Thanks for those links - I started using a something similar to what is described as Smit's algorithm (Slab method) in the Amy Williams paper. My code us subtly different to that usually given because I check the "back face" intersection first -for an early terminate ; this basically involves swapping x_plane_min and x_plane_max early if v.X<0 .. then looking explicitly for the back plane intersection points first.. as well as being slightly more verbose it seems to have the side effect of avoiding the divide by -0 problem described in the paper. (misses ray parallel to plane, catches adjoining edges) I think my version is sub-optimal but not a priority right now.
I've noticed that the divides in all the "slab" methods could be multiplied out - this which inverts the boundary condition if the divisor was negative, but can be caught using if (multiplied_x < undivided_boundary_value) ==(divisor>0) instead of the vanilla. if (x < boundary_value) not only does this avoid any division (hurray!), but also reduces the cumulative float errors - the number of operations on both sides of the condition of is roughly equal, rather than being unbalanced with most on one side in the un-premultuplied version.. It's a little more complicated, but divides are, afaik, still slow even on modern hardware, which is something I suspect may be a bottleneck within the function.. I need to bench this.. It may mitigate but doesn't solve the float precision problem. The second link is a nice optimisation for simd too. Optimisation for this function isn't a big priority for me because I'm primarily using it it get the intersection point for a bounding volume for a "3d texture" (or 'non-sparse voxel array' in other words) - my main routine steps through the voxel grid front to back using a different algorithm. PBRT looks good - wasn't aware of it - found the pre-publication chapter section 3.9.2 "CONSERVATIVE RAY–BOUNDS INTERSECTIONS" - though I didn't proof read it it looks like it covers using calculated expected errors (deltas) well. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
