Original Post
Hi, i´m the ssao guy again :P. I´ve come up with a method to avoid ssao halo artifacts almost to the point of disappearing. I highly doubt it hasn´t been made before but it is so good i don´t get why it isn´t being used in that case. I call it "depth extrapolation" and it´s pretty simple: Halos appear because of big discontinuities in the depth buffer. If your occlusion function decays when depth starts to increase, the halos will be white, if not, your halos will be black. These artifacts would disappear if we were able to use two different depth buffers, one for 'first layer' depth and the second one containing a second layer of depth. This can be achieved using depth peeling but it is a very expensive technique since you have to render your scene several times. So we can´t get that second depth, but we can guess it. To do so , when we encounter a discontinuity, we reverse the sampling 'direction' and we switch depths when comparing depths, effectively performing a linear extrapolation. Some images will help: 
How do we know when we have encountered a discontinuity? when the depth difference is "too big". In my implementation consider that "too big" means > 0, and to get the final occlusion at the discontinuity border, I interpolate the original value with the corrected value. The bigger the gap, the more "correction" we apply. This removes halos almost completely, no matter how hardcore they were on your implementation.Here´s an example using my "gauss SSAO" (see http://www.gamedev.net/community/forums/topic.asp?topic_id=550452), which now looks almost like regular AO, only using the depth buffer: 
The additional cost of this technique depends on how you are taking your samples. The worst case is that it will multiply by 1.5 aprox the number of samples taken, the best case is when your sampling is not randomized and you can store samples the first time you take them, then by adding some math to do the extrapolation, the technique is almost free! (no additional texture fetches). Halos in SSAO are gone forever! wohooo! [Edited by - ArKano22 on October 18, 2009 4:32:03 PM]
How do we know when we have encountered a discontinuity? when the depth difference is "too big". In my implementation consider that "too big" means > 0, and to get the final occlusion at the discontinuity border, I interpolate the original value with the corrected value. The bigger the gap, the more "correction" we apply. This removes halos almost completely, no matter how hardcore they were on your implementation.Here´s an example using my "gauss SSAO" (see http://www.gamedev.net/community/forums/topic.asp?topic_id=550452), which now looks almost like regular AO, only using the depth buffer: The additional cost of this technique depends on how you are taking your samples. The worst case is that it will multiply by 1.5 aprox the number of samples taken, the best case is when your sampling is not randomized and you can store samples the first time you take them, then by adding some math to do the extrapolation, the technique is almost free! (no additional texture fetches). Halos in SSAO are gone forever! wohooo! [Edited by - ArKano22 on October 18, 2009 4:32:03 PM]
