// S4AO (Stupid Simple Screen Space Ambient Occlusion) - Jonathan Dummer (O1S)
// This micro version uses only 3 depth samples, the midpoint and a randomly-rotated, balanced pair.

const mediump float ssao_falloff_frac = 0.25;
// Perform the SSAO.
float s4ao(vec2 UV) {
#ifdef USE_MULTIVIEW
	mediump float depth = texture(depth_buffer_array, vec3(UV, view)).r;
#else
	mediump float depth = texture(depth_buffer, UV).r;
#endif
	mediump float inv_falloff = 1.0f / max(1e-4f, depth * ssao_falloff_frac);
	// Random 2D rotation per pixel (0..1 -> parabola approximating a 180 deg arc)
	mediump float r01 = fract(dot(UV, ssao_prn_UV));
	mediump vec2 duv = vec2(r01 - 0.5f, 2.0f * (r01 - r01 * r01)) * (2.0f * depth * ssao_radius_frac); // 180 degrees.
	// Grab the samples and determine the occlusion.
	mediump float occlusion = 0.0f;
	for (int s = 0; s < 2; ++s) {
#ifdef USE_MULTIVIEW
		mediump float dz = texture(depth_buffer_array, vec3(UV + duv, view)).r - depth;
#else
		mediump float dz = texture(depth_buffer, UV + duv).r - depth;
#endif
		// How 'directly overhead' is it?  Factor in the falloff depth.
		occlusion += normalize(vec3(duv, dz)).z * mix(1.0f, 0.0f, dz * inv_falloff);
		// Mirror the next sample.
		duv = -duv;
	}
	// Adjust the occlusion for intensity, and # samples.
	occlusion = 1.0f - clamp(occlusion * 0.5f * ssao_intensity, 0.0f, 1.0f);
	return occlusion * occlusion;
}