float chebyshevUpperBound()
{
    vec2 moments = texture2D(ShadowMap,ShadowCoordPostW.xy).rg;

    // Surface is fully lit. as the current fragment is before the light occluder
    if (ShadowCoordPostW.z <= moments.x)
        return 1.0 ;

    // The fragment is either in shadow or penumbra. We now use chebyshev's upperBound to check
    // How likely this pixel is to be lit (p_max)
    float variance = moments.y - (moments.x*moments.x);
    variance = max(variance,0.1);

    float d = ShadowCoordPostW.z - moments.x;
    float p_max = (variance / (variance + d*d));

    return max(p_max, 0.4);
}

//Color computation in main()
shadow = chebyshevUpperBound();
fragColor = ((ambient_color + diffuse_color * diffuse_value))*texel;
fragColor.rgb *= shadow;