added in this CL). I think that it should not be declared in a public
header since we are not publicly exposing UPM (except in SkImageDecoder).

+    U8CPU a = SkGetPackedA32(c);
+    U8CPU r = SkGetPackedR32(c);
+    U8CPU g = SkGetPackedG32(c);
+    U8CPU b = SkGetPackedB32(c);
+    return SkPreMultiplyARGB(a, r, g, b);
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 static inline SkScalar ByteToScalar(U8CPU x) {