Luma_Shader.osl

// Adrian Cruceru
// Redshift Rendering Technologies 2020
// This file is licensed under Apache 2.0 license
// luma

shader Luma
[[  string help = "computes the relative luminance = luma of an input in various formats",
    string label = "Luma/Relative Luminance" ]]
(
    // inputs
    color inColor = 0
    [[string label = "In Color"]],
    // various modes to compute the relative Luminance
    int mode = 0
    [[  string label = "Mode",
        string widget = "mapper",
        string options = "Rec. 709 (HD):0|Rec. 2020 (4k):1|Ccir 601 (SD):2|Average:3|Red:4|Green:5|Blue:6"]],
    float gamma = 2.2
    [[  string label = "Gamma",
        float min = 0, float max = 3 ]],
    int preGamma = 1
    [[  string widget = "checkBox",
        string label = "Pre Gamma" ]],
    int postGamma = 1
    [[  string widget = "checkBox" ,
        string label = "Post Gamma" ]],
    // outputs
    output color outColor = 1
    [[  string label = "Out Color" ]]
)
{
    // define various luma standards
    vector rec709   = {0.2126,  0.7152, 0.0722};
    vector rec2020  = {0.2627,  0.678,  0.0593};
    vector ccir601  = {0.299,   0.587,  0.114};
    float R = preGamma*pow(inColor[0],1/gamma)+(1-preGamma)*inColor[0];
    float G = preGamma*pow(inColor[1],1/gamma)+(1-preGamma)*inColor[1];
    float B = preGamma*pow(inColor[2],1/gamma)+(1-preGamma)*inColor[2];
    
    if (mode == 0)
    {
        // internally the luminance function is using REC709
        // outColor = luminance(inColor);
        color C = rec709[0]*R +rec709[1]*G+rec709[2]*B;
        color powC = pow(C,gamma);
        outColor = postGamma*powC+(1-postGamma)*C;
    }
    else if (mode == 1)
    {
        color C = rec2020[0]*R+rec2020[1]*G+rec2020[2]*B;
        color powC = pow(C,gamma);
        outColor = postGamma*powC+(1-postGamma)*C;
    }
    else if (mode == 2)
    {
        color C = ccir601[0]*R+ccir601[1]*G+ccir601[2]*B;
        color powC = pow(C,gamma);
        outColor = postGamma*powC+(1-postGamma)*C;
    }
    else if (mode == 3)
    {
        color C = (R+G+B)/3;
        color powC = pow(C,gamma);
        outColor = postGamma*powC+(1-postGamma)*C;
    }
    else if (mode == 4)
    {
        outColor = postGamma*pow(R,gamma)+(1-postGamma)*R;
    }
    else if (mode == 5)
    {
        outColor = postGamma*pow(G,gamma)+(1-postGamma)*G;
    }
    else if (mode == 6)
    {
        outColor = postGamma*pow(B,gamma)+(1-postGamma)*B;
    }
}