POV rendering errors!

POV rendering errors!

Post by Per Michael Knuds » Sun, 16 Aug 1998 04:00:00



I am fairly new to pov, and have run into a problem:

When i render some of the example scenes that come with POV, for
instance shadow.pov and teapot.pov, i get small black pixels in the
scene. These black pixels are obviously not supposed to be there.
Why do these black pixels appear, is it a bug in the renderer? Is
there any way to get rid of them (i have tried turning on
anti-aliasing and other things all of which didn't help)?
One could of course remove them afterwards in a paint-program, but i
hope there is another solution :)

Per Knudsen

PS. Remove antispamprotect from my email-address if you want to email
me directly.

 
 
 

POV rendering errors!

Post by Jerry Cullingfo » Tue, 18 Aug 1998 04:00:00




Quote:>I am fairly new to pov, and have run into a problem:

>When i render some of the example scenes that come with POV, for
>instance shadow.pov and teapot.pov, i get small black pixels in the
>scene. These black pixels are obviously not supposed to be there.
>Why do these black pixels appear, is it a bug in the renderer? Is
>there any way to get rid of them (i have tried turning on
>anti-aliasing and other things all of which didn't help)?
>One could of course remove them afterwards in a paint-program, but i
>hope there is another solution :)

Hmm. This may be similar to a problem I ran into: POV (or at least the
version I was using at the time) has what I consider a slightly buggy
approach to CSG.

 It doesn't like coincident surfaces - which is
more-or-less understandable when you consider that they *might* have
different characteristics - and sometimes gets over-enthusiastic at eliminating
them. Sometimes you can partially work round this - for example, instead
of defining a CSG L shaped block with overlapping cuboids, you can *
the two cuboids together - but I never found a way to avoid the possibility
of coincident points where two surfaces touch; unfortunately, in the version
I was using at the time, this resulted in *both* surfaces being eliminated,
effectively producing a hole at that pixel position. (This doesn't always
happen, because you have to hit the join exactly - usually you don't.)

Usually, that gives you
a black pixel (you see an unlit inside face); occasionally, if you align the
viewpoint just right (wrong?) you can get the same thing happening on the far
side and see right through the object. To avoid the problem would need a
change to the hidden surface elimination code (If I remember correctly,
both surfaces were satisfying an "is this surface inside another object" test
and being removed) - it'd need additional checks to avoid removing all front
or back points, which would also allow you to have coincident CSG surfaces;
you'd have to make sure they had the same characteristics to get reliable
results - but it would fix the problem of holes at the joins.

My argument is that POV _should_ handle coincident surfaces gracefully - you
can't avoid them if you use CSG, because they're _always_ going to appear at
joins - there's no way to avoid it. It doesn't really matter which one you
keep, as long as you keep one of them - if it's a join, it's a tiny area,
if it's a larger area, then the user should make sure all surfaces are the
same - but removing all the points leaves holes, which are bad :-).

It's always possible that all this has changed in a later version of POV
than the one I was using :-) :-)

--


  \_|_   Hemel Hempstead, UK            PGP key at www.selune.demon.co.uk
\__/    (Speaking only for myself and not the company unless otherwise stated)

 
 
 

POV rendering errors!

Post by Per Michael Knuds » Tue, 18 Aug 1998 04:00:00






>>I am fairly new to pov, and have run into a problem:

>>When i render some of the example scenes that come with POV, for
>>instance shadow.pov and teapot.pov, i get small black pixels in the
>>scene. These black pixels are obviously not supposed to be there.
>>Why do these black pixels appear, is it a bug in the renderer? Is
>>there any way to get rid of them (i have tried turning on
>>anti-aliasing and other things all of which didn't help)?
>>One could of course remove them afterwards in a paint-program, but i
>>hope there is another solution :)

>Hmm. This may be similar to a problem I ran into: POV (or at least the
>version I was using at the time) has what I consider a slightly buggy
>approach to CSG.

First, thanks for answering.

About the CSG thing, these black pixels also appear on simple objects.
The mentioned shadow.pov scene has a simple cube in it, and the the
phenomenon is seen on that one too.

There gotta be some * pov-people in here, who knows why this
happens, and hopefully how to get rid of it.

Please someone :)!!

PS. remove antispamprotect from my email-address if you want to mail
me.

- Show quoted text -

>--


>  \_|_   Hemel Hempstead, UK            PGP key at www.selune.demon.co.uk
>\__/    (Speaking only for myself and not the company unless otherwise stated)

 
 
 

POV rendering errors!

Post by VerseCur » Wed, 19 Aug 1998 04:00:00


Quote:>shadow.pov and teapot.pov, i get small black pixels in the
>scene. These black pixels are obviously not supposed to be there.
>Why do these black pixels appear, is it a bug in the renderer? Is
>there any way to get rid of them (i have tried turning on
>anti-aliasing and other things all of which didn't help)?

What graphics card/driver do you have, and OS (Win 95, etc.)? Did you check
known problems with certain ones in the Help?
By "other things" do you mean max_trace_level too? Although I seriously doubt
any of those things could be the trouble, all demo files I've seen render fine
(which is not all), but shadows.pov and teapot.pov are fine for me.
I don't recall any video card incompatibilities for the 3.02 version.

   "No time like the future." xyz - The Great Unknown
 http://members.aol.com/versecurse/gallery1.htm

 
 
 

POV rendering errors!

Post by Per Michael Knuds » Thu, 20 Aug 1998 04:00:00



Quote:>>shadow.pov and teapot.pov, i get small black pixels in the
>>scene. These black pixels are obviously not supposed to be there.
>>Why do these black pixels appear, is it a bug in the renderer? Is
>>there any way to get rid of them (i have tried turning on
>>anti-aliasing and other things all of which didn't help)?

>What graphics card/driver do you have, and OS (Win 95, etc.)? Did you check
>known problems with certain ones in the Help?
>By "other things" do you mean max_trace_level too? Although I seriously doubt
>any of those things could be the trouble, all demo files I've seen render fine
>(which is not all), but shadows.pov and teapot.pov are fine for me.
>I don't recall any video card incompatibilities for the 3.02 version.

I don't think it has anything to do with videocards or operating
system or even different pov-versions. I have had this problem with
several versions of POV (last pov 3.1 Beta 5) under both MAC and
WINNT.
I tried the max_trace_level and set it for 1000 - it didn't help, but
it was worth a try, thanks for the tip.
Do you really mean that on the front box in shadows.pov you don't get
any black pixels, when tracing it with standard setting?.

Any other tips :)?

Per Knudsen

PS. If you want to mail me directly, remove the antispamprotect from
my emailaddress.

 
 
 

POV rendering errors!

Post by Doug Partridg » Fri, 21 Aug 1998 04:00:00


Have you loaded the image in another image viewer? In other words are
the black dots really there or are they just being caused by some bug in
pov/memmory glich in video card/slight incompatapility.
--

Doug Partridge
~
~

 
 
 

POV rendering errors!

Post by Per Michael Knuds » Fri, 21 Aug 1998 04:00:00


On Thu, 20 Aug 1998 08:01:59 GMT, Doug Partridge


>Have you loaded the image in another image viewer? In other words are
>the black dots really there or are they just being caused by some bug in
>pov/memmory glich in video card/slight incompatapility.
>--

>Doug Partridge
>~
>~

Yes i have view'ed it in several program on both pc's and macintosh
computers.
Help! You don't mean to say that you too don't get these black pixels
when rendering shadow.pov and teapot.pov, or have you not rendered
these particular scenes (they come with the pov-program)?
Could i please ask you (and others) to render these particular scenes,
to find out if it's only me who get these annoying black pixels :)
It can be seen even in 320x200 so it shouldn't take long, please!!!

Thanks in advance.

Per Knudsen

 
 
 

POV rendering errors!

Post by K. Tyle » Fri, 21 Aug 1998 04:00:00



> Could i please ask you (and others) to render these particular scenes,
> to find out if it's only me who get these annoying black pixels :)
> It can be seen even in 320x200 so it shouldn't take long, please!!!

> Thanks in advance.

> Per Knudsen

  Yes sir I rendered teapot.pov.
"I see spots and I see dots. I see them on the spout and I see them
on the rim" said Sam I Am.

Feel better ?

K.Tyler

 
 
 

POV rendering errors!

Post by K. Tyle » Fri, 21 Aug 1998 04:00:00




> > Could i please ask you (and others) to render these particular scenes,
> > to find out if it's only me who get these annoying black pixels :)
> > It can be seen even in 320x200 so it shouldn't take long, please!!!

> > Thanks in advance.

> > Per Knudsen

>   Yes sir I rendered teapot.pov.
> "I see spots and I see dots. I see them on the spout and I see them
> on the rim" said Sam I Am.

> Feel better ?

> K.Tyler

  Now for the fix. I don't know about Shadow.pov because I
haven't looked at it. I believe what you are seeing is an edge
effect of the different segments of the bezier patches used
in it's construction. Fortunatly the fix I have is an easy one.

Open teapot.inc. At the top of the file find a line that says

#declare Flat_Enough = 0.01;

change it to

#declare Flat_Enough = 0.001;

"no more spots, no more dots. No more on the spout and
no more on the rim" said Sam I Am.

K.Tyler

 
 
 

POV rendering errors!

Post by K. Tyle » Fri, 21 Aug 1998 04:00:00



> Yes i have view'ed it in several program on both pc's and macintosh
> computers.
> Help! You don't mean to say that you too don't get these black pixels
> when rendering shadow.pov and teapot.pov, or have you not rendered
> these particular scenes (they come with the pov-program)?
> Could i please ask you (and others) to render these particular scenes,
> to find out if it's only me who get these annoying black pixels :)
> It can be seen even in 320x200 so it shouldn't take long, please!!!

> Thanks in advance.

> Per Knudsen

  In shadows.pov the problem you are experiencing is called coincident
surfaces. It happens when you have two flat surfaces trying to occupy
the same space. When pov projects the light rays into the scene it can't
differentiate which face belongs to which object. In this case the box is
rest on a plane  and subsequently are abutting each other on one face.
This condition generally appears more on transparent objects rather than
solids.

To correct the scene:
On the box object which is the only place I noted the problem with spots
change the translate < ?, 3, ? > to translate < ?, 2.999, ? >. I've forgotten
what the x and z transforms were but they are not important to this
discussion.

Happy Rendering !

K.Tyler

P.S. See my earlier post on c.g.r.r. for the fix on teapot.pov.

 
 
 

POV rendering errors!

Post by Per Michael Knuds » Sat, 22 Aug 1998 04:00:00





>> Could i please ask you (and others) to render these particular scenes,
>> to find out if it's only me who get these annoying black pixels :)
>> It can be seen even in 320x200 so it shouldn't take long, please!!!

>> Thanks in advance.

>> Per Knudsen

>  Yes sir I rendered teapot.pov.
>"I see spots and I see dots. I see them on the spout and I see them
>on the rim" said Sam I Am.

>Feel better ?

>K.Tyler

Good to know one is not (completely) mad :)

Thanks for your time.

 
 
 

POV rendering errors!

Post by Per Michael Knuds » Sat, 22 Aug 1998 04:00:00






>> > Could i please ask you (and others) to render these particular scenes,
>> > to find out if it's only me who get these annoying black pixels :)
>> > It can be seen even in 320x200 so it shouldn't take long, please!!!

>> > Thanks in advance.

>> > Per Knudsen

>>   Yes sir I rendered teapot.pov.
>> "I see spots and I see dots. I see them on the spout and I see them
>> on the rim" said Sam I Am.

>> Feel better ?

>> K.Tyler

>  Now for the fix. I don't know about Shadow.pov because I
>haven't looked at it. I believe what you are seeing is an edge
>effect of the different segments of the bezier patches used
>in it's construction. Fortunatly the fix I have is an easy one.

>Open teapot.inc. At the top of the file find a line that says

>#declare Flat_Enough = 0.01;

>change it to

>#declare Flat_Enough = 0.001;

>"no more spots, no more dots. No more on the spout and
>no more on the rim" said Sam I Am.

>K.Tyler

Thank you very much for the solution.
I guess i could have spend weeks (at least) as a beginner, finding out
that myself.
Thanks again.

With you permission i will also post here your solution (which you
mailed me) to the problem i had with shadows.pov (someone else might
become happy too :))

In shadows.pov the problem you are experiencing is called coincident
surfaces. It happens when you have two flat surfaces trying to occupy
the same space. When pov projects the light rays into the scene it
can't
differentiate which face belongs to which object. In this case the box
is
rest on a plane  and subsequently are abutting each other on one face.
This condition generally appears more on transparent objects rather
than
solids.

To correct the scene:
On the box object which is the only place I noted the problem with
spots
change the translate < ?, 3, ? > to translate < ?, 2.999, ? >. I've
forgotten
what the x and z transforms were but they are not important to this
discussion.
<<

What a beautiful day it is now! :)

Per Knudsen

 
 
 

1. Error codes for PA 8.5 (render errors)

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Thanx in advance for any help with this...

--

Ing. Christoph Soukup
Tech. Development Manager

neo Software Produktions GmbH.
Business Park Vienna
Wienerbergstrasse 7.
A-1100 Viennna
Austria

http://www.neo.at

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