Mittwoch, 27. Mai 2015

Sparse Voxel Octree Raytracing based Occlusion Culling (Theory)

I will share some thoughts on how the next generation of block based sandbox games rendering (triangle based) and creators pipelines might look like. In current games, overdraw is the most limiting factor when drawing high detailed scenes - that means, occlusions are not determined 100% accurate. Further, it is necessary to traverse some data structures on the CPU to create a list of objects to be rendered on the screen.

To improve on this, here the proposal for a completely GPU based solution that uses octree raycasting to determine the visibility.

The first step is to raycast a low res, lets say 128x128 pixel, ID buffer, which takes about 1ms from experiments. This is fast and only used to determine the visibility. In a second step, we create a list of visible objects and further merge with the list of objects that were visible in the past few frames. The list is generated as input buffer for glmultidrawelementsindirect. The steps until now are performed on the GPU using multiple CL or GL compute kernels. The final step is to draw all object with one GL call. The raycasting and grouping of duplicate IDs on the screen can be performed best in opengl compute or opencl. To not miss out distant objects only occupying a few pixels, it is necessary to add jittering to the rays casted. The proposed technology will allow the use of high resolution blocks for sandbox games while the performance is high enough to run on notebooks at high frame rates.

The creation pipeline is as usual:

First, sculpting / creating of the mesh as hi-res, then creating LODs+Normal maps using mesh simplification and transferring the details to a normal map.

Actually, there is already a similar method out there for a while called instant OC. It is not that effeciently integrated however. Its a script addon to Unity - yet, is uses raycasts to determine the visibility (1000-2000 have been the demo settings). 

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