# version 120
 
uniform sampler2D bgl_RenderedTexture;
 
float interp(float xa[6], float ya[6], float x){
    float xi = 0.0;
    int i = 0;
    for(int ii=0; ii<6; ii ++){
        float xix = xa[ii];
        if (xix >= x){
            xi = xix;
            i = ii;
            break;
        }
    }
    float x_min = xa[i-1];
    float y_min = ya[i-1];
    float y_max = ya[i];
    float factor = (x - x_min) / (xi - x_min);
    return y_min + (y_max - y_min) * factor;
}
 
float x_array[6] = float[6](0.0, 0.35, 0.46, 0.53, 0.7, 1.0);
float y_array[6] = float[6](0.0, 0.41, 0.52, 0.52, 0.64, 1.0);
    
void main(void)
{
    vec4 color = texture2D(bgl_RenderedTexture, gl_TexCoord[0].st);
    float xc = interp(x_array, y_array, color.x);
    float yc = interp(x_array, y_array, color.y);
    float zc = interp(x_array, y_array, color.z);
    color = vec4(xc, yc, zc, 1.0);
    
    gl_FragColor = color.rgba;
}

BUMP, seems that my own text got gone.

So my question is:

how to make this be smooth rather than linear?

How does it work?

# version 120
 
uniform sampler2D bgl_RenderedTexture;
 
float interp(float xa[4], float ya[4], float x){
    float xi = 0.0;
    int i = 0;
    for(int ii=0; ii<6; ii ++){
        float xix = xa[ii];
        if (xix >= x){
            xi = xix;
            i = ii;
            break;
        }
    }
    float x_min = xa[i-1];
    float y_min = ya[i-1];
    float y_max = ya[i];
    float factor = smoothstep(x_min, xi, x);
    return y_min + (y_max - y_min) * factor;
}
 
float x_array[4] = float[4](0.0, 0.49, 0.53, 1.0);
float y_array[4] = float[4](0.0, 0.4, 0.6, 1.0);
    
void main(void)
{
    vec4 color = texture2D(bgl_RenderedTexture, gl_TexCoord[0].st);
    float xc = interp(x_array, y_array, color.x);
    float yc = interp(x_array, y_array, color.y);
    float zc = interp(x_array, y_array, color.z);
    color = vec4(xc, yc, zc, 1.0);
    
    gl_FragColor = color.rgba;
}

BUMP, why's nobody telling anything? :(

How does it work?

You did not say what you mean with "it". If you mean "How does the smoothstep function work?", the answer can be found on https://en.wikipedia.org/wiki/Smoothstep and https://www.opengl.org/sdk/docs/man/html/smoothstep.xhtml

Or you could create the color gradient with a painting program and load it as a texture. That would allow for more complex gradients with less math operations.

I'm not experienced in 2D filters. I don't understand much.

However, it seems that the problem with smoothstep is that it doesn't work in 1.20 version. Must try changing to later one.