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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef mozilla_RelativeLuminanceUtils_h
#define mozilla_RelativeLuminanceUtils_h
#include "nsColor.h"
namespace mozilla {
// Utilities for calculating relative luminance based on the algorithm
// defined in https://www.w3.org/TR/WCAG20/#relativeluminancedef
class RelativeLuminanceUtils {
public:
// Compute the relative luminance.
static float Compute(nscolor aColor) {
float r = ComputeComponent(NS_GET_R(aColor));
float g = ComputeComponent(NS_GET_G(aColor));
float b = ComputeComponent(NS_GET_B(aColor));
return ComputeFromComponents(r, g, b);
}
// Adjust the relative luminance of the given color.
static nscolor Adjust(nscolor aColor, float aLuminance) {
float r = ComputeComponent(NS_GET_R(aColor));
float g = ComputeComponent(NS_GET_G(aColor));
float b = ComputeComponent(NS_GET_B(aColor));
float luminance = ComputeFromComponents(r, g, b);
float factor = (aLuminance + 0.05f) / (luminance + 0.05f);
uint8_t r1 =
DecomputeComponent(std::max(0.0f, (r + 0.05f) * factor - 0.05f));
uint8_t g1 =
DecomputeComponent(std::max(0.0f, (g + 0.05f) * factor - 0.05f));
uint8_t b1 =
DecomputeComponent(std::max(0.0f, (b + 0.05f) * factor - 0.05f));
return NS_RGBA(r1, g1, b1, NS_GET_A(aColor));
}
// https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
static float ContrastRatio(nscolor aColor1, nscolor aColor2) {
float l1 = Compute(aColor1);
float l2 = Compute(aColor2);
if (l1 < l2) {
std::swap(l1, l2);
}
return (l1 + 0.05f) / (l2 + 0.05f);
}
private:
static float ComputeComponent(uint8_t aComponent) {
float v = float(aComponent) / 255.0f;
if (v <= 0.03928f) {
return v / 12.92f;
}
return std::pow((v + 0.055f) / 1.055f, 2.4f);
}
static constexpr float ComputeFromComponents(float aR, float aG, float aB) {
return 0.2126f * aR + 0.7152f * aG + 0.0722f * aB;
}
// Inverse function of ComputeComponent.
static uint8_t DecomputeComponent(float aComponent) {
if (aComponent <= 0.03928f / 12.92f) {
aComponent *= 12.92f;
} else {
aComponent = std::pow(aComponent, 1.0f / 2.4f) * 1.055f - 0.055f;
}
return ClampColor(aComponent * 255.0f);
}
};
} // namespace mozilla
#endif // mozilla_RelativeLuminanceUtils_h
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