my new ssao & some help

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Hello! I´m new here. I´ve recently been experimenting with screen space ambient occlusion but i was quite disappointed with the results, its very difficult to get it to look like in Crysis. So i tried to implement it myself without following the crytek method. I ended up creating this shader which implements a type of ssao i have dubbed coherent ssao (cssao for friends). It looks far better than crysis´s (or at least that´s what i think). Now i need help because i want to extend it to full ambient occlusion, (still in image space, but taking into account hidden geometry). I´ll post the shader and explain the method first. The method is quite simple: a convolution kernel which samples pixels from a depth buffer and a normal buffer, calculates occlusion for each pixel using mainly the angular difference between normals, and then modulates it using depth. Usually this would only produce a cartoon outline type shader because it would shade "outside" as well as "inside" creases, but then i apply a cosine difference check between normal and direction of sampling (simple dotproduct between normalized vectors, this is the "coherence" test), that returns >0 for inside creases and <0 for inside ones, so i only occlude concave creases. The resulting AO doesn´t need to be blurred to look good, doesn´t suffer from halos, can be used for local as well as global ao -local looks better ;)-, and is quite fast to compute. A 3x3 kernel is more than enough for a game. Now for the code (needs to be cleaned up): GLSL:
#define NUM_SAMPLES 8

uniform sampler2D som; //the depth buffer
uniform sampler2D normal; //the normal buffer

//noise producing function to reduce banding (got it from someone else´s shader):
float rand(vec2 co){

return 0.5+(fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453))*0.5;

}

void main()
{
float sum = 0.0;

float zFar = 80.0;
float zNear = 0.5;

float prof = texture2D(som, gl_TexCoord[0].st).x;
prof = zFar * zNear / (prof * (zFar - zNear) - zFar);  //linearize z sample

vec3 norm = normalize(vec3(texture2D(normal,gl_TexCoord[0].st).xyz)*2.0-vec3(1.0));

int hf = NUM_SAMPLES/2;

//calculate sampling rates:
float incx = (1.0/160.0)*8;//8 is the radius
float incy = (1.0/120.0)*8;

for(int i=-hf; i < hf; i++){
for(int j=-hf; j < hf; j++){

vec2 coords = vec2(i*incx,j*incy)/prof;

float prof2 = texture2D(som,gl_TexCoord[0].st+coords*rand(gl_TexCoord[0])).x;
prof2 = zFar * zNear / (prof2 * (zFar - zNear) - zFar);  //linearize z sample

if (prof2>prof){

vec3 norm2 = normalize(vec3(texture2D(normal,gl_TexCoord[0].st+coords*rand(gl_TexCoord[0])).xyz)*2.0-vec3(1.0));

//calculate approximate pixel distance:
vec3 dist = vec3(coords,prof-prof2);

//calculate normal and sampling direction coherence:
float coherence = dot(normalize(-coords),normalize(vec2(norm2.xy)));

//if there is coherence, calculate occlusion:
if (coherence > 0){
//approximate form factor:
float pformfactor = 0.5*((1.0-dot(norm,norm2)))/(3.1416*pow(abs(length(dist*4)),2.0)+0.5);//4 is depthscale
sum += clamp(pformfactor*2,0.0,1.0);//2 is ao intensity;
}

}
}
}

float occlusion = 1.0-(sum/NUM_SAMPLES);
gl_FragColor = vec4(occlusion ,occlusion ,occlusion ,1.0);
}


Now the question: I have a vague idea of using a second camera to capture the back faces of the geometry, or using a second pass with front culling and a reverse normal buffer to calculate occlusion from the hidden parts of the image, but i´ve tried to imagine it and write it on paper and i have a feeling that it won´t work. Any ideas? thanks and sorry for my weird english PD: normals are supplied in camera space and the zbuffer is not linear. images with 3x3 filter and radius=7: [Edited by - ArKano22 on December 8, 2008 5:51:31 PM]

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Another idea to increase performance would be to reduce kernel size (along with radius, which already shrinks) when depth increases, i´ll try to add it to the code.

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That seems like a very interesting implementation - thanks for sharing.

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Can we get a bigger version of the full-color shot? Thanks. :)

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Quote:
 Original post by n00body Can we get a bigger version of the full-color shot? Thanks. :)

Yes, i had trimmed the size a bit in order to keep the post relatively small. Note that i´m multiplying the ao on top of the scene, so it can look weird in less illuminated areas. I will use in in the lighting equation once i complete the shader.
Here are another two screenshots:
with very local ao (radius = 2):

with more global ao (radius = 9). Contrast increased in photoshop:

Both images keep the kernel size as 3x3. More samples yield more quality. With this settings (k=3x3,redius=9) it runs at 190-210 fps in a 8800GT, in a moderately complex scene (terrain+skybox+sponza atrium+physics going on). Any ideas about the backface occlusion thing?

Thanks :)

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I was messing around with the shader, and i tried to convert it to a color bleeding shader(global illumination is too big to speak about here :P)

The results are quite nifty for a convolution, i think:

Settings are the same as with global ssao. The changes made to the shader are minimal, I will post it tomorrow. Hopefully i will find a way to merge both shaders into one, trying to use common computations only once (different kernel sizes for ao & color bleeding are a problem).

Still thinking about a way to take invisible geometry into account...

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Quite impressive. Sounds like it runs at a decent speed as well. I'm very interested in seeing it when you combine the two.

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Good job!

I am developing similar technique (called ISR - 'image-space radiosity') for my PhD thesis - I hope I will able to share my work soon.

For back faces you can try the second pass with inverted culling - it should give you more precise results.

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for the backgeometry, you'd need something like "deep shadow maps". order independent transparency needs that feature, too.

depth peeling, that was the name.

should help a bit to google around.

i really like your work. is it free to use? :)

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Quote:
 i really like your work. is it free to use? :)

Yes, it is free to use. Just share the results if you achieve something better :). The depth peeling idea sounds promising, i had heard about it before but never really implemented it, or found a practical use for it aside of transparency. I´ll google for info.

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