// by IQ
// just another march, this time to accumulate the difference
// between points across the normal (i.e. little means dark means occluded)
// requires a fair bit of fucking around to make this look good
float calcAO( in vec3 pos, in vec3 nor, in float strength)
{
    float occ = 0.0;
    float sca = 1.0;
    for( int i=0; i<16; i++ )
    {
        float h = 0.001 + strength*float(i)/4.0;
        float d = fScene( pos + h*nor );
        occ += (h-d)*sca;
        sca *= 0.95;
    }
    return clamp( 1.0 - 1.5*occ /* was 1.5 */, 0.0, 1.0 );
}