GranTurismoTonemapper

简介

Gran Turismo色调映射是一种颜色变换技术,通过改变HSV色彩空间的色调H值并采样色调映射纹理来产生平滑的颜色变化,将颜色从HDR转为LDR

实现步骤

1. Shader中将输入的RGB颜色空间转换到HSV颜色空间
2. 改变HSV空间的色调H值,可以通过时间或其他参数进行变化。色调H值的变化范围是0到1
3. 根据变化的色调H值采样色调映射纹理,得到新的RGB颜色
4. 将原始RGB颜色和新的RGB颜色进行线性插值,得到最终输出颜色。插值因子由外部设定,控制颜色变换的强度

float4 Frag(Varyings input) : SV_Target
{
    // Lut space
    // We use Alexa LogC (El 1000) to store the LUT as it provides a good enough range
    // (~58.85666) and is good enough to be stored in fp16 without losing precision in the
    // darks
    float3 colorLutSpace = GetLutStripValue(input.uv, _Lut_Params);

    // Color grade & tonemap
    float3 gradedColor = ColorGrade(colorLutSpace);
    gradedColor = Tonemap(gradedColor);

    return float4(gradedColor, 1.0);
}

_CurveHueVsSat/_CurveSatVsSat/_CurveLumVsSat 参数由 Color Adjustments 提供

// HSV operations
float satMult;
float3 hsv = RgbToHsv(colorLinear);
{
    // Hue Vs Sat
    satMult = EvaluateCurve(_CurveHueVsSat, hsv.x) * 2.0;

    // Sat Vs Sat
    satMult *= EvaluateCurve(_CurveSatVsSat, hsv.y) * 2.0;

    // Lum Vs Sat
    satMult *= EvaluateCurve(_CurveLumVsSat, Luminance(colorLinear)) * 2.0;

    // Hue Shift & Hue Vs Hue
    float hue = hsv.x + _HueSatCon.x;
    float offset = EvaluateCurve(_CurveHueVsHue, hue) - 0.5;
    hue += offset;
    hsv.x = RotateHue(hue, 0.0, 1.0);
}
colorLinear = HsvToRgb(hsv);

参考

GT Tonemap

效果

Tonemapping 为 None

Tonemapping 为 Neutral

Tonemapping 为 Aces

Tonemapping 为 GranTurismo

用的曲线公式最大值不超过1，所以 Bloom 效果无法生效，原先大于1的颜色值被映射到小于1的值，可以调整相关参数

代码

#ifndef GRAN_TURISMO_LIB
//https://www.desmos.com/calculator/gslcdxvipg?lang=es

#define E 2.718281828459

half W(half x,half e0,half e1){
    if(x<=e0){
        return 0;
    }
    if(x>=e1){
        return 1;
    }
    half temp=(x-e0)/(e1-e0);
    return temp*temp*(3-2*temp);
}

half H(half x,half e0,half e1){
    if(x<=e0){
        return 0;
    }
    if(x>=e1){
        return 1;
    }
    return (x-e0)/(e1-e0);
}

half GranTurismoColor(half x){    
    half P=1;
    half a=1;
    half m=0.22;
    half l=0.4;
    half c=1.33;
    half b=0;
    half l0=0.312;
    half L0=0;
    half L1=1;
    half Lx=m+a*(x-m);
    half Tx=m*(pow(x/m,c))+b;
    half S0=0.532;
    half S1=0.532;
    half C2=2.13675213675;
    half Sx=P-(P-S1)*pow(E,-(C2*(x-S0)/P));
    half W0x=1-W(x,0,m);
    half W2x=H(x,m+l0,m+l0);
    half W1x=1-W0x-W2x;
    return Tx*W0x+Lx*W1x+Sx*W2x;
}

static const float e = 2.71828;

float W_f(float x,float e0,float e1) {
    if (x <= e0)
        return 0;
    if (x >= e1)
        return 1;
    float a = (x - e0) / (e1 - e0);
    return a * a*(3 - 2 * a);
}
float H_f(float x, float e0, float e1) {
    if (x <= e0)
        return 0;
    if (x >= e1)
        return 1;
    return (x - e0) / (e1 - e0);
}

float GranTurismoTonemapper(float x) {
    float P = 1;
    float a = 1;
    float m = 0.22;
    float l = 0.4;
    float c = 1.33;
    float b = 0;
    float l0 = (P - m)*l / a;
    float L0 = m - m / a;
    float L1 = m + (1 - m) / a;
    float L_x = m + a * (x - m);
    float T_x = m * pow(x / m, c) + b;
    float S0 = m + l0;
    float S1 = m + a * l0;
    float C2 = a * P / (P - S1);
    float S_x = P - (P - S1)*pow(e,-(C2*(x-S0)/P));
    float w0_x = 1 - W_f(x, 0, m);
    float w2_x = H_f(x, m + l0, m + l0);
    float w1_x = 1 - w0_x - w2_x;
    float f_x = T_x * w0_x + L_x * w1_x + S_x * w2_x;
    return f_x;
}

half3 GranTurismoTonemap(half3 color){
    
    // half r=GranTurismoTonemapper(color.r);
    // half g=GranTurismoTonemapper(color.g);
    // half b=GranTurismoTonemapper(color.b);

    half r=GranTurismoColor(color.r);
    half g=GranTurismoColor(color.g);
    half b=GranTurismoColor(color.b);
    return half3(r,g,b);
}

#endif