#ifdef GL_FRAGMENT_PRECISION_HIGH
  // we need high precision for the depth values
  precision highp float;
#else
  precision mediump float;
#endif

uniform sampler2D uDepthTexIndex;
uniform sampler2D uFogTexIndex;
uniform vec2 uInverseTexSize;   //in 1/pixels, e.g. 1/512 if the texture is 512px wide
uniform float uEffectStrength;
uniform float uNearPlane;
uniform float uFarPlane;

varying vec2 vTexCoord;

/* Retrieves the depth value 'offset' pixels away from 'pos' from texture 'uDepthTexIndex'. */
float getDepth(vec2 pos, ivec2 offset)
{
  float z = texture2D(uDepthTexIndex, pos + float(offset) * uInverseTexSize).x;
  return (2.0 * uNearPlane) / (uFarPlane + uNearPlane - z * (uFarPlane - uNearPlane)); // linearize depth
}


/* getOcclusionFactor() determines a heuristic factor (from [0..1]) for how
 * much the fragment at 'pos' with depth 'depthHere'is occluded by the
 * fragment that is (dx, dy) texels away from it.
 */
float getOcclusionFactor(float depthHere, vec2 pos, ivec2 offset) {
    float depthThere = getDepth(pos, offset);
    /* if the fragment at (dx, dy) has no depth (i.e. there was nothing rendered there),
     * then 'here' is not occluded (result 1.0) */
    if (depthThere == 0.0)
      return 1.0;

    /* if the fragment at (dx, dy) is further away from the viewer than 'here', then
     * 'here is not occluded' */
    if (depthHere < depthThere )
      return 1.0;

    float relDepthDiff = depthThere / depthHere;
    float depthDiff = abs(depthThere - depthHere) * uFarPlane;
    /* if the fragment at (dx, dy) is closer to the viewer than 'here', then it occludes
     * 'here'. The occlusion is the higher the bigger the depth difference between the two
     * locations is.
     * However, if the depth difference is too high, we assume that 'there' lies in a
     * completely different depth region of the scene than 'here' and thus cannot occlude
     * 'here'. This last assumption gets rid of very dark artifacts around tall buildings.
     */
    return depthDiff < 50.0 ? mix(0.99, 1.0, 1.0 - clamp(depthDiff, 0.0, 1.0)) : 1.0;
}

/* This shader approximates the ambient occlusion in screen space (SSAO).
 * It is based on the assumption that a pixel will be occluded by neighboring
 * pixels iff. those have a depth value closer to the camera than the original
 * pixel itself (the function getOcclusionFactor() computes this occlusion
 * by a single other pixel).
 *
 * A naive approach would sample all pixels within a given distance. For an
 * interesting-looking effect, the sampling area needs to be at least 9 pixels
 * wide (-/+ 4), requiring 81 texture lookups per pixel for ambient occlusion.
 * This overburdens many GPUs.
 * To make the ambient occlusion computation faster, we do not consider all
 * texels in the sampling area, but only 16. This causes some sampling artifacts
 * that are later removed by blurring the ambient occlusion texture (this is
 * done in a separate shader).
 */
void main() {
  float depthHere = getDepth(vTexCoord, ivec2(0, 0));
  float fogIntensity = texture2D(uFogTexIndex, vTexCoord).w;

  if (depthHere == 0.0)
  {
	//there was nothing rendered 'here' --> it can't be occluded
    gl_FragColor = vec4(1.0);
    return;
  }

  float occlusionFactor = 1.0;

  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-1,  0));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+1,  0));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2( 0, -1));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2( 0, +1));

  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-2, -2));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+2, +2));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+2, -2));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-2, +2));

  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-4,  0));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+4,  0));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2( 0, -4));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2( 0, +4));

  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-4, -4));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+4, +4));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(+4, -4));
  occlusionFactor *= getOcclusionFactor(depthHere, vTexCoord, ivec2(-4, +4));

  occlusionFactor = pow(occlusionFactor, 4.0) + 55.0/255.0; // empirical bias determined to let SSAO have no effect on the map plane
  occlusionFactor = 1.0 - ((1.0 - occlusionFactor) * uEffectStrength * (1.0-fogIntensity));
  gl_FragColor = vec4(vec3(occlusionFactor), 1.0);
}