On Wed, 30 Dec 1998 02:35:58 -0500, Steven Pigeon
<pig### [at] iroumontrealca> wrote:

>
>
>Matthew Corey Brown - XenoArch wrote:
>
>> Due to prescion limitations of Float math, Povray is compiled
>> by default with the following:
>>
>> Anything below 0.001 is considered 0.0
>> Anythung beyond +/- 1e7 is non exsistant
>
>Well, pov uses DOUBLEs, and not FLOATs, and the epsilonis 1e-6...
>not 1e-3.  I've checked the source.

Check the source again.  EPSILON varies with the platform.  For 
example, the Windows source to version 3.1a uses 

#define EPSILON 1.0e-5

The default (if not specified in config.h) in frame.h is

#define EPSILON 1.0e-10

Still not 1e-3, I'll admit, but not 1e-6, either.

HOWEVER:

What Matthew said is entirely correct, if the comments in the source
are to be believed.  Note this fragment from frame.h.  Note too that
it's platform-independent:

/*
 * These values determine the minumum and maximum distances
 * that qualify as ray-object intersections.
 */

#define Small_Tolerance 0.001
#define Max_Distance 1.0e7


>However, after toying with objects, I've discovered that
>the bug is related to spheres and ellipsoids. A cube a couple
>of thousand miles wide set at a quarter millon miles is visible
>(as a pixel, granted, but still visible). A sphere, 6 k miles wide,
>(assuming a pov unit is 1 meter) at 292 000 miles is not visible.
>
>Why?
>
>I have not been able to pinpoint the bug in the source. It
>seems OK to me... still, it is not. My guess is that the epsilon
>is used in an additive way instead of multiplicative way in
>the sphere intersection routine (should be something like
>eps * dist rather than just eps).

It has been stated before, perhaps in .programming, that the sphere
intersection routine was suboptimal WRT its use of EPSILON.  I'd go
look up the exact discussion, but I'm too lazy. :)

Post a reply to this message

>The sphere is one of the weakest, it changes size and
>disappears altogether rather easily if scaled too large or
small. The
>box primitive tends to distort quite a bit before suffering
the same
>fate. This happens regardless of camera being scaled to
view objects.
>I could put my scale test file at the scene-files.text
group if I can
>locate it here.


 Yes, the object type matters a lot. I remember some posting
about half a year ago that tried to scale a sphere and a
cylinder down to .... well, almost nothing.
From a certain scale value, the sphere started to shrink
faster than the cylinder.

Julius

Post a reply to this message

Hmm
I've ben experimenting a bit, and at certain sizes, pov says that a sphere is
infinite...
Why so ? (camera inside the sphere...)

//Spider

Julius Klatte wrote:
> 
> >The sphere is one of the weakest, it changes size and
> >disappears altogether rather easily if scaled too large or
> small. The
> >box primitive tends to distort quite a bit before suffering
> the same
> >fate. This happens regardless of camera being scaled to
> view objects.
> >I could put my scale test file at the scene-files.text
> group if I can
> >locate it here.
> 
>  Yes, the object type matters a lot. I remember some posting
> about half a year ago that tried to scale a sphere and a
> cylinder down to .... well, almost nothing.
> From a certain scale value, the sphere started to shrink
> faster than the cylinder.
> 
> Julius

Post a reply to this message

Spider wrote:
> 
> Hmm
> I've ben experimenting a bit, and at certain sizes, pov says that a sphere is
infinite...
> Why so ? (camera inside the sphere...)
> 
> //Spider
 
Because it's always been that way. They call a plane
an infinite object too but it fits inside my screen
with no problem.

-- 
Ken Tyler

tyl### [at] pacbellnet

Post a reply to this message

> 
> Because it's always been that way. They call a plane
> an infinite object too but it fits inside my screen
> with no problem.
Now that isn't the right answer to give *grin*
"because it has always been....." *yuch*

but I see your point, I know it isn't a bug in output, but It got me confused as hell
the
first time, two more planes(infinite) then I had used...

I only wondered why it was done, because of the sizes or something else ???



> 
> --
> Ken Tyler
> 
> tyl### [at] pacbellnet

Post a reply to this message

Well, I do know the "sky sphere" object is "infinite" in that it remains the same
distance
'away' no matter how much you move stuff around, AND it makes the 'scene' the center
of the
sphere no matter where the cartesian system SAYS the scene is located.

Was unaware that normal spheres did that if you put the camera inside them.... will
have to
experiment.

As for planes, well... they are a finite distance "away" along one axis, but extend
infinitely in the other two.  We could also argue that the x, y, and z 'vectors' are
also
infinite, but being one-dimensional, wouldn't do us much good as objects...

(I think i should pick up that old copy of Euclid and see if I can't render a few of
the
proofs... that should keep me safely in the padded cell for months. :)

Spider wrote:

> >
> > Because it's always been that way. They call a plane
> > an infinite object too but it fits inside my screen
> > with no problem.
> Now that isn't the right answer to give *grin*
> "because it has always been....." *yuch*
>
> but I see your point, I know it isn't a bug in output, but It got me confused as
hell the
> first time, two more planes(infinite) then I had used...
>
> I only wondered why it was done, because of the sizes or something else ???
>
> >
> > --
> > Ken Tyler
> >
> > tyl### [at] pacbellnet

--
 george erhard
 microsoft certified system engineer
 http://home.pacbell.net/dcnblues

 remove "nospam" from domain when replying

Post a reply to this message

george erhard wrote:
> 
> Well, I do know the "sky sphere" object is "infinite" in that it remains the same
distance
> 'away' no matter how much you move stuff around, AND it makes the 'scene' the center
of the
> sphere no matter where the cartesian system SAYS the scene is located.
> 
> Was unaware that normal spheres did that if you put the camera inside them.... will
have to
> experiment.
> 
> As for planes, well... they are a finite distance "away" along one axis, but extend
> infinitely in the other two.  We could also argue that the x, y, and z 'vectors' are
also
> infinite, but being one-dimensional, wouldn't do us much good as objects...
Well, those are SUPPOSED to be infinite... The sphere was what confused me..

 
> (I think i should pick up that old copy of Euclid and see if I can't render a few of
the
> proofs... that should keep me safely in the padded cell for months. :)
Show me the results, please..

//Spider

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