Tek wrote:

> It seems that there's a maximum value that pov won't trace beyond.

Yep, from frame.h I found the Max_Distance constant, whose value is 1e7. 
Apparently it is used in all intersection calculations as an upper limit; 
intersections found farther away are just ignored.

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Johannes Dahlstrom wrote:

> intersection
> intersections

Ray-object-intersections, that is, not the CSG ones ;)

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Johannes Dahlstrom <sad### [at] tkukoulufi> wrote in message
news:3de4c9e2@news.povray.org...
> Johannes Dahlstrom wrote:
>
> > intersection
> > intersections
>
> Ray-object-intersections, that is, not the CSG ones ;)
>

:) Yeah I realised what you meant.

Speaking as a programmer this sounds like rather shoddy coding! Okay I'm sure
values near the limits of floating point numbers would cause problems, but the
limit is around 1e38, so I'd expect it could get much bigger than 1e7 without
any problems. Ah well, I'll just use a global scale in all my big scenes...

BTW, does anyone know if there's a similar lower accuracy limit? i.e. the
shortest distance below which two surfaces will be judged to be in the same
place?

--
Tek
http://www.evilsuperbrain.com

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On Wed, 27 Nov 2002 18:56:13 -0000, "Tek" <tek### [at] evilsuperbraincom>
wrote:

>Speaking as a programmer this sounds like rather shoddy coding! Okay I'm sure
>values near the limits of floating point numbers would cause problems, but the
>limit is around 1e38, so I'd expect it could get much bigger than 1e7 without
>any problems.

Don't forget that some shapes require solving polynomials of a very
high degree, and large values, even though way below the theoretical
fp limit, can easily cause considerable problems for the root solver
when used in high powers.

>BTW, does anyone know if there's a similar lower accuracy limit? i.e. the
>shortest distance below which two surfaces will be judged to be in the same
>place?

Yes, there is. Any intersection closer than EPSILON to the current one
is ignored. The reason is simple - it can easily be the same
intersection :)


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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Tek wrote:

> Speaking as a programmer this sounds like rather shoddy coding! Okay I'm
> sure values near the limits of floating point numbers would cause
> problems, but the limit is around 1e38, so I'd expect it could get much
> bigger than 1e7 without any problems. Ah well, I'll just use a global
> scale in all my big scenes...

Hmm... It might have something to do with the nature of floating point 
numbers - that a number's accuracy decreases as the magnitude increases 
("the dot is shifted", so to say). Thus, a unit-radius sphere a billion 
units away could be prone to artifacts - but what's the point of using that 
small objects that far away anyway?-)


> BTW, does anyone know if there's a similar lower accuracy limit? i.e. the
> shortest distance below which two surfaces will be judged to be in the
> same place?

There seem to be several different constants to be used with different 
calculations. Particularly, each object type has its own "depth_tolerance" 
value for ignoring intersection points too close to the ray origin. For a 
sphere it is 1e-6, but for a blob as much as 1e-2.

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