Convert SkPMFloat to [0,1] range and prune its API.

src/effects/SkColorMatrixFilter.cpp

Index: src/effects/SkColorMatrixFilter.cpp
diff --git a/src/effects/SkColorMatrixFilter.cpp b/src/effects/SkColorMatrixFilter.cpp
index 894a7b089d378820fa81ec2cae3a01511f9ddf0b..78d4797e44c9bce37bd3cfa8e16c2d5241dd3279 100644
--- a/src/effects/SkColorMatrixFilter.cpp
+++ b/src/effects/SkColorMatrixFilter.cpp
@@ -239,25 +239,9 @@ uint32_t SkColorMatrixFilter::getFlags() const {
     return this->INHERITED::getFlags() | fFlags;
 }
 
-/**
- *  Need inv255 = 1 / 255 as a constant, so when we premul a SkPMFloat, we can do this
- *
- *      new_red = old_red * alpha * inv255
- *
- *  instead of (much slower)
- *
- *      new_red = old_red * alpha / 255
- *
- *  However, 1.0f/255 comes to (in hex) 0x3B808081, which is slightly bigger than the "actual"
- *  value of 0x3B808080(repeat 80)... This slightly too-big value can cause us to compute
- *  new_red > alpha, which is a problem (for valid premul). To fix this, we use a
- *  hand-computed value of 0x3B808080, 1 ULP smaller. This keeps our colors valid.
- */
-static const float gInv255 = 0.0039215683f; //  (1.0f / 255) - ULP == SkBits2Float(0x3B808080)
-
 static Sk4f premul(const Sk4f& x) {
-    float scale = SkPMFloat(x).a() * gInv255;
-    Sk4f pm = x * Sk4f(scale, scale, scale, 1);
+    float scale = SkPMFloat(x).a();
+    Sk4f pm = x * SkPMFloat(1, scale, scale, scale);
 
 #ifdef SK_DEBUG
     SkPMFloat pmf(pm);
@@ -268,12 +252,12 @@ static Sk4f premul(const Sk4f& x) {
 }
 
 static Sk4f unpremul(const SkPMFloat& pm) {
-    float scale = 255 / pm.a(); // candidate for fast/approx invert?
-    return pm * Sk4f(scale, scale, scale, 1);
+    float scale = 1 / pm.a(); // candidate for fast/approx invert?
+    return pm * SkPMFloat(1, scale, scale, scale);
 }
 
-static Sk4f clamp_0_255(const Sk4f& value) {
-    return Sk4f::Max(Sk4f::Min(value, Sk4f(255)), Sk4f(0));
+static Sk4f clamp_0_1(const Sk4f& value) {
+    return Sk4f::Max(Sk4f::Min(value, Sk4f(1)), Sk4f(0));
 }
 
 void SkColorMatrixFilter::filterSpan(const SkPMColor src[], int count, SkPMColor dst[]) const {
@@ -292,14 +276,16 @@ void SkColorMatrixFilter::filterSpan(const SkPMColor src[], int count, SkPMColor
 #endif
 
     if (use_floats) {
+        // c0-c3 are already in [0,1].
         const Sk4f c0 = Sk4f::Load(fTranspose + 0);
         const Sk4f c1 = Sk4f::Load(fTranspose + 4);
         const Sk4f c2 = Sk4f::Load(fTranspose + 8);
         const Sk4f c3 = Sk4f::Load(fTranspose + 12);
-        const Sk4f c4 = Sk4f::Load(fTranspose + 16);  // translates
+        // c4 (the translate vector) is in [0, 255].  Bring it back to [0,1].
+        const Sk4f c4 = Sk4f::Load(fTranspose + 16)*Sk4f(1.0f/255);
 
         // todo: we could cache this in the constructor...
-        SkPMColor matrix_translate_pmcolor = SkPMFloat(premul(clamp_0_255(c4))).roundClamp();
+        SkPMColor matrix_translate_pmcolor = SkPMFloat(premul(clamp_0_1(c4))).round();
 
         for (int i = 0; i < count; i++) {
             const SkPMColor src_c = src[i];
@@ -323,7 +309,7 @@ void SkColorMatrixFilter::filterSpan(const SkPMColor src[], int count, SkPMColor
             Sk4f dst4 = c0 * r4 + c1 * g4 + c2 * b4 + c3 * a4 + c4;
 
             // clamp, re-premul, and write
-            dst[i] = SkPMFloat(premul(clamp_0_255(dst4))).round();
+            dst[i] = SkPMFloat(premul(clamp_0_1(dst4))).round();
         }
     } else {
         const State& state = fState;