color: Towards ColorTransform optimizations and code generation

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) {
+  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 SkTransferFnIsApproximatelyIdentity(const SkColorSpaceTransferFn& fn) {
+  const float kEpsilon = 1.f / 256.f;
+  if (fn.fD > 0.f) {
+    if (std::abs(fn.fC - 1.f) > kEpsilon)
+      return false;
+    if (std::abs(fn.fF) > kEpsilon)
+      return false;
+  }
+  if (std::abs(fn.fA - 1.f) > kEpsilon)
+    return false;
+  if (std::abs(fn.fB) > kEpsilon)
+    return false;
+  if (std::abs(fn.fE) > kEpsilon)
+    return false;
+  if (std::abs(fn.fG - 1.f) > kEpsilon)
+    return false;
+  return true;
+}
+
+bool SkTransferFnsApproximatelyCancel(const SkColorSpaceTransferFn& a,
+                                      const SkColorSpaceTransferFn& b) {
+  const float kStep = 1.f / 256.f;
+  SkColorSpaceTransferFn b_inv = SkTransferFnInverse(b);
+
+  // Evaluate the linear sections explicitly.
+  float linear_segment_max = std::max(a.fD, b.fD);
+  for (float x = 0.f; x < linear_segment_max; x += kStep) {
+    float a_of_x = EvalSkTransferFn(a, x);
+    float b_inv_of_x = EvalSkTransferFn(b, a_of_x);
+    if (std::abs(a_of_x - b_inv_of_x) > kEpsilon)
+      return false;
+  }
+
+  // Compare the coefficients of the pow sections.
+  if (std::abs(a.fA - b_inv.fA) > kEpsilon)
+    return false;
+  if (std::abs(a.fB - b_inv.fB) > kEpsilon)
+    return false;
+  if (std::abs(a.fE - b_inv.fE) > kEpsilon)
+    return false;
+  if (std::abs(a.fG - b_inv.fG) > kEpsilon)
+    return false;
+
+  return true;
+}
+
+bool SkMatrixIsApproximatelyIdentity(const SkMatrix44& m) {
+  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