[Bf-committers] OpenGL accelerated Nodes

Timothy Baldridge tbaldridge at gmail.com
Sat Sep 8 18:52:44 CEST 2007


Another thing to remember, is that you won't always have the ability
to do GPU FP rendering on some cards. Anything lower than a GeForceFX
will simply convert the data to a float when its uploaded to the card.
And anything below a GeForce8 will truncate the floats to 16bits.

That being said, the artifacts you mention because of non IEEE float
compliance won't be an issues here. You really only run into issues
when you start talking about the way a GPU rounds the data. So yeah,
in ray tracing, 5.000000 and 5.000001 could be totally different if
you're firing a ray across a room, but deciding if a pixel is 0.05 or
0.051 isn't even going to be noticeable to the human eye.

Timothy

On 9/8/07, Brecht Van Lommel <brechtvanlommel at pandora.be> wrote:
>
> Hi,
>
> There may be differences in the results depending on the graphics
> card, but I don't think that is a big issue, rather something you
> would fix as problems come up. It will mostly show in corner cases
> anyway, like very small or big numbers, inf/nan, hard to test in advance.
>
> A tricky thing about accelerating a few nodes is that it won't
> necessarily make things faster, since moving data to and from the GPU
> has a lot of latency, by the time you send something to the GPU, have
> it compute, and get it back it may be much faster to just do it on the
> CPU. So you need a good integration with the existing node execution
> system to make sure that there is the least GPU-CPU transfer possible,
> and that as much as possible stays on the GPU.
>
> Not sure which nodes would be desired to have on the GPU, I guess
> people would just want nodes in general to run faster. I think rgb
> curves, mix and blur are used most often. In my experience, most of
> the work would be coding the system, if that is done well, adding more
> nodes should not be much work.
>
> Brecht.
>
> Bob Holcomb wrote:
> > My 2 cents:
> >
> > I was under the impression that the differences between floating point
> > numbers representation on most graphics cards and on the cpu was
> > different enough to cause artifacts in rendering?  Would this only be
> > for preview or for actual rendering?  I do know that some of the latest
> > video cards (my nvidia 8800 GTX for example) does support the same IEEE
> > 754 floating point standard, for representation, however I'm not sure
> > that the process of the mathematics is exactly the same, so it may
> > return slightly different results (I could run some quick tests for
> > those interested).  If there is a desire for accelerated nodes, which
> > ones specifically?  Some will be harder than others to put on GPU due to
> > instruction limitations (again, depending on GPU).  This would be a good
> > use case for a plugin-in system, which should encompass more than just
> > nodes.
> >
> > Cheers,
> >
> > Bob
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> >
> >
>
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