How to debug a GLSL shader?

Question

I need to debug a GLSL program but I don\'t know how to output intermediate result. Is it possible to make some debug traces (like with printf) with GLSL ?

Answer 1

If you want to visualize the variations of a value across the screen, you can use a heatmap function similar to this (I wrote it in hlsl, but it is easy to adapt to glsl):

float4 HeatMapColor(float value, float minValue, float maxValue)
{
    #define HEATMAP_COLORS_COUNT 6
    float4 colors[HEATMAP_COLORS_COUNT] =
    {
        float4(0.32, 0.00, 0.32, 1.00),
        float4(0.00, 0.00, 1.00, 1.00),
        float4(0.00, 1.00, 0.00, 1.00),
        float4(1.00, 1.00, 0.00, 1.00),
        float4(1.00, 0.60, 0.00, 1.00),
        float4(1.00, 0.00, 0.00, 1.00),
    };
    float ratio=(HEATMAP_COLORS_COUNT-1.0)*saturate((value-minValue)/(maxValue-minValue));
    float indexMin=floor(ratio);
    float indexMax=min(indexMin+1,HEATMAP_COLORS_COUNT-1);
    return lerp(colors[indexMin], colors[indexMax], ratio-indexMin);
}

Then in your pixel shader you just output something like:

return HeatMapColor(myValue, 0.00, 50.00);

And can get an idea of how it varies across your pixels:

Of course you can use any set of colors you like.