Implement fast, correct gamma conversion for color xforms

src/core/SkColorSpaceXform.h

Index: src/core/SkColorSpaceXform.h
diff --git a/src/core/SkColorSpaceXform.h b/src/core/SkColorSpaceXform.h
index 34726435199ddf10905cb6dae3ad65fed114e870..1ea608094f252cd421b24b6530a28a1553c57ea8 100644
--- a/src/core/SkColorSpaceXform.h
+++ b/src/core/SkColorSpaceXform.h
@@ -26,23 +26,36 @@ public:
 
     /**
      *  Apply the color conversion to a src buffer, storing the output in the dst buffer.

[mtklein, 2016/06/15 20:39:42]

Just curious, is there any real reason we need to require RGB1 input to produce
RGB1 output?  Can't these be RGBx -> RGB1 with the same performance?  Ideally
we'll never even look at the source alpha.

This kinda trickles through the various API names and comments.  I'm not sure
it's important.

[msarett, 2016/06/15 21:20:24]

It became RGB1 because I wanted to take advantage of always writing 0xFF to the
alpha channel...

It could be renamed RGBx, I just don't know where we'd ever get RGBx input from.

I think I prefer it as is, but don't feel strongly.  It'll get extended and
possibly renamed anyway when we add RGBA unpremul and premul.

-     *  The src is stored in RGBA_8888 and the dst is stored in 8888 platform format.
-     *  The output is not premultiplied.
+     *  The src is opaque and stored in RGBA_8888, and the dst is also opaque and stored
+     *  in 8888 platform format.
      */
-    virtual void xform_RGBA_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const = 0;
+    virtual void xform_RGB1_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const = 0;
 
     virtual ~SkColorSpaceXform() {}
 };
 
-class Sk2Dot2Xform : public SkColorSpaceXform {
+class SkSRGBTo2Dot2Xform : public SkColorSpaceXform {
 public:
 
-    void xform_RGBA_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const override;
+    void xform_RGB1_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const override;
 
 private:
-    Sk2Dot2Xform(const SkMatrix44& srcToDst);
+    SkSRGBTo2Dot2Xform(const SkMatrix44& srcToDst);
 
-    float fSrcToDst[16];
+    float fSrcToDst[12];
+
+    friend class SkColorSpaceXform;
+};
+
+class Sk2Dot2To2Dot2Xform : public SkColorSpaceXform {
+public:
+
+    void xform_RGB1_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const override;
+
+private:
+    Sk2Dot2To2Dot2Xform(const SkMatrix44& srcToDst);
+
+    float fSrcToDst[12];
 
     friend class SkColorSpaceXform;
 };
@@ -53,7 +66,7 @@ private:
 class SkDefaultXform : public SkColorSpaceXform {
 public:
 
-    void xform_RGBA_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const override;
+    void xform_RGB1_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const override;
 
 private:
     SkDefaultXform(const sk_sp<SkGammas>& srcGammas, const SkMatrix44& srcToDst,