gfx/src/RelativeLuminanceUtils.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_RelativeLuminanceUtils_h
   8 #define mozilla_RelativeLuminanceUtils_h
   9
  10 #include "nsColor.h"
  11
  12 namespace mozilla {
  13
  14 // Utilities for calculating relative luminance based on the algorithm
  15 // defined in https://www.w3.org/TR/WCAG20/#relativeluminancedef
  16 class RelativeLuminanceUtils {
  17  public:
  18   // Compute the relative luminance.
  19   static float Compute(nscolor aColor) {
  20     float r = ComputeComponent(NS_GET_R(aColor));
  21     float g = ComputeComponent(NS_GET_G(aColor));
  22     float b = ComputeComponent(NS_GET_B(aColor));
  23     return ComputeFromComponents(r, g, b);
  24   }
  25
  26   // Adjust the relative luminance of the given color.
  27   static nscolor Adjust(nscolor aColor, float aLuminance) {
  28     float r = ComputeComponent(NS_GET_R(aColor));
  29     float g = ComputeComponent(NS_GET_G(aColor));
  30     float b = ComputeComponent(NS_GET_B(aColor));
  31     float luminance = ComputeFromComponents(r, g, b);
  32     float factor = (aLuminance + 0.05f) / (luminance + 0.05f);
  33     uint8_t r1 =
  34         DecomputeComponent(std::max(0.0f, (r + 0.05f) * factor - 0.05f));
  35     uint8_t g1 =
  36         DecomputeComponent(std::max(0.0f, (g + 0.05f) * factor - 0.05f));
  37     uint8_t b1 =
  38         DecomputeComponent(std::max(0.0f, (b + 0.05f) * factor - 0.05f));
  39     return NS_RGBA(r1, g1, b1, NS_GET_A(aColor));
  40   }
  41
  42   // https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
  43   static float ContrastRatio(nscolor aColor1, nscolor aColor2) {
  44     float l1 = Compute(aColor1);
  45     float l2 = Compute(aColor2);
  46     if (l1 < l2) {
  47       std::swap(l1, l2);
  48     }
  49     return (l1 + 0.05f) / (l2 + 0.05f);
  50   }
  51
  52  private:
  53   static float ComputeComponent(uint8_t aComponent) {
  54     float v = float(aComponent) / 255.0f;
  55     if (v <= 0.03928f) {
  56       return v / 12.92f;
  57     }
  58     return std::pow((v + 0.055f) / 1.055f, 2.4f);
  59   }
  60
  61   static constexpr float ComputeFromComponents(float aR, float aG, float aB) {
  62     return 0.2126f * aR + 0.7152f * aG + 0.0722f * aB;
  63   }
  64
  65   // Inverse function of ComputeComponent.
  66   static uint8_t DecomputeComponent(float aComponent) {
  67     if (aComponent <= 0.03928f / 12.92f) {
  68       aComponent *= 12.92f;
  69     } else {
  70       aComponent = std::pow(aComponent, 1.0f / 2.4f) * 1.055f - 0.055f;
  71     }
  72     return ClampColor(aComponent * 255.0f);
  73   }
  74 };
  75
  76 }  // namespace mozilla
  77
  78 #endif  // mozilla_RelativeLuminanceUtils_h