Something to make you go "hmmmmm..."

I was trying to make this image using a bent-up bicubic patch, and
noticed some odd bright and dark spots on it.
If you have say, a single light source, a bicubic patch, and an object
between them, you can see the shadow of the middle object on the "dark
side" of the bicubic patch.

The pigment was a conventional color rgb < 1, 0, 0 > , with _no_
transmit or filter in it. Ambient was 0.0, and no other finish was
applied, no radiosity.

I took several looks at it, rotating the camera around and so on, and
noticed that there were bright areas on the (supposedly) dark side,
corresponding to where you'd expect a bright area on the illuminated
side.

It's as if the patch was transluscent, like ground glass, or something.
Not like filter or transmit, because light doesnt pass through it to
other objects, nor can you see through it.

If one bicubic patch is partly shading a second patch, the top one casts
its shadow on the second one, (as expected) and this shadow 

I tried placing two instances of exactly the same bicubic patch in the
same exact place, but the transluscent effect

The patch did not fold back in on itself or anything too outlandish.

Similar infinitely thin objects, like clipped planes, don't do seem to
do this.

I noticed this phenomenon in a Win version (3.something, Win95) of
POV-Ray, as well as with version 3.0.2a for Mac.

Any ideas?

-jr-

/* Of course the "spamBgone." has to go */

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J. L. Rose <jro### [at] spamBgonenbnetnbca> wrote:
: I was trying to make this image using a bent-up bicubic patch, and
: noticed some odd bright and dark spots on it.

  From the povray documentation:

As POV-Ray processes the Bezier patch it makes a test of the current piece
of the patch to see if it is flat enough to just pretend it is a rectangle.
The statement that controls this test is flatness. Typical flatness values
range from 0 to 1 (the lower the slower).
...
There are both advantages and disadvantages to using a non-zero flatness.
...
The biggest disadvantage is that if POV-Ray stops subdividing at a particular
level on one part of the patch and at a different level on an adjacent part
of the patch there is the potential for cracking. This is typically visible
as spots within the patch where you can see through. How bad this appears
depends very highly on the angle at which you are viewing the patch.

-- 
                                                           - Warp. -

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eerie...

I just posted the same problem on povray.general  - I uploaded a couple of images
- take a look.
Same problem? (nb my pipes are bicubic patches - from sPatch)

I used a standard texture T_Silver_5B (ok, I changed the roughness from 1/60 to
1/20 - but thats all)

I doubt the problem is that which Nieminen describes. All my patches have
flatness 0 and we're not getting spots or cracks, its a nice smooth (but
unwanted) light

David

Nieminen Mika wrote:

> J. L. Rose <jro### [at] spamBgonenbnetnbca> wrote:
> : I was trying to make this image using a bent-up bicubic patch, and
> : noticed some odd bright and dark spots on it.
>
>   From the povray documentation:
>
> As POV-Ray processes the Bezier patch it makes a test of the current piece
> of the patch to see if it is flat enough to just pretend it is a rectangle.
> The statement that controls this test is flatness. Typical flatness values
> range from 0 to 1 (the lower the slower).
> ...
> There are both advantages and disadvantages to using a non-zero flatness.
> ...
> The biggest disadvantage is that if POV-Ray stops subdividing at a particular
> level on one part of the patch and at a different level on an adjacent part
> of the patch there is the potential for cracking. This is typically visible
> as spots within the patch where you can see through. How bad this appears
> depends very highly on the angle at which you are viewing the patch.
>
> --
>                                                            - Warp. -

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David Greaves wrote:

> eerie...
>
> I just posted the same problem on povray.general  - I uploaded a couple of images
> - take a look.
> Same problem? (nb my pipes are bicubic patches - from sPatch)
>
> I used a standard texture T_Silver_5B (ok, I changed the roughness from 1/60 to
> 1/20 - but thats all)
>
> I doubt the problem is that which Nieminen describes. All my patches have
> flatness 0 and we're not getting spots or cracks, its a nice smooth (but
> unwanted) light
>
> David

In light of the fact you are experiencing problems with
patches it wouldn't hurt to try a non zero value.
Try a value of  0.01 - 0.0001 and see what kinds of
differences there are.
It also wouldn't hurt to double check your spatch
source file to make sure it doesn't contain overlooked
errors. I haven't used it much myself yet, but it is a
common occurrence in 3D modeling packages, to
think you have all of your vertices properly connected,
only to find out later, when looked at from a different
angle, things are not what they appeared to be.

Ken Tyler

-- I trace Rays but those guys can really hide ! --

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On Wed, 30 Sep 1998 23:12:20 +0100, David Greaves 
	<dav### [at] telekinesyscouk> wrote:
>eerie...
>
>I just posted the same problem on povray.general  - I uploaded a couple of images
>- take a look.
>Same problem? (nb my pipes are bicubic patches - from sPatch)
>
>I used a standard texture T_Silver_5B (ok, I changed the roughness from 1/60 to
>1/20 - but thats all)
>
>I doubt the problem is that which Nieminen describes. All my patches have
>flatness 0 and we're not getting spots or cracks, its a nice smooth (but
>unwanted) light

I can't seem to find it now, but I know I've heard about this problem
before.  It's not something wrong with your code, it's actually a bug
(or at least a limitation) in POV.  I'm not sure whether it is gone in 
3.1 or not.

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I have also noticed this phenomenon, and have used it to my advantage in a few
scenes... the bicubic patch is the only object that I know of that exhibits
this behavior.  Making shadow-puppets behind a sheet is practically impossible
without this.  Maybe every object should have a light-both-sides option.

I don't remember hearing about it being changed in POV 3.1, either.

-Nathan

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Nathan Kopp <nk8### [at] ltuedu> wrote:
: the bicubic patch is the only object that I know of that exhibits
: this behavior.

  Have you tried it with smooth triangles?

-- 
                                                           - Warp. -

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Hi!
Nieminen Mika wrote in message <3614b221.0@news.povray.org>...
>Nathan Kopp <nk8### [at] ltuedu> wrote:
>: the bicubic patch is the only object that I know of that exhibits
>: this behavior.
>
>  Have you tried it with smooth triangles?
>
>--
>                                                           - Warp. -

I believe simple triangles have the problem... but I have no longer the
source to prove it (sorry).

BTW, I already posted this message (almost) to the same thread on another
newsgroup.But I do not know how to do a cross posting reply with Outlook
Express, except by remenbering all the newsgroup the thread is in and
manually adding them to the post :-( Help?

Povingly,


Philippe

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Nieminen Mika wrote:
>   Have you tried it with smooth triangles?
> 
> --
>                                                            - Warp. -

Yes.  It does work with bicubic patches and smooth triangles (not regular
triangles).  Actually, it is caused by a flag that is available in all POV
objects.  The flag is set for bicubic patches and smooth triangles and not for
anything else.  I think it would be nice if the user had some control over this
flag (similar to the no_shadow flag).

-Nathan

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