Source/WebCore/platform/graphics/ColorInterpolation.cpp

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#include "config.h"
#include "ColorInterpolation.h"

#include "Color.h"

namespace WebCore {

std::pair<float, float> fixupHueComponentsPriorToInterpolation(HueInterpolationMethod method, float component1, float component2)
{
    auto normalizeAnglesUsingShorterAlgorithm = [] (auto theta1, auto theta2) -> std::pair<float, float> {
        // https://drafts.csswg.org/css-color-4/#hue-shorter
        auto difference = theta2 - theta1;
        if (difference > 180.0)
            return { theta1 + 360.0, theta2 };
        if (difference < -180.0)
            return { theta1, theta2 + 360.0 };
        return { theta1, theta2 };
    };

    auto normalizeAnglesUsingLongerAlgorithm = [] (auto theta1, auto theta2) -> std::pair<float, float> {
        // https://drafts.csswg.org/css-color-4/#hue-longer
        auto difference = theta2 - theta1;
        if (difference > 0.0 && difference < 180.0)
            return { theta1 + 360.0, theta2 };
        if (difference > -180.0 && difference < 0)
            return { theta1, theta2 + 360.0 };
        return { theta1, theta2 };
    };

    auto normalizeAnglesUsingIncreasingAlgorithm = [] (auto theta1, auto theta2) -> std::pair<float, float> {
        // https://drafts.csswg.org/css-color-4/#hue-increasing
        if (theta2 < theta1)
            return { theta1, theta2 + 360.0 };
        return { theta1, theta2 };
    };

    auto normalizeAnglesUsingDecreasingAlgorithm = [] (auto theta1, auto theta2) -> std::pair<float, float> {
        // https://drafts.csswg.org/css-color-4/#hue-decreasing
        if (theta1 < theta2)
            return { theta1 + 360.0, theta2 };
        return { theta1, theta2 };
    };

    auto normalizeAnglesUsingSpecifiedAlgorithm = [] (auto theta1, auto theta2) -> std::pair<float, float> {
        // https://drafts.csswg.org/css-color-4/#hue-specified
        return { theta1, theta2 };
    };

    switch (method) {
    case HueInterpolationMethod::Shorter:
        return normalizeAnglesUsingShorterAlgorithm(component1, component2);
    case HueInterpolationMethod::Longer:
        return normalizeAnglesUsingLongerAlgorithm(component1, component2);
    case HueInterpolationMethod::Increasing:
        return normalizeAnglesUsingIncreasingAlgorithm(component1, component2);
    case HueInterpolationMethod::Decreasing:
        return normalizeAnglesUsingDecreasingAlgorithm(component1, component2);
    case HueInterpolationMethod::Specified:
        return normalizeAnglesUsingSpecifiedAlgorithm(component1, component2);
    }
    RELEASE_ASSERT_NOT_REACHED();
}

Color interpolateColors(ColorInterpolationMethod colorInterpolationMethod, Color color1, double color1Multiplier, Color color2, double color2Multiplier)
{
    return WTF::switchOn(colorInterpolationMethod.colorSpace,
        [&] (auto& colorSpace) {
            using ColorType = typename std::remove_reference_t<decltype(colorSpace)>::ColorType;
            switch (colorInterpolationMethod.alphaPremultiplication) {
            case AlphaPremultiplication::Premultiplied:
                return makeCanonicalColor(interpolateColorComponents<AlphaPremultiplication::Premultiplied>(colorSpace, color1.toColorTypeLossy<ColorType>(), color1Multiplier, color2.toColorTypeLossy<ColorType>(), color2Multiplier));
            case AlphaPremultiplication::Unpremultiplied:
                return makeCanonicalColor(interpolateColorComponents<AlphaPremultiplication::Unpremultiplied>(colorSpace, color1.toColorTypeLossy<ColorType>(), color1Multiplier, color2.toColorTypeLossy<ColorType>(), color2Multiplier));
            }
            RELEASE_ASSERT_NOT_REACHED();
        }
    );
}

}