#define LIGHT_WORLD_SIZE 0.005
#define LIGHT_FRUSTUM_WIDTH 3.75
#define LIGHT_SIZE_UV (LIGHT_WORLD_SIZE / LIGHT_FRUSTUM_WIDTH)
#define NEAR_PLANE 9.5

#define NUM_SAMPLES 17
#define NUM_RINGS 11
#define BLOCKER_SEARCH_NUM_SAMPLES NUM_SAMPLES
#define PCF_NUM_SAMPLES NUM_SAMPLES

vec2 poissonDisk[NUM_SAMPLES];

void initPoissonSamples( const in vec2 randomSeed ) {
  float ANGLE_STEP = PI2 * float( NUM_RINGS ) / float( NUM_SAMPLES );
  float INV_NUM_SAMPLES = 1.0 / float( NUM_SAMPLES );

  // jsfiddle that shows sample pattern: https://jsfiddle.net/a16ff1p7/
  float angle = rand( randomSeed ) * PI2;
  float radius = INV_NUM_SAMPLES;
  float radiusStep = radius;

  for( int i = 0; i < NUM_SAMPLES; i ++ ) {
    poissonDisk[i] = vec2( cos( angle ), sin( angle ) ) * pow( radius, 0.75 );
    radius += radiusStep;
    angle += ANGLE_STEP;
  }
}

float penumbraSize( const in float zReceiver, const in float zBlocker ) { // Parallel plane estimation
  return (zReceiver - zBlocker) / zBlocker;
}

float findBlocker( sampler2D shadowMap, const in vec2 uv, const in float zReceiver ) {
  // This uses similar triangles to compute what
  // area of the shadow map we should search
  float searchRadius = LIGHT_SIZE_UV * ( zReceiver - NEAR_PLANE ) / zReceiver;
  float blockerDepthSum = 0.0;
  int numBlockers = 0;

  for( int i = 0; i < BLOCKER_SEARCH_NUM_SAMPLES; i++ ) {
    float shadowMapDepth = unpackRGBAToDepth(texture2D(shadowMap, uv + poissonDisk[i] * searchRadius));
    if ( shadowMapDepth < zReceiver ) {
      blockerDepthSum += shadowMapDepth;
      numBlockers ++;
    }
  }

  if( numBlockers == 0 ) return -1.0;

  return blockerDepthSum / float( numBlockers );
}

float PCF_Filter(sampler2D shadowMap, vec2 uv, float zReceiver, float filterRadius ) {
  float sum = 0.0;
  for( int i = 0; i < PCF_NUM_SAMPLES; i ++ ) {
    float depth = unpackRGBAToDepth( texture2D( shadowMap, uv + poissonDisk[ i ] * filterRadius ) );
    if( zReceiver <= depth ) sum += 1.0;
  }
  for( int i = 0; i < PCF_NUM_SAMPLES; i ++ ) {
    float depth = unpackRGBAToDepth( texture2D( shadowMap, uv + -poissonDisk[ i ].yx * filterRadius ) );
    if( zReceiver <= depth ) sum += 1.0;
  }
  return sum / ( 2.0 * float( PCF_NUM_SAMPLES ) );
}

float PCSS ( sampler2D shadowMap, vec4 coords ) {
  vec2 uv = coords.xy;
  float zReceiver = coords.z; // Assumed to be eye-space z in this code

  initPoissonSamples( uv );
  // STEP 1: blocker search
  float avgBlockerDepth = findBlocker( shadowMap, uv, zReceiver );

  //There are no occluders so early out (this saves filtering)
  if( avgBlockerDepth == -1.0 ) return 1.0;

  // STEP 2: penumbra size
  float penumbraRatio = penumbraSize( zReceiver, avgBlockerDepth );
  float filterRadius = penumbraRatio * LIGHT_SIZE_UV * NEAR_PLANE / zReceiver;

  // STEP 3: filtering
  //return avgBlockerDepth;
  return PCF_Filter( shadowMap, uv, zReceiver, filterRadius );
}