src/core/SkColorSpacePriv.h

/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include <math.h>

#include "SkColorSpace_Base.h"

#define SkColorSpacePrintf(...)

static constexpr float gSRGB_toXYZD50[] {
    0.4360747f, 0.3850649f, 0.1430804f, // Rx, Gx, Bx
    0.2225045f, 0.7168786f, 0.0606169f, // Ry, Gy, Gz
    0.0139322f, 0.0971045f, 0.7141733f, // Rz, Gz, Bz
};

static constexpr float gAdobeRGB_toXYZD50[] {
    0.6097559f, 0.2052401f, 0.1492240f, // Rx, Gx, Bx
    0.3111242f, 0.6256560f, 0.0632197f, // Ry, Gy, Gz
    0.0194811f, 0.0608902f, 0.7448387f, // Rz, Gz, Bz
};

static inline bool color_space_almost_equal(float a, float b) {
    return SkTAbs(a - b) < 0.01f;
}

static inline float add_epsilon(float v) {
    return v + FLT_MIN;
}

static inline bool is_zero_to_one(float v) {
    // Because we allow a value just barely larger than 1, the client can use an
    // entirely linear transfer function.
    return (0.0f <= v) && (v <= add_epsilon(1.0f));
}

static inline bool is_valid_transfer_fn(const SkColorSpaceTransferFn& coeffs) {
    if (SkScalarIsNaN(coeffs.fA) || SkScalarIsNaN(coeffs.fB) ||
        SkScalarIsNaN(coeffs.fC) || SkScalarIsNaN(coeffs.fD) ||
        SkScalarIsNaN(coeffs.fE) || SkScalarIsNaN(coeffs.fF) ||
        SkScalarIsNaN(coeffs.fG))
    {
        return false;
    }

    if (!is_zero_to_one(coeffs.fD)) {
        return false;
    }

    if (coeffs.fD == 0.0f) {
        // Y = (aX + b)^g + e  for always
        if (0.0f == coeffs.fA || 0.0f == coeffs.fG) {
            SkColorSpacePrintf("A or G is zero, constant transfer function "
                               "is nonsense");
            return false;
        }
    }

    if (coeffs.fD >= 1.0f) {
        // Y = cX + f          for always
        if (0.0f == coeffs.fC) {
            SkColorSpacePrintf("C is zero, constant transfer function is "
                               "nonsense");
            return false;
        }
    }

    if ((0.0f == coeffs.fA || 0.0f == coeffs.fG) && 0.0f == coeffs.fC) {
        SkColorSpacePrintf("A or G, and C are zero, constant transfer function "
                           "is nonsense");
        return false;
    }

    if (coeffs.fC < 0.0f) {
        SkColorSpacePrintf("Transfer function must be increasing");
        return false;
    }

    if (coeffs.fA < 0.0f || coeffs.fG < 0.0f) {
        SkColorSpacePrintf("Transfer function must be positive or increasing");
        return false;
    }

    return true;
}

static inline bool is_almost_srgb(const SkColorSpaceTransferFn& coeffs) {
    return color_space_almost_equal(1.0f / 1.055f,   coeffs.fA) &&
           color_space_almost_equal(0.055f / 1.055f, coeffs.fB) &&
           color_space_almost_equal(1.0f / 12.92f,   coeffs.fC) &&
           color_space_almost_equal(0.04045f,        coeffs.fD) &&
           color_space_almost_equal(0.00000f,        coeffs.fE) &&
           color_space_almost_equal(0.00000f,        coeffs.fF) &&
           color_space_almost_equal(2.40000f,        coeffs.fG);
}

static inline bool is_almost_2dot2(const SkColorSpaceTransferFn& coeffs) {
    return color_space_almost_equal(1.0f, coeffs.fA) &&
           color_space_almost_equal(0.0f, coeffs.fB) &&
           color_space_almost_equal(0.0f, coeffs.fE) &&
           color_space_almost_equal(2.2f, coeffs.fG) &&
           coeffs.fD <= 0.0f;
}

static inline bool is_almost_linear(const SkColorSpaceTransferFn& coeffs) {
    // OutputVal = InputVal ^ 1.0f
    const bool linearExp =
           color_space_almost_equal(1.0f, coeffs.fA) &&
           color_space_almost_equal(0.0f, coeffs.fB) &&
           color_space_almost_equal(0.0f, coeffs.fE) &&
           color_space_almost_equal(1.0f, coeffs.fG) &&
           coeffs.fD <= 0.0f;

    // OutputVal = 1.0f * InputVal
    const bool linearFn =
           color_space_almost_equal(1.0f, coeffs.fC) &&
           color_space_almost_equal(0.0f, coeffs.fF) &&
           coeffs.fD >= 1.0f;

    return linearExp || linearFn;
}

static inline void value_to_parametric(SkColorSpaceTransferFn* coeffs, float exponent) {
    coeffs->fA = 1.0f;
    coeffs->fB = 0.0f;
    coeffs->fC = 0.0f;
    coeffs->fD = 0.0f;
    coeffs->fE = 0.0f;
    coeffs->fF = 0.0f;
    coeffs->fG = exponent;
}

static inline bool named_to_parametric(SkColorSpaceTransferFn* coeffs,
                                       SkGammaNamed gammaNamed) {
    switch (gammaNamed) {
        case kSRGB_SkGammaNamed:
            coeffs->fA = 1.0f / 1.055f;
            coeffs->fB = 0.055f / 1.055f;
            coeffs->fC = 1.0f / 12.92f;
            coeffs->fD = 0.04045f;
            coeffs->fE = 0.0f;
            coeffs->fF = 0.0f;
            coeffs->fG = 2.4f;
            return true;
        case k2Dot2Curve_SkGammaNamed:
            value_to_parametric(coeffs, 2.2f);
            return true;
        case kLinear_SkGammaNamed:
            coeffs->fA = 0.0f;
            coeffs->fB = 0.0f;
            coeffs->fC = 1.0f;
            // Make sure that we use the linear segment of the transfer function even
            // when the x-value is 1.0f.
            coeffs->fD = add_epsilon(1.0f);
            coeffs->fE = 0.0f;
            coeffs->fF = 0.0f;
            coeffs->fG = 0.0f;
            return true;
        default:
            return false;
    }
}