clipka <ano### [at] anonymousorg> wrote:
> Am 30.04.2010 21:09, schrieb Edouard:
>
> > It hangs when my settings are "subsurface { samples 1, 1 }", so I think it's
> > gotten stuck with something. When I try to quit POV-Ray I get a crash.
>
> Um... yes, that sounds suspicious indeed.
>
> After having had a closer look at the stack trace, I think I found the
> portion of the code where it hangs.
>
> There is a loop at the beginning of
> Trace::ComputeSingleScatteringContribution() that is intended to trace
> the ray a random distance into the object, using a special random
> distribution that may theoretically yield infinite distances; of course
> the ray should only be traced up to the next intersection point with the
> object's surface, so the algorithm will roll the dice again if that
> condition isn't met.
>
> That said, if the ray happens to hit an exceptionally thin part of the
> object, the algorithm will roll the dice again and again and again
> and... well, I guess you get the point.
>
> The crucial thing here is the ratio between the object "thickness" and
> the sum of the two coefficients used to specify the material's SSLT
> properties. The smaller the coefficients and/or the "thinner" the
> object, the more likely is a lockup to occur.
>
> Note that you can reduce the probability of a lockup without changing
> the scene geometry or material properties, by increasing the
> "mm_per_unit" global setting (though of course this will reduce the SSLT
> effect as if your whole scene was increased in size).

Is it the loop with the comment "calculate s_prime_out"?

It looks like the random number can simply be scaled so that you don't need the
loop at all...

At the very worst, can you make a change to ensure it doesn't hang - some sort
of bailout condition? Do you want me to add a bug to flyspray for it?

Cheers,
Edouard.

Post a reply to this message