Dissapointing building times from lbvh gpu based builder

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outofspace
Posts: 11
Joined: Fri Jan 13, 2012 4:44 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby outofspace » Mon Jan 16, 2012 4:45 pm

spectral: I was not referring to Kirill's frustum paper (which however seems pretty clear to me), but to our
joint work on HLBVH (and to his separate work on "Grid SAH", which I also plan to include in nih over time).

As to SBVH: while it's certainly the algorithm which produces the best trees for a given memory budget,
I believe it's actually possible to get very close to it with a little pre-splitting (see the Grid SAH paper above),
which can also be done effectively in real-time.
So I wouldn't personally spend much time on optimizing SBVH itself: at best, it will give a 5-10% improvement
at a 200+% build cost.

straaljager: it's just yet another item on my (lower priority) todo list.
I plan to write something about it (probably a technical report), but I don't know when,
and most probably it won't be in the next 3 months.
For now, I can just say it's based on an efficient CUDA implementation of parallel range queries.

dbz
Posts: 46
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Location: the Netherlands

Re: Dissapointing building times from lbvh gpu based builder

Postby dbz » Tue Jan 17, 2012 10:03 pm

outofspace wrote:

Thanks for your comments.
outofspace wrote:And then now that real-time ray tracing is a solved problem (thanks to Kirill), there are plenty of other
interesting things in my todo list. ;)

A Whitted integrator may run in real-time but I don't think ray tracing in general can be done in real time. Some global illumination
effects still take many hours to render on either cpu or gpu. Or did Nvidia invent something that renders physically correct global illumination
in less than 10 ms as well?

outofspace
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Re: Dissapointing building times from lbvh gpu based builder

Postby outofspace » Wed Jan 18, 2012 10:17 am

Have I ever mentioned global illumination or full light transport simulation ? ;)

I just said ray tracing. It's now possible to solve primary and a little secondary visibility
on fully dynamic models at rates that were previously possible only with rasterization, that's it.

The rest is pretty much a matter of time, letting processors and sampling/reconstruction
algorithms evolve. From a purely computational point of view, I suspect it will take at least
10 years before we'll have the raw compute power needed to run fully realistic light transport
in real-time (and that's without taking into account increasing display resolution).
Possibly 15 or 20.

What I am saying is just that I am now a little less interested in research on basic ray tracing.

p.s.
This has nothing to do with this thread, but: I am getting really bored by the words
"physically correct" - Feynman is probably laughing really hard whenever graphics folks say that -
shouldn't we all switch to "physically based", "physically inspired" or "statistically correct" instead?

spectral
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Re: Dissapointing building times from lbvh gpu based builder

Postby spectral » Wed Jan 18, 2012 11:22 am

outofspace wrote:This has nothing to do with this thread, but: I am getting really bored by the words
"physically correct" - Feynman is probably laughing really hard whenever graphics folks say that -
shouldn't we all switch to "physically based", "physically inspired" or "statistically correct" instead?


I agree... we all know that there is no physically-correct engine... even if some claims :-P They just approach to the physically correct...
Like you... I prefer 'physically based'... it is more appropriate :-)

But anyway it is just a question of terminology, but you're right !
Spectral
OMPF 2 global moderator

dr_eck
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Joined: Mon Dec 05, 2011 7:35 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby dr_eck » Wed Jan 18, 2012 5:18 pm

@outofspace Thanks for the pointer to Garanzha's HLBVH paper. http://garanzha.com/default.aspx The BVH build times are remarkable.

You can also add me to the list of supporters for "physically based".

ingenious
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Location: London, UK
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Re: Dissapointing building times from lbvh gpu based builder

Postby ingenious » Wed Jan 18, 2012 10:16 pm

How about a list of "unbiased" haters? Count me it :twisted:
Image Click here. You'll thank me later.

graphicsMan
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Joined: Mon Nov 28, 2011 7:28 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby graphicsMan » Thu Jan 19, 2012 1:27 am

I don't hate unbiased. I think it's a useful concept. The trouble is when people think that unbiased means anything without stating the problem being solved. Or to assume that biased solutions must be worse.

dr_eck
Posts: 45
Joined: Mon Dec 05, 2011 7:35 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby dr_eck » Thu Jan 19, 2012 4:22 pm

Both consistency and bias are important concepts for a physically based ray tracer. Just for review: a consistent ray tracer will converge to the correct solution if enough rays are traced. Fewer rays will only result in noise. An unbiased ray tracer will give a result that is statistically accurate, even if a "small" number of rays is traced.

For example, photon mapping is biased. As Jensen writes on p. 53 of his book, "The price we pay for using density estimation to compute illumination statistics is that the method is no longer unbiased. This means that the average expected value of the method may not be the correct value. However, the technique is consistent which means that it will converge to the correct result as more points/photons are used."

outofspace
Posts: 11
Joined: Fri Jan 13, 2012 4:44 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby outofspace » Thu Jan 19, 2012 5:20 pm

Way too much importance is given to unbiasedness.
Consistency is completely fine if there is a way to control the error.

Remember that "statiscally accurate" doesn't buy you anything if all you get is noise.

In other words, if the amount of error you get in the form of bias+variance is the same
or less than the error you'd get in the form of variance otherwise, then simple consistency
can even be a win.

And in practice, robust light transport simulation can only be done with consistent methods -
unbiased sampling techniques (based on local path sampling) cannot capture all transport paths.
So as long as your renderer supports specular reflection and transmission, you cannot even claim
statistical correctness with any unbiased method - or at least not on all the scenes a user could create
(or on any of the scenes you see in daily life - any light bulb in any room will generate tons of
SDS paths - or paths that for all practical matters are SDS, even if you model the real camera aperture
and the real tungsten filaments - or the sun, the probabilities are just close to infinitesimal if you use
bidirectional path tracing).

graphicsMan
Posts: 156
Joined: Mon Nov 28, 2011 7:28 pm

Re: Dissapointing building times from lbvh gpu based builder

Postby graphicsMan » Thu Jan 19, 2012 5:56 pm

On the other hand, understanding bias and it's sources is important. For example, proper importance sampling is unbiased, and doing it wrong can have important consequences. Even in algorithms such as photon mapping that have bias, we still try to limit the sources of our bias, and reduce those that remain to present the smallest amount of bias possible while retaining the benefits of introducing bias in the first place. If you're not careful, consistent also goes out the window.


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