src/sksl/sksl_rt_shader.sksl

layout(builtin=15) float4 sk_FragCoord;

// Color conversion functions used in gradient interpolation, based on
// https://www.w3.org/TR/css-color-4/#color-conversion-code
// TODO(skia:13108): For all of these, we can eliminate any linear math at the beginning
// (by removing the corresponding linear math at the end of the CPU code).
$pure half3 $css_Lab_to_XYZ(half3 Lab) {
    const half k = 24389 / 27.0;
    const half e = 216 / 24389.0;

    half3 f;
    f[1] = (Lab[0] + 16) / 116;
    f[0] = (Lab[1] / 500) + f[1];
    f[2] = f[1] - (Lab[2] / 200);

    half3 f_cubed = pow(f, half3(3));

    half3 xyz = half3(
        f_cubed[0] > e ? f_cubed[0] : (116 * f[0] - 16) / k,
        Lab[0] > k * e ? f_cubed[1] : Lab[0] / k,
        f_cubed[2] > e ? f_cubed[2] : (116 * f[2] - 16) / k
    );

    const half3 D50 = half3(0.3457 / 0.3585, 1.0, (1.0 - 0.3457 - 0.3585) / 0.3585);
    return xyz * D50;
}

// Also used by OKLCH -> OKLab
$pure half3 $css_LCH_to_Lab(half3 LCH) {
    return half3(
        LCH[0],
        LCH[1] * cos(radians(LCH[2])),
        LCH[1] * sin(radians(LCH[2]))
    );
}

$pure half3 $css_LCH_to_XYZ(half3 LCH) {
    return $css_Lab_to_XYZ($css_LCH_to_Lab(LCH));
}

$pure half3 $css_OKLab_to_XYZ(half3 OKLab) {
    const half3x3 LMStoXYZ = half3x3(
         1.2268798733741557,  -0.5578149965554813,  0.28139105017721583,
        -0.04057576262431372,  1.1122868293970594, -0.07171106666151701,
        -0.07637294974672142, -0.4214933239627914,  1.5869240244272418
    );
    // TODO(skia:13794): Switch this back to half3x3 when it no longer triggers sk_Caps errors
    const float3x3 OKLabToLMS = half3x3(
        0.99999999845051981432,  0.39633779217376785678,   0.21580375806075880339,
        1.0000000088817607767,  -0.1055613423236563494,   -0.063854174771705903402,
        1.0000000546724109177,  -0.089484182094965759684, -1.2914855378640917399
    );
    half3 LMSnl = half3(OKLabToLMS * OKLab);
    return LMStoXYZ * pow(LMSnl, half3(3));
}

$pure half3 $css_OKLCH_to_XYZ(half3 OKLCH) {
    return $css_OKLab_to_XYZ($css_LCH_to_Lab(OKLCH));
}

// TODO(skia:13108): Use our optimized version (though it has different range)
// Doing so might require fixing (re-deriving?) the math for the HWB version below
$pure half3 $css_hsl_to_sRGB(half3 hsl) {
    hsl.x = mod(hsl.x, 360);
    if (hsl.x < 0) {
        hsl.x += 360;
    }

    hsl.yz /= 100;

    half3 k = mod(half3(0, 8, 4) + hsl.x/30, 12);
    half a = hsl.y * min(hsl.z, 1 - hsl.z);
    return hsl.z - a * clamp(min(k - 3, 9 - k), -1, 1);
}

$pure half3 $css_hwb_to_sRGB(half3 hwb) {
    hwb.yz /= 100;
    if (hwb.y + hwb.z >= 1) {
        half gray = hwb.y / (hwb.y + hwb.z);
        return half3(gray);
    }
    half3 rgb = $css_hsl_to_sRGB(half3(hwb.x, 100, 50));
    rgb *= (1 - hwb.y - hwb.z);
    rgb += hwb.y;
    return rgb;
}