I try to implement a glsl shader for screen-space ambient occlusion.

I tried somthing similar to this:

https://gist.github.com/fisch0920/6770311

But there are some strange black dots in the render. What is wrong ?

No AO: http://s18.postimg.org/k21sqjgav/SAO1.jpg

With AO : http://s3.postimg.org/7lxraqehd/SAO2.jpg

Code:

#version 330

uniform sampler2D ColorSampler;
uniform sampler2D NormalSampler;
uniform sampler2D WorldPosSampler;
uniform mat4 ViewMatrix;
uniform mat4 ProjMatrix;

out float FragAO;

float SampleRadius = 20.0;
float PI = 3.14159265359;

float Random(vec4 seed4)
{
    float dot_product = dot(seed4, vec4(12.9898,78.233,45.164,94.673));
    return fract(sin(dot_product) * 43758.5453);
}

float GetAOSample(vec3 SamplePos, vec3 fNormal, vec4 fViewPos)
{
    vec4 SampleCoord = ProjMatrix * vec4(SamplePos, 1.0);
    SampleCoord.xy /= SampleCoord.w;
    SampleCoord.xy = SampleCoord.xy * 0.5 + 0.5;
        
    vec4 sWorldPos = vec4(texture(WorldPosSampler, SampleCoord.xy, 0).xyz, 1.0);
    vec4 sViewPos = ViewMatrix*sWorldPos; 
    
    vec3 sampleDir = sViewPos.xyz-fViewPos.xyz;
    float NdotS = max(dot(fNormal, sampleDir), 0.0);
    float dist2 = dot(sampleDir,sampleDir);
    
    float SampleRadius2 = SampleRadius * SampleRadius;
    float f = max(SampleRadius2 - dist2, 0.0) / SampleRadius2;
    return f * f * f * NdotS / (0.0001 + dist2);
}

void main(void)
{
    ivec2 FragCoord = ivec2(gl_FragCoord.xy);
    vec4 fWorldPos = vec4(texelFetch(WorldPosSampler, FragCoord, 0).xyz, 1.0);
    
    vec3 fNormal = texelFetch(NormalSampler, FragCoord, 0).xyz * 2.0 - 1.0;
    vec4 fViewPos = ViewMatrix*fWorldPos;
    fNormal.z = sqrt(1.0-dot(fNormal.xy, fNormal.xy));
    
    float AOValue = 0.0;
    float thetaRand = Random(vec4(gl_FragCoord.yyxx))*2.0*PI;
    
    for(int i=0; i<32; i++)
    {
        float iStep = float(i) / 32.0;
        float radius = iStep*SampleRadius;
        float theta = thetaRand + iStep * 4.0*PI;
        
        vec3 offset = vec3(radius*cos(theta), radius*sin(theta), 0.0);
        offset *= sign(dot(offset,fNormal));
        
        vec3 SamplePos = fViewPos.xyz + offset;
        AOValue += GetAOSample(SamplePos, fNormal, fViewPos);
    }
    FragAO = max( AOValue/32.0, 0.0);
} 

Then in my final shader I use:

float AO = pow(1.0 - texelFetch(AOSampler, FragCoord, 0).x, 1.0 + AOCoeff * 8.0);

I fixed the dots by adding a bias to the radius:

float radius = iStep*SampleRadius;

Is now

float radius = iStep*SampleRadius + 1.0;

--

But I still have some strange artefacts: http://s18.postimg.org/6mb3q3gav/SAO3.jpg

I'm also having issues with the SAO shader. For me, it's something about the normals.

Your images arent working though, reuppload them.

The banding artifact is very clearly visible in the image you posted.

Have a look here: http://mtnphil.wordpress.com/2013/06/26/know-your-ssao-artifacts/

Theres a good explanation how to get rid of those bandings by using mixups when sampling.

Also are you blurring the SSAO texture. A 3x3 gaussian blur can help a lot against the high frequency noise in there.