source: data/data_extern/programs/OLD/hdr_downscale2x2luminence.glsl

uniform sampler2D inRTT;
uniform vec2 texelSize;

varying vec2 uv;

void main(void)
{
        
    vec4 accum = vec4(0.0, 0.0, 0.0, 0.0);
        vec4 LUMINENCE_FACTOR  = vec4(0.27, 0.67, 0.06, 0.0);

    // Get colour from source
    accum += texture2D(inRTT, uv + texelSize * vec2(-0.5, -0.5));
    accum += texture2D(inRTT, uv + texelSize * vec2(-0.5, 0.5));
    accum += texture2D(inRTT, uv + texelSize * vec2(0.5, 0.5));
    accum += texture2D(inRTT, uv + texelSize * vec2(0.5, -0.5));
    
        // Adjust the accumulated amount by lum factor
        float lum = dot(accum, LUMINENCE_FACTOR);
        // take average of 4 samples
        lum *= 0.25;
        gl_FragColor = vec4(lum, lum, lum, 1.0);

}