I’ve joined a few friends and former schoolmates at Firaxis. Actually, my graduate school adviser, Marc Olano, has been here for the last year on sabbatical. Some of his work on filtering specular highlights from normal maps for ocean rendering in Civilization V was published at I3D 2010 under the title LEAN Mapping. Speaking of, he’s been posting a number of graphics tricks over at the UMBC Games and Interactive Media blog. Trick 1 reformulates the computation of a normal from a heightfield and Trick 2 deals with calculating the size of a mip chain via closed form equations. He’ll be posting a few more in the future, so add the RSS feed and check back in a week or so.

Many of the blogs I used to read on the regular have either gone silent or their authors have primarily moved to Twitter. I read a few people’s tweets but mostly I just haven’t made (or plan to make) that transition. I find it hard enough to post anything here without over-analyzing whether or not its even worth reading, let alone posting 8 times a day. And frankly as a reader I find the signal:noise ratio difficult to manage. A few of the good old fashioned web blogs I’ve been reading lately are Miles Macklin’s blog, Game Angst by Adrian Stone,  and Smash’s direct to video. On the less graphics focused front, The Witness blog starring Jon Blow and Ignacio Castaño, the Wolfire Games blog and Charles Bloom’s cbloom rants (as always). As a total hypocrite, I would like to call-out and encourage Brian Karis, Tim Farrar and Christer Ericsson to update their damn blogs.

Anyway, I’ve been reading more papers lately so maybe I’ll have some impetus to update this dumb blog. And if you still have my RSS feed in your reader, thanks!

As a side note, Civilization V was announced several weeks ago and has been available for pre-order on Steam. Go team!

This paper presents two techniques, a simple line sampling technique and a more involved area sampling technique.

Line integral technique – Depth values are sampled in a uniform disc around each screen-space pixel.  This disc has a radius that is constant in object space. For each sample, a line integral of the occupancy for that sample is computed. In plain speak, this is the ratio of stuff to non-stuff between the view-ray intersection of the unit sphere around the pixel and the aforementioned disc (see illustration). Therefore depth is considered analytically in the calculation of AO and as a result the computed AO should be less noisy during camera movement. The authors also posit that suitable results can be achieved with less samples because the samples are generated in screen-space vs. in object space, where object space sampling can result in multiple samples being close to each other in screen-space (which is where depth is ultimately sampled). This results in a more accurate consideration of nearby occluding surfaces.

Area integral technique- Very similar in spirit to Angelo Pesce’s “Variance Methods for Screen-Space Ambient Occlusion” in ShaderX7. A mip-map of depth average and variance is computed for the depth buffer. This
statistical information is then used to compute an AO value without undersampling artifacts. The authors state that only one sample can be considered before the performance becomes prohibitive. I’m probably misunderstanding their technique because it seems to me that most of the overhead is in generating the hierarchical statistical information about the depth buffer. In the case of one sample, you figure out the screen-space area of your sampling disc and sample the mip-map level in which a texel covers about the same amount of screen-space. If you decide to use five samples instead of one, each sample would represent about 1/5th the amount of screen-space as a single sample, so you would in turn sample the mip-map at the level in which a texel is about 1/5th the screen-space area of the sampling disc.I’ll have to reread this section later.
The authors also present a thickness model that is supposed to treat surfaces in the depth buffer as if they aren’t part of a relief image, i.e. each surface has a certain thickness, so nearby pixels shouldn’t be occluded by them if they are a certain z-distance away. I’m pretty sure that just about every SSAO technique already does this. It’s usually covered by a world-space distance falloff or depth thresholding.

The paper also discusses computing AO at different frequencies and combining them, which I believe is what is done in the article “Multi-layered, Dual Resolution Ambient Occlusion” by the guys at NVIDIA.

Volumetric Ambient Occlusion – László Szirmay-Kalos, Tamás Umenhoffer, Balázs Tóth, László Szécsi, and Mateu Sbert. IEEE Computer Graphics and Applications. 2009.

Though this paper is a little dense, it boils down to a few simple ideas. The most important is a method for generating occlusion test samples that are above the tangent plane, without actually having to do tangent plane calculations (ala Horizon-Based Ambient Occlusion). This is done by sampling within a sphere of radius R/2, with its center R/2 units away from the shaded point along the normal. This  is the largest sphere above the tangent plane and also contained within the original sampling sphere (of radius R). Occlusion is calculated via an integral similar to that used in the Volumetric Obscurance paper discussed above.

Ambient Occlusion Volumes – Morgan McGuire. Technical Report. 2009.

This tech report is a much more fully developed implementation of the ideas I discussed in the article “Deferred Occlusion by Analytic Surfaces” in ShaderX7. The occlusion due to a triangle is computed analytically and splatted onto the scene in a deferred manner. The same equation was used for computing form-factors in Baum’s 1989 radiosity paper and was previously used for AO by Hoberock and Jia in the GPU Gems 3 article “High-Quality Ambient Occlusion”. A 3D bounding volume of the influence of each primitive is generated via the Geometry Shader to compute the occlusion value to be splatted. The reported framerates are surprisingly good, though I believe it is mentioned that AO is computed at a drastically reduced resolution. At least I think that is what is being referred to by “15×15 subsampling”. There is probably a lot of room for performance improvements to this algorithm. The author states that all AO Volumes are precomputed (dynamically outputting geometry from the GS adds 25% more render time). I don’t think is an unreasonable assumption.. most scene geometry in games is static. But considering that all bounding geometry is essentially a cube, an instanced stream of cubes could be rendered, where each cube fetches the primitive’s vertices it is bounding and computes its own vertex positions in the vertex shader. This would eliminate the use of the GS. Also, doing an early out in the pixel shader based on depth would help with fill in situations where the volume is covering parts of the screen that are in the distance and couldn’t possibly be occluded.

Hybrid Ambient Occlusion – Christoph K. Reinbothe, Tamy Boubekeur and Marc Alexa. Eurographics 2009.

Computes ambient occlusion by raycasting a voxelized representation of the scene per-pixel and filtering the results. This uses everyone’s (especially mine) favorite scene voxelization technique. Didn’t read this one too in-depth, but it seems to be worth a closer look.

SIGGRAPH ASIA 2009 paper acceptance notification was sent out – here’s a paper list at the usual place: Ke-Sen Huang. No papers links yet.

Approximating Higher Order Texture Filtering @ Iñigo Quílez’s website

Assembly 2009 Seminar Videos

Maciej Sinilo’s SIGGRAPH 2009 link collection

Still waiting for waiting for Advances in Real-time Rendering course notes to pop up on Bungie’s website.. come on already

Anyway, there were a few good replies to his post with derivations of the normalization terms required to conserve energy provided by Fabian “ryg” Giesen of Farbrausch. The lighting models used by most games won’t account for energy conservation across diffuse and specular terms but adding a normalization term is a simple step to take.

Me and My Leaf Node

Now that I’ve got all of these major life changes out of the way, I’m hoping to get back to regular updates on the blog. In the meantime, everyone should check out the Trine demo. Great looking 3D side-scroller with physics-based gameplay.

Chris Wyman has posted a link to the preprint of the EGSR 2009 paper with Greg Nichols and myself. The paper builds on the multiresolution technique described in the I3D 2009 paper “Multiresolution Splatting for Indirect Illumination” by pairing it with two virtual point light clustering techniques. The first technique clusters VPLs that sample the same surface. This VPL similarity is determined using the same multiresolution discontinuity detection method from the last paper. The other clusters lights hierarchically and performs a traversal based on VPL contribution to the final image. Clustering VPLs allows two advantages: Performance scales with light-space scene complexity instead of the number of initial VPLs and flickering in indirect illumination due to temporal incoherence in reflective shadow map sampling is reduced.

The paper also offers a stencil buffer based implementation of the multiresolution illumination calculation technique detailed in the previous paper that offers dramatic performance improvement over the original geometry shader implementation.

Greg Nichols, Jeremy Shopf, and Chris Wyman. “Hierarchical Image-Space Radiosity for Interactive Global Illumination.” Accepted to the Eurographics Symposium on Rendering. (June 2009)

Paper link

Related blog posts: Imperfect Shadow Maps – I3D 2009 Day 1 - Mixed Resolution Rendering