SSAO

This is my SSAO (Screen Space Ambient Occlusion) implementation and it’s both fast and gives good result. It’s inspired by the Crysis SSAO algorithm but also the Starcraft II implementation and a little from Nvidia’s implementation.

To use the SSAO shader, render a fullscreen quad with the SSAO shader applied to it. This shader will trace 10-16 rays for every fragment (pixel when there are no multi-sampling) on the screen. The rays are shot in a random direction in a hemisphere around the normal of the current fragment. By doing texture lookups, the depth and the normals are compared between the current fragment and the traced ones. The comparison is a simple step-formula and from the result a SSAO term can be evaluated. This SSAO term is saved to the red channel to the texture. This result must be blurred before combined with the original scene render. The blur can for example be a bilateral blur. Both the SSAO term, and the blurring can be done in a much lower resolution (half or fourth the size) than the screen to save some clock cycles. Then after blurring, one can upsample it to full screen size and get a some blur for free.

The combination with the original screen can be as simple as just multiplying this AO term with the already rendered screen.

The shader is written to save the AO in the red channel so render it to a single-channel texture to save memory.

The SSAO term is saved only in the red channel

The shader requires that the normalMap sampler holds a renderable texture with the normals of the scene in screen space in the RGB channels, and the linear depth (scaled to 0..1) in the A-channel. NOTE: These maps are not the same maps as when doing deferred lighting. For example, the normals should be pure face normals rendered to the screen (no normalmapping).

The rnm sampler should hold a texture with random normals in the RGB-channel, and can for example be the picture below.

These are the properties that worked well for me (but it’s different from setup to setup what gives best result):

uniform float totStrength = 1.38;
uniform float strength = 0.07;
uniform float offset = 18.0;
uniform float falloff = 0.000002;
uniform float rad = 0.006;

Here’s the SSAO GLSL fragment shader:

uniform sampler2D rnm;
uniform sampler2D normalMap;
varying vec2 uv;
uniform float totStrength;
uniform float strength;
uniform float offset;
uniform float falloff;
uniform float rad;
#define SAMPLES 16 // 10 is good
const float invSamples = 1.0/16.0;
void main(void)
{
// these are the random vectors inside a unit sphere
vec3 pSphere[16] = vec3[](vec3(0.53812504, 0.18565957, -0.43192),vec3(0.13790712, 0.24864247, 0.44301823),vec3(0.33715037, 0.56794053, -0.005789503),vec3(-0.6999805, -0.04511441, -0.0019965635),vec3(0.06896307, -0.15983082, -0.85477847),vec3(0.056099437, 0.006954967, -0.1843352),vec3(-0.014653638, 0.14027752, 0.0762037),vec3(0.010019933, -0.1924225, -0.034443386),vec3(-0.35775623, -0.5301969, -0.43581226),vec3(-0.3169221, 0.106360726, 0.015860917),vec3(0.010350345, -0.58698344, 0.0046293875),vec3(-0.08972908, -0.49408212, 0.3287904),vec3(0.7119986, -0.0154690035, -0.09183723),vec3(-0.053382345, 0.059675813, -0.5411899),vec3(0.035267662, -0.063188605, 0.54602677),vec3(-0.47761092, 0.2847911, -0.0271716));
//const vec3 pSphere[8] = vec3[](vec3(0.24710192, 0.6445882, 0.033550154),vec3(0.00991752, -0.21947019, 0.7196721),vec3(0.25109035, -0.1787317, -0.011580509),vec3(-0.08781511, 0.44514698, 0.56647956),vec3(-0.011737816, -0.0643377, 0.16030222),vec3(0.035941467, 0.04990871, -0.46533614),vec3(-0.058801126, 0.7347013, -0.25399926),vec3(-0.24799341, -0.022052078, -0.13399573));
//const vec3 pSphere[12] = vec3[](vec3(-0.13657719, 0.30651027, 0.16118456),vec3(-0.14714938, 0.33245975, -0.113095455),vec3(0.030659059, 0.27887347, -0.7332209),vec3(0.009913514, -0.89884496, 0.07381549),vec3(0.040318526, 0.40091, 0.6847858),vec3(0.22311053, -0.3039437, -0.19340435),vec3(0.36235332, 0.21894878, -0.05407306),vec3(-0.15198798, -0.38409665, -0.46785462),vec3(-0.013492276, -0.5345803, 0.11307949),vec3(-0.4972847, 0.037064247, -0.4381323),vec3(-0.024175806, -0.008928787, 0.17719103),vec3(0.694014, -0.122672155, 0.33098832));
//const vec3 pSphere[10] = vec3[](vec3(-0.010735935, 0.01647018, 0.0062425877),vec3(-0.06533369, 0.3647007, -0.13746321),vec3(-0.6539235, -0.016726388, -0.53000957),vec3(0.40958285, 0.0052428036, -0.5591124),vec3(-0.1465366, 0.09899267, 0.15571679),vec3(-0.44122112, -0.5458797, 0.04912532),vec3(0.03755566, -0.10961345, -0.33040273),vec3(0.019100213, 0.29652783, 0.066237666),vec3(0.8765323, 0.011236004, 0.28265962),vec3(0.29264435, -0.40794238, 0.15964167));
   // grab a normal for reflecting the sample rays later on
   vec3 fres = normalize((texture2D(rnm,uv*offset).xyz*2.0) - vec3(1.0));
 
   vec4 currentPixelSample = texture2D(normalMap,uv);
 
   float currentPixelDepth = currentPixelSample.a;
 
   // current fragment coords in screen space
   vec3 ep = vec3(uv.xy,currentPixelDepth);
 // get the normal of current fragment
   vec3 norm = currentPixelSample.xyz;
 
   float bl = 0.0;
   // adjust for the depth ( not shure if this is good..)
   float radD = rad/currentPixelDepth;
 
   vec3 ray, se, occNorm;
   float occluderDepth, depthDifference, normDiff;
 
   for(int i=0; i&lt;SAMPLES;++i)
   {
      // get a vector (randomized inside of a sphere with radius 1.0) from a texture and reflect it
      ray = radD*reflect(pSphere[i],fres);
 
      // if the ray is outside the hemisphere then change direction
      se = ep + sign(dot(ray,norm) )*ray;
 
      // get the depth of the occluder fragment
      vec4 occluderFragment = texture2D(normalMap,se.xy);
 
      // get the normal of the occluder fragment
      occNorm = occluderFragment.xyz;
 
      // if depthDifference is negative = occluder is behind current fragment
      depthDifference = currentPixelDepth-occluderFragment.a;
 
      // calculate the difference between the normals as a weight
 
      normDiff = (1.0-dot(occNorm,norm));
      // the falloff equation, starts at falloff and is kind of 1/x^2 falling
      bl += step(falloff,depthDifference)*normDiff*(1.0-smoothstep(falloff,strength,depthDifference));
   }
 
   // output the result
   float ao = 1.0-totStrength*bl*invSamples;
   gl_FragColor.r = ao;
 
}

This is an optimized version of the same shader, but a little harder to read and understand

uniform sampler2D rnm;
uniform sampler2D normalMap;
varying vec2 uv;
const float totStrength = 1.38;
const float strength = 0.07;
const float offset = 18.0;
const float falloff = 0.000002;
const float rad = 0.006;
#define SAMPLES 10 // 10 is good
const float invSamples = -1.38/10.0;
void main(void)
{
// these are the random vectors inside a unit sphere
vec3 pSphere[10] = vec3[](vec3(-0.010735935, 0.01647018, 0.0062425877),vec3(-0.06533369, 0.3647007, -0.13746321),vec3(-0.6539235, -0.016726388, -0.53000957),vec3(0.40958285, 0.0052428036, -0.5591124),vec3(-0.1465366, 0.09899267, 0.15571679),vec3(-0.44122112, -0.5458797, 0.04912532),vec3(0.03755566, -0.10961345, -0.33040273),vec3(0.019100213, 0.29652783, 0.066237666),vec3(0.8765323, 0.011236004, 0.28265962),vec3(0.29264435, -0.40794238, 0.15964167));
 
   // grab a normal for reflecting the sample rays later on
   vec3 fres = normalize((texture2D(rnm,uv*offset).xyz*2.0) - vec3(1.0));
 
   vec4 currentPixelSample = texture2D(normalMap,uv);
 
   float currentPixelDepth = currentPixelSample.a;
 
   // current fragment coords in screen space
   vec3 ep = vec3(uv.xy,currentPixelDepth);
  // get the normal of current fragment
   vec3 norm = currentPixelSample.xyz;
 
   float bl = 0.0;
   // adjust for the depth ( not shure if this is good..)
   float radD = rad/currentPixelDepth;
 
   //vec3 ray, se, occNorm;
   float occluderDepth, depthDifference;
   vec4 occluderFragment;
   vec3 ray;
   for(int i=0; i&lt;SAMPLES;++i)
   {
      // get a vector (randomized inside of a sphere with radius 1.0) from a texture and reflect it
      ray = radD*reflect(pSphere[i],fres);
 
      // get the depth of the occluder fragment
      occluderFragment = texture2D(normalMap,ep.xy + sign(dot(ray,norm) )*ray.xy);
    // if depthDifference is negative = occluder is behind current fragment
      depthDifference = currentPixelDepth-occluderFragment.a;
 
      // calculate the difference between the normals as a weight
 // the falloff equation, starts at falloff and is kind of 1/x^2 falling
      bl += step(falloff,depthDifference)*(1.0-dot(occluderFragment.xyz,norm))*(1.0-smoothstep(falloff,strength,depthDifference));
   }
 
   // output the result
   gl_FragColor.r = 1.0+bl*invSamples;
 
}

To use the SSAO effect. Render a fullscreen quad over the screen with the following vertex shader and the previous SSAO fragment shader.

varying vec2  uv;
 
void main(void)
{
gl_Position = ftransform();
gl_Position = sign( gl_Position );
 
// Texture coordinate for screen aligned (in correct range):
uv = (vec2( gl_Position.x, - gl_Position.y ) + vec2( 1.0 ) ) * 0.5;
}

Here’s the source for a school project we did in six weeks (halftime work). It’s a simple FPS game, but does demonstrate the SSAO. Use the “O”-button to toggle the different SSAO modes. This source is unfortunately hard to build since it uses a lot of third party libraries (Ogre3D, boost, fmod, ode …)

http://sourceforge.net/svn/?group_id=244295

And here’s the compiled version. Just unzip it and run the exe.

Download zip

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