I was working on my thesys and I did have a very strange response form rtrace.. Am I missing something or is it a dangerous bug?

I did cast two times the same call and I did have sistematically two different results.. WHY ?

So I did calculate the same thing from different files:

prova.dat containing :

2.5 -60 1.5 -1.667 61.1 -1.5

2.5 -60 1.5 0 61.1 -1.5

and prova1.dat containing just the second line :

2.5 -60 1.5 0 61.1 -1.5

I did cast the command :

cat prova.dat | rtrace -h- tmp/U0.oct

obtaining :

5.804947e-03 5.804947e-03 5.804947e-03

5.987884e-03 5.987884e-03 5.987884e-03

and

cat prova1.dat | rtrace -h- tmp/U0.oct

obtaining :

5.996149e-03 5.996149e-03 5.996149e-03

second line of first result, if I'm not wrong, should be identical to second result... Why is it not ?

Lucio Boscolo

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Hello Lucio,

It is normal for rtrace (and other Monte Carlo ray-tracing programs) to return slightly different results for the same ray, due to randomness in the sampling process. The goal is to produce the correct result on average in a reasonably unbiased estimation process, rather than producing the same systematically inaccurate result each time, as would be produced by a completely deterministic algorithm.

I hope this is helpful.

-Greg

## ยทยทยท

From: "loscotec\@libero\.it" <[email protected]>

Date: December 28, 2007 3:35:34 PM PST

I was working on my thesys and I did have a very strange response form rtrace.. Am I missing something or is it a dangerous bug?

I did cast two times the same call and I did have sistematically two different results.. WHY ?

So I did calculate the same thing from different files:

prova.dat containing :

2.5 -60 1.5 -1.667 61.1 -1.5

2.5 -60 1.5 0 61.1 -1.5

and prova1.dat containing just the second line :

2.5 -60 1.5 0 61.1 -1.5

I did cast the command :

cat prova.dat | rtrace -h- tmp/U0.oct

obtaining :

5.804947e-03 5.804947e-03 5.804947e-03

5.987884e-03 5.987884e-03 5.987884e-03

and

cat prova1.dat | rtrace -h- tmp/U0.oct

obtaining :

5.996149e-03 5.996149e-03 5.996149e-03

second line of first result, if I'm not wrong, should be identical to second result... Why is it not ?

Lucio Boscolo

Thanks Greg for such fast reply :o)) .. In the beginning I thought it was something like that, but I was confused when I saw that every time I was casting rtrace from the same input file it was giving exactly the same result..

I did also think that non-deterministic behaviour was just for indirect calculations.. well, it looks like it is not !! ;o)

Ok then, I feel better knowing that the 'error' is someway intentional and it will be probably not big.

Considering I'm working with luminances the 'imprecision' is in the third decimal number .. microluxes, than.. probably it will not affect much calculations..

Thank you again, I will read the chapter about stocastic and deterministic raytracers, in 'Rendering with Radiance' that could help me understanding probably..

Sorry for giving alarm !!

Lucio Boscolo

.Hello Lucio,

.

.It is normal for rtrace (and other Monte Carlo ray-tracing programs)

.to return slightly different results for the same ray, due to

.randomness in the sampling process. The goal is to produce the

.correct result on average in a reasonably unbiased estimation

.process, rather than producing the same systematically inaccurate

.result each time, as would be produced by a completely deterministic

.algorithm.

.

.I hope this is helpful.

.-Greg

.

.> From: "loscotec\@libero\.it" <[email protected]>

.> Date: December 28, 2007 3:35:34 PM PST

.>

.> I was working on my thesys and I did have a very strange response

.> form rtrace.. Am I missing something or is it a dangerous bug?

.>

.> I did cast two times the same call and I did have sistematically

.> two different results.. WHY ?

.>

.> So I did calculate the same thing from different files:

.>

.> prova.dat containing :

.>

.> 2.5 -60 1.5 -1.667 61.1 -1.5

.> 2.5 -60 1.5 0 61.1 -1.5

.>

.> and prova1.dat containing just the second line :

.>

.> 2.5 -60 1.5 0 61.1 -1.5

.>

.>

.> I did cast the command :

.>

.> cat prova.dat | rtrace -h- tmp/U0.oct

.>

.> obtaining :

.>

.> 5.804947e-03 5.804947e-03 5.804947e-03

.> 5.987884e-03 5.987884e-03 5.987884e-03

.>

.> and

.>

.> cat prova1.dat | rtrace -h- tmp/U0.oct

.>

.> obtaining :

.>

.> 5.996149e-03 5.996149e-03 5.996149e-03

.>

.> second line of first result, if I'm not wrong, should be identical

.> to second result... Why is it not ?

.>

.> Lucio Boscolo

.

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