On Wed, May 17, 2017 at 6:20 PM, Marco Domingues <marcodomingue...@gmail.com
> wrote:
> Hi,
>
> When looking at Boolean evaluation code i noticed the use of the struct
> bu_bitv (struct bu_bitv *solidbits) to test if a given region is ready to
> be evaluated, by checking if every solid in the region has been intersected
> (function ‘bool_partition_eligible’).
>
> From my understanding, these tests are not necessary to be ported to
> OpenCl, because at the moment of boolean evaluation, every solid is
> guaranteed to be intersected, considering the sequence of OCL kernels:
>
> count_hits()
> store_segs()
> weave_segs()*
> eval_partitions()*
> shade_segs()
>
> *yet to be implemented
>
> Is this correct? Or the struct bu_bitv is used for something else in the
> background other than just checking if every solid has been intersected?
>
Right. If I remember correctly that bitvector is used to ensure that we do
not intersect each object more than once. It is basically a performance
optimization because, if the acceleration structure is a spatial partition
like a kd-tree or a grid, it is possible that you will compute multiple
intersections with the same object in the same ray otherwise. But the thing
is, I implemented a BVH object partition acceleration structure for the
OpenCL version of the raytracer code. In a BVH you will never compute an
intersection with any object more than once along the same ray. That's just
how it is. So the final list should have no duplicates already.
Hence we should not need the bitvector at all. That is why I used a BVH in
the first place. Having a per thread bitvector with one bit per primitive
in a GPU, with possibly thousands of threads in flight, would use a lot of
cache space which would make the intersection computations really slow. You
want to minimize the amount of temporaries per thread on a GPU so you can
have more threads in flight without waiting for accesses to DRAM which.are
really painfully slow in comparison to the cache.
Regards,
--
Vasco Alexandre da Silva Costa
PhD in Computer Engineering (Computer Graphics)
Instituto Superior Técnico/University of Lisbon, Portugal
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
BRL-CAD Developer mailing list
brlcad-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/brlcad-devel
