greyscale to rgb

grey2rgb.hlsl

float interpolate(float val, float y0, float x0, float y1, float x1) {
    return (val - x0)*(y1 - y0) / (x1 - x0) + y0;
}
float blue(float grayscale) {
    if (grayscale < -0.33) return 1.0;
    else if (grayscale < 0.33) return interpolate(grayscale, 1.0, -0.33, 0.0, 0.33);
    else return 0.0;
}
float green(float grayscale) {
    if (grayscale < -1.0) return 0.0; // unexpected grayscale value
    if (grayscale < -0.33) return interpolate(grayscale, 0.0, -1.0, 1.0, -0.33);
    else if (grayscale < 0.33) return 1.0;
    else if (grayscale <= 1.0) return interpolate(grayscale, 1.0, 0.33, 0.0, 1.0);
    else return 1.0; // unexpected grayscale value
}
float red(float grayscale) {
    if (grayscale < -0.33) return 0.0;
    else if (grayscale < 0.33) return interpolate(grayscale, 0.0, -0.33, 1.0, 0.33);
    else return 1.0;
}

float4 torgb(float val)
{
    return float4(red(val), green(val), blue(val), 1);
}