gistfile1.c

uniform sampler2D reference;
uniform sampler2D comparison;

uniform int comparisonCountX;
uniform int comparisonCountY;

in vec2 imageIndex;

void main (void)
{
    vec2 comparisonSize = vec2(1.0 / comparisonCountX, 1.0 / comparisonCountY);
    vec2 comparisonOffset = comparisonSize * imageIndex;
    
    float score = 0;
    
    for (float x = 0.0; x < 1.0; x += 1.0 / 32.0)
    {
        for (float y = 0.0; y < 1.0; y += 1.0 / 32.0)
        {
            vec2 sample = vec2(x,y);
            
            vec3 referenceColour = texture(reference, sample);
            vec3 comparisonColour = texture(comparison, comparisonOffset + sample * comparisonSize);
            
            score += abs(referenceColour.r - comparisonColour.r) + abs(referenceColour.g - comparisonColour.g) + abs(referenceColour.b - comparisonColour.b); 
        }
    }
    
    gl_FragColor = score;
}