There’s been quite a lot of interest lately in the realtime graphics world to do with Sparse Voxel Octrees (SVOs), so I thought it was about time I had a look at them. Although I’ve experimented with voxels before, in particular in the Geo project I worked on a few years ago (http://jwhigham.wordpress.com/category/geo) I didn’t really make use of octrees in those cases and more significantly I converted the voxel data to geometry prior to rendering using the classic Marching Cubes algorithm - now generally superseded by better techniques such as Dual Contouring.
Having been down that route before I wanted to try something new so this time I’m endeavouring to render by ray-casting directly against the SVO rather than converting to geometry and rasterising. I don’t believe that ray-casting (or ray-tracing if you want to consider more than one bounce) is going to replace rasterisation any time soon as the genesis of GPUs has left them understandably specialised for that particular problem and therefore very efficient at it. I do believe however that when used in conjunction with rasterisation ray casting or tracing is a tool too powerful to ignore and can be an ideal solution for effects such as reflection, refraction, shadows and global illumination that are difficult to simulate with rasterisation, especially at a global scale.
|128x128x128 SVO ray-cast on the GPU with simple dot-product lighting|
So while it’s technically perfectly possible to convert the voxels to geometry and then intersect rays against the resulting polygonal soup it flies in the face of this belief and doesn’t strike me as a particularly sensible thing to do. I also want to see if ray casting against the voxel structures directly can help alleviate some of the more challenging problems with geometry based voxelisation, particularly transitions between multiple levels of detail which I struggled with on Geo. Also like I say, it’s not something I’ve done before which is really reason enough.
|128x128x128 Sphere rendered with linear distance filtering|
|The same sphere rendered with point distance sampling to make the individual voxels clearer|
Ultimately I would like to recreate my virtual planet out of landscape blocks of varying resolutions right down from global scale viewed from orbit to standing on the surface with reasonable fidelity. I’ll also need to have a think about how to procedurally generate the data as the flexibility to have full 3D makes the problem far more interesting than just the heightfield used by Isis (http://jwhigham.wordpress.com/tag/isis/) and Osiris (http://johnwhigham.blogspot.co.uk/search/label/Osiris). I want to avoid if possible having the floating blobs of landscape that plagued Geo’s simplistic 3D noise based solution while still having features that make the 3D nature of the data obvious such as realistic overhangs and caves.
As with most new experiments the first thing to do is investigate the existing body of work on the subject, which as I mentioned earlier is quite active at the moment with the dramatic increase in GPU performance and especially their flexibility in recent years. See the references section at the end for a few links to papers and videos I discovered in my internet SVO safari, it’s by no means an exhaustive list but they are some of the more interesting items I found.
Once I felt I had a sufficient understanding of current techniques I jumped in with both feet and embarked on a bit of coding. While the aim is to produce a real time GPU based solution my experience of the somewhat patchy tools available for GPU debugging steered me towards producing a reference implementation in C++ on the CPU first. While it would run far more slowly this would let me debug the code effectively and help me verify that my algorithm and data structures were correct before sending them down to the GPU.
This turned out to be a really useful process and with modern CPUs being pretty powerful themselves (and ray casting being a parallel friendly technique) it was possible to produce a C++ implementation that ran acceptably quickly even in debug builds. Having it render a quarter resolution version during camera movements and a full resolution one once the camera stopped also helped.
The first technique I attempted was the Laine & Karras one. While there was sample source code available for this I am of the belief that to properly understand a technique you have to at least have a go at coding it up from first principles so that’s what I did. After a while though my spider sense started to tell me that storing down to the individual voxel level wasn’t going to be best suited to planetary scales so I switched to the Cyril Crassin paper where he stores an octree down to a certain level then stores bricks of voxels rather than individual ones for the final few levels.
|The 16x16x16 grid of voxel "bricks" that make up the actual Octree|
This system felt more scalable and better suited to GPU application, plus there were advantages to do with cone tracing through the structure for shadows and global illumination applications that felt worth having available for the future.
It’s pretty clear that a voxel is a cube shaped blob of space, games such as MineCraft have made the voxel pretty famous these days, but there are a couple of key considerations when rendering them. Primarily is what each voxel represents, in it’s most simplisitic form a voxel could be considered to be either solid or empty which is easy to understand but limits rendering to producing blocky effects dependent entirely on the size of the voxels in use. Rather than storing a flag indicating solid or empty with a voxel a better way is to store a distance value representing how far the centre of the voxel is from the surface that is to be rendered.
Under this representation for a landscape voxels above the ground would have increasingly positive values while those underground would have increasingly negative ones. Rendering the isosurface where the distance is zero would produce the intended surface. The primary benefit of this scheme is that the distance value can be interpolated accurately between voxels to produce a better approximation of the surface. The quality of the output is still highly dependent upon the resolution of the data but undersampling artifacts show up as lumpy blobs rather than hard square edges which I think look generally better.
The images below show the progression from highest detail to lowest through the four levels of octree produced by my 128x128x128 noise-blob data. Note that I've turned off the linear filtering of distance here in all but the final image to make the individual voxels clearer:
|The 128x128x128 noise-blob at octree level 4 (highest detail)|
|The same 128x128x128 blob at octree level 3, each voxel is effectively twice the size|
|Down to octree level 2 and while the shape remains the detail is largely gone|
|Finally at the lowest detail level on the crude outline remains. The 128x128x128 original data is now effectively being rendered at 16x16x16 resolution.|
|The same lowest detail level but with linear distance filtering - not a terrible approximation of the blob considering it's only using 1/512th of the data|
On top of the distance value there are also other required fields such as material types which I have some ideas for but I’ll get in to that in later posts.
From CPU to GPU
Once I was happy that everything seemed to be working on the CPU I set about converting the code to run on the GPU using a pixel shader. I toyed briefly with using a compute shader instead as I've heard there are some benefits to converting some traditionally pixel based tasks to the more flexible compute pipeline but I'll save that for a rainy day. Ray casting seems a natural fit for per-pixel so I'm sticking with a pixel shader for now.
Thankfully as it had always been my intent to run it on the GPU I had written it with that in mind so storing the data structures in textures and moving the code to HLSL was fairly straight forwards. It did take a few hours to work out the inevitable teething problems compounded by the typically unstable nature of GPU debugging tools but considering what it's doing it didn't take too long.
The benefit of course is that instead of taking a couple of hundred milliseconds to render it now rendered in just a few milliseconds giving me true real-time frame rates. Being able to dynamically reload the HLSL code and GPU states also meant I could make changes and see the results virtually instantly without having to re-run the program, saving a considerably amount of time. If you are creating a graphics framework for your own projects I strongly recommend adding this ability!
|Two-sphere distance field showing the geometry that is actually sent to the GPU (i.e. 12 triangles). The back-faces of the cube are rendered instead of the front to allow the viewpoint to move inside the cube and still render|
So I have a single sparse voxel octree I can render via ray casting on the GPU complete with surface normals, what's next? Well the first thing to do is to be able to render multiple SVOs at different points in space, also to have them abut neatly with continuous surface normals so there are no shading seams.
After this I want to add some automatic level of detail control so the tree is traversed less deeply as the voxel bricks become smaller on screen, having this controllable from the user interface would also be useful to allow performance to be measured against visual fidelity. Next I want to add a new distance generation function to produce blocks that are a bit more landscape-like plus having the materials to go with them.
So, plenty to do then!
As mentioned above, this is a fairly arbitrary collection of some of the links I came across while researching SVOs. I am sure there are many more so do feel free to report any good ones you find in the comments!
Efficient Sparse Voxel Octrees – Analysis, Extensions, and Implementation, Samuli Laine Tero Karras, NVIDIA Research
Sparse voxel octree
Sparse Voxel Octree Ray Tracing on the GPU, Kristof Römisch, Masters Thesis
GigaVoxels: A Voxel-Based Rendering Pipeline For Efficient Exploration Of Large And Detailed Scenes, Cyril Crassin, PhD thesis
Sparse Voxel Octree (SVO) Demo by Jon Olick