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// Simple GLSL fragment program to attenuate the input fragment color as a
// function of the distance of the fragment position from the center
// of the window

const float windowSize = 700.0; // TODO: this should be a built-in parameter!

void main()
{
    // Compute distance from center in range [0.0, 1.0]
    vec2 dist = min(abs((gl_FragCoord.xy - (windowSize)/2.0) / windowSize), 1.0);
    vec2 invDist = 1.0 - dist;

    // Compute attenuation
    float atten = invDist.x * invDist.y;
    vec4 outcolor = (gl_Color + gl_SecondaryColor) * atten;

    gl_FragColor = outcolor;
}