Greetings,
In my deferred shading pipeline, I am trying to add screen-space ambient occlusion. I'm (of course) creating it after my G-Buffer is created so I have the normals in view space and can reconstruct the position in view space from the depth buffer. I originally implemented it in my forward renderer following the example from Frank Luna's Direct3D 11 book, and it worked well, but I am running into some issues trying to adapt that to my deferred shading approach.
My G-Buffer normal render target is a R32G32B32A32_FLOAT format and my depth buffer is D24S8, so precision for calculations isn't an issue. As mentioned, I'm reconstructing the view space position from depth as described here by MJP.
The results I'm getting are shown below. As you can see in the first image, some of the occlusion looks correct, particularly where the box sits over the ground and the corner of the upper-middle box touches the lower-middle box, as well as the occlusion occurring on the sphere behind the left-most box. You'll notice in the first image that there is some occlusion being generated in the gap between the upper two boxes, but not as much as I would probably expect. Furthermore, when I move the camera to the right very slightly (second image), the occlusion value between them basically disappears, which makes me think I'm doing something wrong somewhere in view space. Notice that for those two boxes, too, the faces facing the screen have nothing else in front of them, so I would imagine they would receive no occlusion at all.
The third image is the same scene rendered from a different angle, this time showing that the sphere is somehow getting occlusion on faces that have no other geometry in front of them.
Also, the images are intentionally unblurred so we can see them for what they are and hopefully get a better idea of what is happening. I've tried it with bilateral blur passes enabled and the blurring works fine, but it's still just blurring the same incorrect data. Any ideas that might help in fixing this issue are very welcome and appreciated.
Here is the shader code I am using.
Vertex Shader:
struct VertexIn
{
float3 posL : POSITION;
float2 tex : TEXCOORD;
};
struct VertexOut
{
float4 posH : SV_POSITION;
float3 viewRay : VIEWRAY;
float2 tex : TEXCOORD;
};
cbuffer cbPerFrame : register(cb0)
{
float4x4 inverseProjectionMatrix;
};
VertexOut main(VertexIn vIn)
{
VertexOut vOut;
// already in NDC space
vOut.posH = float4(vIn.posL, 1.0f);
float3 positionV = mul(float4(vIn.posL, 1.0f), inverseProjectionMatrix).xyz;
vOut.viewRay = float3(positionV.xy / positionV.z, 1.0f);
// pass to pixel shader
vOut.tex = vIn.tex;
return vOut;
}
Pixel Shader:
struct VertexOut
{
float4 posH : SV_POSITION;
float3 viewRay : VIEWRAY;
float2 tex : TEXCOORD;
};
cbuffer cbPerFrame : register(cb0)
{
float4x4 gViewToTexSpace; // proj * texture
float4 gOffsetVectors[14];
float gOcclusionRadius; //0.5f
float gOcclusionFadeStart; // 0.2f
float gOcclusionFadeEnd; // 2.0f
float gSurfaceEpsilon; // 0.05f
// for reconstructing position from depth
float projectionA;
float projectionB;
float2 _padding;
};
Texture2D normalTexture : register(t0);
Texture2D depthStencilTexture : register(t1);
Texture2D randomVecMap : register(t2);
SamplerState samNormalDepth : register(s0);
SamplerState samRandomVec : register(s1);
// determines how much the sample point q occludes the point p as a function of distZ
float occlusionFunction(float distZ)
{
float occlusion = 0.0f;
if(distZ > gSurfaceEpsilon)
{
float fadeLength = gOcclusionFadeEnd - gOcclusionFadeStart;
// linearly decrease occlusion from 1 to 0 as distZ goes from fade start to end
occlusion = saturate((gOcclusionFadeEnd - distZ) / fadeLength);
}
return occlusion;
}
float4 main(VertexOut pIn) : SV_TARGET
{
float3 normal = normalize(normalTexture.SampleLevel(samNormalDepth, pIn.tex, 0.0f).xyz);
float depth = depthStencilTexture.SampleLevel(samNormalDepth, pIn.tex, 0.0f).r;
float linearDepth = projectionB / (depth - projectionA);
float3 position = pIn.viewRay * linearDepth;
// extract random vector from map from [0,1] to [-1, 1]
float3 randVec = 2.0f * randomVecMap.SampleLevel(samRandomVec, 4.0f * pIn.tex, 0.0f).rgb - 1.0f;
float occlusionSum = 0.0f;
// sample neighboring points about position in the hemisphere oriented by normal
[unroll]
for(int i = 0; i < 14; ++i)
{
// offset vectors are fixed and uniformly distributed - reflecting them about a random vector gives a random, uniform distribution
float3 offset = reflect(gOffsetVectors.xyz, randVec);
// flip offset vector if it is behind the plane define by (position, normal)
float flip = sign(dot(offset, normal));
// sample a point near position within the occlusion radius
float3 q = position + flip * gOcclusionRadius * offset;
// project q and generate projective tex-coords
float4 projQ = mul(float4(q, 1.0f), gViewToTexSpace);
projQ.xy /= projQ.w;
// find nearest depth value along ray from eye to q
float rz = depthStencilTexture.SampleLevel(samNormalDepth, projQ.xy, 0.0f).r;
// reconstruct full view space position r = (rx, ry, rz)
linearDepth = projectionB / (rz - projectionA);
float3 r = pIn.viewRay * linearDepth;
// test whether r occludes position
float distZ = position.z - r.z;
float dp = max(dot(normal, normalize(r - position)), 0.0f);
float occlusion = dp * occlusionFunction(distZ);
occlusionSum += occlusion;
}
occlusionSum /= 14;
float access = 1.0f - occlusionSum;
// sharpen the contrast of the SSAO map to make the effect more dramatic
return saturate(pow(access, 4.0f));
}
Here is the application code for setting projectionA and projectionB (from Matt's post).
float clipDiff = farClipDistance - nearClipDistance;
float projectionA = farClipDistance / clipDiff;
float projectionB = (-farClipDistance * nearClipDistance) / clipDiff;
Thanks!
