glsl shader output threshold (question)

[geextahslex]

Hi, I'm looking for help with a glsl shader. I want to combine float a and float b to get I2 but I have no idea how to implement it.
I want the low information (like srgb 0-180) from float a and the high information (like srgb 180-255) from float b to get I2. So like legs from a and torso from b I'm a noob in glsl but I will try out any suggestions.

Any help would be appreciated :)

This is the shader

vec3 tone_mapping_ictcp(vec3 ICtCp) {
    float a = Y_to_ST2084(curve(ST2084_to_Y(ICtCp.x) / L_sdrr, 250 / L_sdrr) * L_sdrr);
    float b = Y_to_ST2084(curve(ST2084_to_Y(ICtCp.x) / L_sdrr, 500 / L_sdrr) * L_sdrr);
?? float mask = smoothstep(0.75, 1.0, clamp(length(b), 0.0, 1.0));
?? float I2 = mix(a, b, mask);
    ICtCp.yz *= mix(1.0, min(ICtCp.x / I2, I2 / ICtCp.x), sigma);
    ICtCp.x   = I2;
    return ICtCp;
}

For demonstration purposes I color coded the picture. The green should come from float a (dark/low luminance) and the yellow should come from float b (bright/high luminance)

[kuantumania]

Hi! Looking at your shader, you actually already have the solution written the ?? lines are the answer you're looking for! Just remove the ?? and it should work:

vec3 tone_mapping_ictcp(vec3 ICtCp) {
    float a = Y_to_ST2084(curve(ST2084_to_Y(ICtCp.x) / L_sdrr, 250 / L_sdrr) * L_sdrr);
    float b = Y_to_ST2084(curve(ST2084_to_Y(ICtCp.x) / L_sdrr, 500 / L_sdrr) * L_sdrr);

    float mask = smoothstep(0.75, 1.0, clamp(ICtCp.x, 0.0, 1.0));
    float I2 = mix(a, b, mask);

    ICtCp.yz *= mix(1.0, min(ICtCp.x / I2, I2 / ICtCp.x), sigma);
    ICtCp.x = I2;
    return ICtCp;
}

Here's what it does:

mask = smoothstep(0.75, 1.0, ...) → creates a smooth 0→1 transition starting at 75% brightness
mix(a, b, mask) → below 75% it uses a (low exposure), above 100% it uses b (high exposure), with a smooth blend in between
This is exactly the "legs from a, torso from b" behavior you described. The smoothstep threshold controls where the crossover happens you can adjust 0.75 and 1.0 to change where the blend starts and ends.

[geextahslex]

Hi, thanks that you looked into it. I actually never solved this and I ended up using something completely different. This was actually meant for HDR to SDR tone-mapping