color: Add analytic transfer functions in shaders

ui/gfx/skia_color_space_util.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/skia_color_space_util.h"
 
 #include <algorithm>
 #include <cmath>
 
 namespace gfx {
 
 namespace {
-
-const float kEpsilon = 1.f / 256.f;
 }
 
-float EvalSkTransferFn(const SkColorSpaceTransferFn& fn, float x) {
+float SkTransferFnEval(const SkColorSpaceTransferFn& fn, float x) {
   if (x < 0.f)
     return 0.f;
   if (x < fn.fD)
     return fn.fC * x + fn.fF;
   return std::pow(fn.fA * x + fn.fB, fn.fG) + fn.fE;
 }
 
 SkColorSpaceTransferFn SkTransferFnInverse(const SkColorSpaceTransferFn& fn) {
   SkColorSpaceTransferFn fn_inv = {0};
   if (fn.fA > 0 && fn.fG > 0) {
     double a_to_the_g = std::pow(fn.fA, fn.fG);
     fn_inv.fA = 1.f / a_to_the_g;
     fn_inv.fB = -fn.fE / a_to_the_g;
     fn_inv.fG = 1.f / fn.fG;
   }
   fn_inv.fD = fn.fC * fn.fD + fn.fF;
   fn_inv.fE = -fn.fB / fn.fA;
   if (fn.fC != 0) {
     fn_inv.fC = 1.f / fn.fC;
     fn_inv.fF = -fn.fF / fn.fC;
   }
   return fn_inv;
 }
 
+bool SkTransferFnsApproximatelyCancel(const SkColorSpaceTransferFn& a,
+                                      const SkColorSpaceTransferFn& b) {
+  const float kStep = 1.f / 8.f;
+  const float kEpsilon = 2.5f / 256.f;
+  for (float x = 0; x <= 1.f; x += kStep) {
+    float a_of_x = SkTransferFnEval(a, x);
+    float b_of_a_of_x = SkTransferFnEval(b, a_of_x);
+    if (std::abs(b_of_a_of_x - x) > kEpsilon)
+      return false;
+  }
+  return true;
+}
+
+bool SkTransferFnIsApproximatelyIdentity(const SkColorSpaceTransferFn& a) {
+  const float kStep = 1.f / 8.f;
+  const float kEpsilon = 2.5f / 256.f;
+  for (float x = 0; x <= 1.f; x += kStep) {
+    float a_of_x = SkTransferFnEval(a, x);
+    if (std::abs(a_of_x - x) > kEpsilon)
+      return false;
+  }
+  return true;
+}
+
 bool SkMatrixIsApproximatelyIdentity(const SkMatrix44& m) {
+  const float kEpsilon = 1.f / 256.f;
   for (int i = 0; i < 4; ++i) {
     for (int j = 0; j < 4; ++j) {
       float identity_value = i == j ? 1 : 0;
       float value = m.get(i, j);
       if (std::abs(identity_value - value) > kEpsilon)
         return false;
     }
   }
   return true;
 }
 
 }  // namespace gfx