WebGL GLSL SSAO (Screen-Space Ambient Occlusion) fragment shader. Converts the g-buffer to an occlusion buffer which estimates local ambient occlusion at each fragment in screen-space. This SSAO version uses per-fragment depth and normal data to integrate local visibility over a normal-oriented hemisphere in world space by comparing the linear d…

billboard_ssao.frag

#define SAMPLE_COUNT        {{ sampleCount }}
#define USE_ACTUAL_NORMALS  {{ useActualNormals }}

uniform sampler2D sGBuffer;
uniform sampler2D sNoise;

uniform float uSampleRadius;
uniform float uIntensity;
uniform vec2  uNoiseScale;
uniform vec3  uKernel[SAMPLE_COUNT];

// reconstructs view-space unit normal from view-space position
vec3 reconstructNormalVS(vec3 positionVS) {
  return normalize(cross(dFdx(positionVS), dFdy(positionVS)));
}

void main() {
  gBufferGeomComponents gBufferValue = decodeGBufferGeom(sGBuffer, varyingTexCoords, clipFar);
  
  vec3 cameraToPositionRay = normalize(varyingCameraFarPlaneWorldSpace - cameraPositionWorldSpace);
  vec3 origin = cameraToPositionRay * gBufferValue.depth + cameraPositionWorldSpace;
  
#if USE_ACTUAL_NORMALS
  vec3 normal = gBufferValue.normal;
#else
  vec3 originVS = (viewMatrix * vec4(origin, 1.0)).xyz;
  vec3 normal   = reconstructNormalVS(originVS);
#endif
  normal = normalize(normal * normalMatrix);
  
  vec3 rvec    = texture2D(sNoise, varyingTexCoords * uNoiseScale).xyz * 2.0 - 1.0;
  vec3 tangent = normalize(rvec - normal * dot(rvec, normal));
  vec3 bitangent  = cross(normal, tangent);
  mat3 tbn        = mat3(tangent, bitangent, normal);
  
  float occlusion = 0.0;
  
  for (int i = 0; i < SAMPLE_COUNT; ++i) {
    vec3 sample = origin + (tbn * uKernel[i]) * uSampleRadius;
    vec4 offset = viewProjectionMatrix * vec4(sample, 1.0);
    
    offset.xy   = (offset.xy / offset.w) * 0.5 + 0.5;
    float depth = length(sample - cameraPositionWorldSpace);
    
    float sampleDepth = decodeGBufferDepth(sGBuffer, offset.xy, clipFar);
    
    float rangeDelta  = abs(gBufferValue.depth - sampleDepth);
    float rangeCheck  = smoothstep(0.0, 1.0, uSampleRadius / rangeDelta);
    
    occlusion += rangeCheck * step(sampleDepth, depth);
  }
  
  occlusion = 1.0 - occlusion / float(SAMPLE_COUNT);
  occlusion = clamp(pow(occlusion, uIntensity), 0.0, 1.0);
  gl_FragColor = vec4(occlusion, occlusion, occlusion, 1.0);
}