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

src/effects/SkColorMatrixFilter.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkColorMatrixFilter.h"
 #include "SkColorMatrix.h"
 #include "SkColorPriv.h"
@@ skipping 221 matching lines @@
 
 SkColorMatrixFilter::SkColorMatrixFilter(const SkScalar array[20]) {
     memcpy(fMatrix.fMat, array, 20 * sizeof(SkScalar));
     this->initState(array);
 }
 
 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);
     SkASSERT(pmf.isValid());
 #endif
 
     return pm;
 }
 
 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 {
     Proc proc = fProc;
     if (NULL == proc) {
         if (src != dst) {
             memcpy(dst, src, count * sizeof(SkPMColor));
         }
         return;
     }
 
 #ifdef SK_SUPPORT_LEGACY_INT_COLORMATRIX
     const bool use_floats = false;
 #else
     const bool use_floats = true;
 #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];
             if (0 == src_c) {
                 dst[i] = matrix_translate_pmcolor;
                 continue;
             }
 
             SkPMFloat srcf(src_c);
 
             if (0xFF != SkGetPackedA32(src_c)) {
                 srcf = unpremul(srcf);
             }
 
             Sk4f r4 = Sk4f(srcf.r());
             Sk4f g4 = Sk4f(srcf.g());
             Sk4f b4 = Sk4f(srcf.b());
             Sk4f a4 = Sk4f(srcf.a());
 
             // apply matrix
             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;
         int32_t result[4];
         const SkUnPreMultiply::Scale* table = SkUnPreMultiply::GetScaleTable();
 
         for (int i = 0; i < count; i++) {
             SkPMColor c = src[i];
 
             unsigned r = SkGetPackedR32(c);
@@ skipping 243 matching lines @@
     str->append("matrix: (");
     for (int i = 0; i < 20; ++i) {
         str->appendScalar(fMatrix.fMat[i]);
         if (i < 19) {
             str->append(", ");
         }
     }
     str->append(")");
 }
 #endif