Monday, October 6, 2008

Dynamic voxels

In the past couple of weeks, I've learned that there are different methods to efficiently store voxels for GPU raycasting:

- Octrees

- Geometry images (Hoppe 2002, Carr et al., 2006)

- Spatial hashes

- Hybrid aceleration structures such as an octree with bricks (Crassin et al., 2008)

Jules Urbach said in an article on TechCrunch (

We store voxel data in several ways, including geometry maps (see our Siggraph or Iceland presentations, where we show this method applied to the Ligthstage 5 structured light data, courtesy Andrew Jones ICT/Graphics lab)

Lightstage 5 is being used to capture performances of real actors in polygon based animations, which are then converted to voxels and stored in geometry maps (or geometry images, see A Brief Overview of Geometry Maps). So there is at least one way to render characters and dynamic objects through voxel raycasting, without the need for hybrid techniques. The paper from Carr et al. (Fast GPU Ray Tracing of Dynamic Meshes using Geometry Images) shows that "interactive" raycasting of dynamic objects is possible "at no extra cost". They use geometry maps to store triangles however instead of voxels. With this method, it's feasible to have extremely detailed characters raycasted in realtime.
Spatial hash maps could possibly be used as for dynamics as well.

In the TechCrunch article Jules Urbach gives some more info on the rendering methods behind OTOY and the Ruby voxel demo:

- The datasets from the BCN and Ruby city scenes contain up to 64 data layers per voxel, including diffuse albedo, fresnel reflectance values, irradiance data, UV coordinates (up to 8 sets), normals, and, for static scenes, look up vectors for 1-20 bounces of light from up to 252 evenly distributed viewpoints (it is important to note that this data is always 100% optional, as the raycaster can do this procedurally when the voxels are close and reflection precision is more important than speed; however, with cached reflectance data, you might see the scene rendering at 100s-1000s of fps when the scene isn’t changing).

- A note on raytracing vs. rasterization: amplifying the tree trunk in Fincher’s Bug Snuff demo to 28 million polys using the GPU tessellator turned out to be faster than rendering a 28 million voxel point cloud for this object. So there is a threshold where voxels become faster than rasterziation at about 100 million polys. At least in our engine, on R7xx GPUs, using full precision raycasting at 1280×720. Below that point, traditional rasterization using the GPU tessellator seems to be faster for a single viewport.

- The engine can convert a 1 million poly mesh into voxel data in about 1/200th second on R770 (60 fps on R600 and 8800 GTX). This is useful for baking dense static scenes that are procedurally generated once, or infrequently, on the GPU. That is why some of the OTOY demos require the GPU tessellator to look right.

- Hard shadows in OTOY were done using rasterization until we got R770 in May. Now hard shadows, like reflections, can be calculated using raycasting, although shadow masks are still very useful, and raycasting with voxel data can still give you aliasing.

- We can use the raycaster with procedurally generated data (perlin generated terrain or clouds, spline based objects etc.). At Jon Peddie’s Siggraph event, we showed a deformation applied in real time to the Ruby street scene. It was resolution independent, like a Flash vector object, so you could get infinitely close to it with no stair stepping effects, and likewise, the shadow casting would work the same way.

- The voxel data is grouped into the rough equivalent of ‘triangle batches’ (which can be indexed into per object or per material groups as well). This allows us to work with subsets of the voxel data in the much the same way we do with traditional polygonal meshes.

- The reflections in the march 2007 ‘Treo’ video are about 1/1000th as precise/fast as the raycasting we now use for the Ruby demo on R770/R700.

- One R770 GPU can render about 100+ viewports at the quality and size shown in the ‘Treo’ video. When scenes are entirely voxel based, the number of simultaneous viewports is less important than the total rendered area of all the viewports combined.

- The server side rendering system is currently comprised of systems using 8x R770 GPUs ( 8 Gb VRAM, 1.5 Kw power per box).

The full Ruby demo:

High quality download: