g80/Cuda for raytracing ?

hi,
i am also doing volume raycasting/-tracing, but i can not reconstruct your problems. could you explain what artifacts you encountered opposed to the slice based rendering?

 

It's true. What you described is exactly discussed in a paper several years, but I forget where I found it.

For fluid with explicit free surface (liquid-air boundary), it seems raytracing is the only way to render it with believable appearance. Otherwise, I believe slice-based volume rendering methods are faster and less artifacted.

 

The main artifact is zebra strips when encountering the thin but high density parts of the volume. It seems much more obvious than rendering with slices. Even in some thick area the low frequency strips can be seen due to the variant number of sampling points. Another annoying one is the undersampling alias trends to form different pattern along each face of the bounding box, that would really become unacceptable when moving the camera. I tried to use jittered sampling position but that didn't help a lot if the sampling points not being increased (at that time I used 20~40 sampling points per ray on a 64x64x64 volume texture). There were also other artifacts, but I can't clearly remember because I did it in 2004, the time 6800 just came out.

Also, if you use ray-marching to render a volume texture, can you apply the volumetric lighting? When rendering with the slices, you can use half-angle oriented slices to compute the absorption, extinction etc.

 

yes, i encountered these artifacts too. one important thing to counter the second problem you mentioned would be to set the first sample position along the ray not where the ray intersects the bounding volume but set it to the first whole numbered multiple of the sampling sep size. this way you sample the volume on discrete shells (as opposed to the slices of the slice base approaches).

that was a very big improvement on my work in visual quality. the zebra stripes can only be countered by higher sampling distances or preintegration but they also occure in slice based rendering.