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//
// Copyright 2020 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#include "image_color_utils.h"
#include <assert.h>
#include <math.h>
#include <algorithm>
#include "gtx_types.h"
namespace gtx {
namespace {
// Returns color component in the appropriate range to calculate the luminance
// of an RGB color.
float AdjustColorComponentRangeForLuminance(float component) {
// Based on
// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
if (component < 0.03928f) {
return component / 12.92f;
} else {
return static_cast<float>(pow((component + 0.055f) / 1.055f, 2.4f));
}
}
} // namespace
namespace image_color_utils {
float Luminance(float red, float blue, float green) {
// Based on
// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
return (0.2126f * AdjustColorComponentRangeForLuminance(red) +
0.0722f * AdjustColorComponentRangeForLuminance(blue) +
0.7152f * AdjustColorComponentRangeForLuminance(green));
}
float ContrastRatio(float first_luminance, float second_luminance) {
assert(first_luminance >= 0);
assert(second_luminance >= 0);
float brighter_luminance = std::max(first_luminance, second_luminance);
float darker_luminance = std::min(first_luminance, second_luminance);
// Based on https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
return (brighter_luminance + 0.05f) / (darker_luminance + 0.05f);
}
} // namespace image_color_utils
} // namespace gtx