A GLSL module for color levels manipulation

levels.glsl

float levelChannel(float inPixel, float inBlack, float inGamma, float inWhite, float outBlack, float outWhite) {
    return (pow(((inPixel * 255.0) - inBlack) / (inWhite - inBlack), inGamma) * (outWhite - outBlack) + outBlack) / 255.0;
}

vec3 levels(vec3 inPixel, float inBlack, float inGamma, float inWhite, float outBlack, float outWhite) {
    vec3 o = vec3(1.0);
    o.r = levelChannel(inPixel.r, inBlack, inGamma, inWhite, outBlack, outWhite);
    o.g = levelChannel(inPixel.g, inBlack, inGamma, inWhite, outBlack, outWhite);
    o.b = levelChannel(inPixel.b, inBlack, inGamma, inWhite, outBlack, outWhite);
    return o;
}