POV-Ray : Newsgroups : povray.unofficial.patches : media method 3 jitter 0 performance branch off release/v3.8.0. : media method 3 jitter 0 performance branch off release/v3.8.0. Server Time
4 Dec 2023 00:49:51 EST (-0500)
  media method 3 jitter 0 performance branch off release/v3.8.0.  
From: William F Pokorny
Date: 18 Feb 2018 10:40:30
Message: <5a899e6e$1@news.povray.org>
For those building their own POV-Ray versions see:


I'm working on other more complicated, longer term alternatives, but the 
commit for the scene changes and the commit for method 3 performance 
contained in this branch are simple, safe and reasonably well tested. 
Enough so that I think it worth considering this branch for adoption in 
release/v3.8.0. Opinions?

Bill P.

--- Details for those interested ---
For the usual Jitter=0 media the performance gain tops out at about 
5.5%. For (abs(Jitter))!=0 there the penalty for the conditional tops 
out at a performance loss of 1.5%.  Not possible at this point, but if 
we could dump the jitter option with method 3 we'd top out at a 7% 
performance gain for complex medias/densities.

Note: I recommend jitter alway be 0 given our current implementation 
unless a noisy, expensive to AA result is what you want.

Note: Performance is always better in my testing where method 3 is 
really used however many of our sample scenes will see less performance 
gain (1-1.5%) because they almost all use constant media (PLAIN_PATTERN 
densities) and so trigger a fast path into method 1 for part of the 
media evaluation. In other words many of our sample scenes partly bypass 
the method 3 code and so the change herein.

While I reformatted the functions 
MediaFunction::ComputeMediaAdaptiveSampling and 
MediaFunction::ComputeOneMediaSampleRecursive to the 120 character max, 
the code change for performance is quite small. Instead of doing the 
jitter calculattion on all ComputeOneMediaSample() calls it changes that 
parameter to:

(IMedia->Jitter==0.0 ? 0.0 : dd * IMedia->Jitter * 
(randomNumberGenerator() - 0.5))

or similar.  In addition the top of ComputeOneMediaSampleRecursive the 
jdist calc becomes:

         jdist = d2;
         jdist = d2 + Jitter * (d3 - d1) * (randomNumberGenerator() - 0.5);

Aside: It neatly happens the performance improvement here speeds up our 
benchmark scene in my testing almost enough to counter the 
FixedSimpleVector elimination degrade.

--- Scene file changes. Both fix artifacts/bugs/set-up issues.

1) media3.pov - Result looked really bad with existing samples setting 
so increased it until result looked good.

2) micro.pov - Was using "intervals 2" to no benefit.  It was also using 
an assumed_gamma of 1.5.  Changed that to 1.0 and updated the emitting 
media colors for I believe is a much better result.  The exact result 
here not mattering much in any case as just demonstrating technique.

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