Why is BMRT so slow ?

Why is BMRT so slow ?

Post by Johan ?ber » Fri, 15 Aug 1997 04:00:00



How come BMRT is so slow compared to PRMan?

Things that take 2 minutes in PRman takes 15 min on BMRT. And I'm not
using reflections, transparancy etc. Just normal metal-shaded objects
with antialiasing.

To speed up renderings, is it possible to use shadowmaps in BMRT?

Besides of the speed, I like BMRT very much. Thanks Larry for a great
product!

Sincerely,

John

 
 
 

Why is BMRT so slow ?

Post by Michael B. Johns » Fri, 15 Aug 1997 04:00:00


: How come BMRT is so slow compared to PRMan?

because prman is so fast, not because BMRT is slow... :-)

: Things that take 2 minutes in PRman takes 15 min on BMRT. And I'm not
: using reflections, transparancy etc. Just normal metal-shaded objects
: with antialiasing.

now you begin to see why people use algorithms other than ray tracing.  
For most non-huge scenes, other algorithms will always handily beat a
ray tracer.  

: To speed up renderings, is it possible to use shadowmaps in BMRT?

sure.

--

-->  Media Arts Technologist, Pixar Animation Studios (Eastern Office)
-->  alumnus, MIT Media Lab, Computer Graphics & Animation Group
-->  http://wave.www.media.mit.edu/people/wave/

 
 
 

Why is BMRT so slow ?

Post by Dwight M Eve » Fri, 15 Aug 1997 04:00:00


: How come BMRT is so slow compared to PRMan?
For one thing, PRman is a scanline renderer. BMRT is a full Raytracer
with reflections, refraction, and radiosity.

The other thing is that it is written entirely in C++ (gcc to compile
it). Larry has not optimised it for any specific processor.

There is a shareware scanline renderer for Windows 95/Nt that support the
majority of Renderman Interfaces. No word on a shader compiler or
viewers, yet. renderdotc its called.

 Not to speak for Larry.

-Dwight

 
 
 

Why is BMRT so slow ?

Post by Noah Lee Gibb » Fri, 15 Aug 1997 04:00:00


Excerpts from netnews.comp.graphics.rendering.renderman: 14-Aug-97 Re:

Quote:> now you begin to see why people use algorithms other than ray tracing.  
> For most non-huge scenes, other algorithms will always handily beat a
> ray tracer.  

    And for huge scenes, other algorithms will almost always handily
beat a ray-tracer.  If you're just measuring speed, a ray-tracer is the
wrong tool for the job.

                                               Noah

       I am Bob, the great and powerful!   Pay no attention
              to the man behind the keyboard!

 
 
 

Why is BMRT so slow ?

Post by Matt Phar » Sat, 16 Aug 1997 04:00:00



Quote:> > now you begin to see why people use algorithms other than ray tracing.  
> > For most non-huge scenes, other algorithms will always handily beat a
> > ray tracer.  

>     And for huge scenes, other algorithms will almost always handily
> beat a ray-tracer.  If you're just measuring speed, a ray-tracer is the
> wrong tool for the job.

So I'm assuming that Larry is watching this thread to see where the
specularion leads before offering some more specifics.

The original poster mentioned anti-aliasing, and that's most likely the
cause of the speed difference.  In BMRT, if you double the number of
samples in a pixel, the surface shader is executed twice as many times, two
times as many shadow rays are traced, etc.  As you up the anti-aliasing,
this gets expensive quickly.  In prman, however, shading is done at a fixed
rate across each primitive, regardless of the number of samples taken in
the pixel.  This means that doubling the pixel sampling causes only a
slight increase in running time, since sampling the shaded geometry is much
cheaper than running the shader more times.

This is one of those deep magic things about prman that makes it so
effective at making high quiality anti-aliased pictures, compared to ray
tracing algorithms.

-matt
--
Matt Pharr

<URL:http://www-graphics.stanford.edu/~mmp>

 
 
 

Why is BMRT so slow ?

Post by Holger Waterma » Sat, 16 Aug 1997 04:00:00




:>
:> How come BMRT is so slow compared to PRMan?
:>
:> Things that take 2 minutes in PRman takes 15 min on BMRT. And I'm not
:> using reflections, transparancy etc. Just normal metal-shaded objects
:> with antialiasing.
:>
:> To speed up renderings, is it possible to use shadowmaps in BMRT?
:>
:> Besides of the speed, I like BMRT very much. Thanks Larry for a great
product!
:>
:> Sincerely,
:>
:> John

hi john

think about these facts: the prman only has knowlege about the lights &
shadows in his
own world - but the Blue Moon REnderer is really an raytracer, or even more an
raydosity tracer.
These Facts result in an much bigger Time to check all reflections and colour
changes on just
every pixel, so imho. i am really impressed about the speed larrys app comes
back with an
superb approximation in this kind of realistic picturing.

you can`t take an so called "realtime renderer" in any hardware card with the
results of prman
or the more math intensive bmrt - all these algorithms do only cheap
approximated stuff, but
they do try to do this in really fast time - this will hopefully result in
shorter modelling
time, an i will like to be one of the first people if i will get an
accellerator card with support
of motion blur to get really perfect results to be able to sell to the
outside.

keep on rendering - Holger

________HolgerWatermann_VisualDataResearch_Linz/Austria_____________

 
 
 

Why is BMRT so slow ?

Post by Klaus Elmquist Nielse » Sat, 16 Aug 1997 04:00:00




> : How come BMRT is so slow compared to PRMan?

> because prman is so fast, not because BMRT is slow... :-)

I believe that part of the reason for BMRT's speed is that shaders
are evaluated using an intrepreter. Intrepretation can be very slow.
The combination of intrepretation, SL and raytracing will almost
certainly be expensive because it is only possible to evaluate a
single shading sample at a time and because of the ``size'' of the
operations in SL is small compared to the intrepretation overhead.

Klaus
--
"We are Microsoft.  Resistance Is Futile.  You Will Be Assimilated."

 
 
 

Why is BMRT so slow ?

Post by Mike Klei » Sat, 16 Aug 1997 04:00:00


        [ stuff about rendering speed of BMRT and others]

This is a bit off topic, but I thought that I'd ask anyway,
as long as we're talking about comparing various rendering speeds.

A while ago, I heard about 4 standard RIB scenes to use for
benchmarks.  I believe they were used to calculate a number
called RenderMarks.  Does anybody (at Pixar, or otherwise)
have or know anything about these files, or their availiability,
or usage?

-- Mike Klein

P.S.

BMRT would still be great at half it's speed.  Thanks, Larry!

--

http://www.alumni.caltech.edu/~mklein

 
 
 

Why is BMRT so slow ?

Post by Johan ?ber » Sun, 17 Aug 1997 04:00:00


Hi!

Thanks for all the interestings answers! And thank you Larry for lighten
things up.

I'm very impressed with BMRT and the fact that one person wrote it by
himself.

Thanks,

Johan

 
 
 

Why is BMRT so slow ?

Post by Pete War » Wed, 20 Aug 1997 04:00:00


*SNIPPED Larry's explanantions*

Quote:> >There is a shareware scanline renderer for Windows 95/Nt that support the
> >majority of Renderman Interfaces. No word on a shader compiler or
> >viewers, yet. renderdotc its called.

> Incidentally, a colleague performed some time trials with renderdotc
> vs. BMRT (running rdc on Win, BMRT on Linux, but on the very same
> Pentium machine).  Guess what?  BMRT was faster, which is
> extraordinary because BMRT is a ray tracer but rdc is scanline.  This
> was an earlier version of rdc, so I don't know if it's still that
> slow.  I encourage anybody who has access to both to run similar tests
> and report the results back to this newsgroup.

        Not to put down BMRT (I have used it a bit and think it is
        awesome), but one of the major factors in contributing to
        BMRT running faster on Linux than rdc on Win/NT is the OS.
        I run a triple boot system, Linux/NT/95. I have noticed that
        my machine SCREAMS when using Linux, and appears somewhat
        sluggish under Win/NT. I am sure some of the people on
        comp.os.linux.advocacy can give a better technical outlook
        than me *8)

        Pete

 
 
 

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