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17 Aug 2022 21:32:30 EDT (-0400)
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From: Chris R
Subject: Weird problem with transparent box
Date: 23 Jun 2022 13:50:00
Message: <web.62b4a6ddb354b68f4ebf5035cc1b6e@news.povray.org>
The attached image shows two renderings of the same scene.  The right image is
the same as the left, except that I added a completely transparent box, whose
purpose was to support media with some ground fog:

box {
   -size/2, size/2
   texture { pigment { color rgbt 1 } }
   translate <0, size.y/2, 0>
}

As you can see, the box shows up as a mirror instead of being transparent.

The scene is fairly complex, but here are a few details that might help point
out to the experts what the problem might be:

1.  The whole scene is inside of a mesh-based sky sphere, but I verified that
the problem shows up even if that is turned off.
2.  I have enabled radiosity, but I verified that the problem shows up without
it as well.
3.  The light source is a sun emulator using a parallel area_light at a far
distance from the scene

I also tried using rgbf 1.0 as the pigment for the box, but I get the same
problem.

Any ideas?  It's driving me crazy

-- Chris R.


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Attachments:
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Preview of image 'scene-no-media-box.png'
scene-no-media-box.png


 

From: jr
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 14:00:00
Message: <web.62b4a9273b7b3290d6bf93276cde94f1@news.povray.org>
hi,

"Chris R" <car### [at] comcastnet> wrote:
> ...
> Any ideas?  It's driving me crazy

<https://wiki.povray.org/content/Reference:No_Image_Object_Modifier> ?


regards, jr.


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From: Chris R
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 14:30:00
Message: <web.62b4b0253b7b32904ebf5035cc1b6e@news.povray.org>
"jr" <cre### [at] gmailcom> wrote:
> hi,
>
> "Chris R" <car### [at] comcastnet> wrote:
> > ...
> > Any ideas?  It's driving me crazy
>
> <https://wiki.povray.org/content/Reference:No_Image_Object_Modifier> ?
>
>
> regards, jr.

Thanks, but I don't think that's what I need.  Ultimately I will be adding a
media statement to the interior of the object and I want the media to show up.

I have created other scenes with completely transparent objects before and you
can't see the edges of them, and they don't reflect the scene around them.  I
just can't figure out why it's happening here.

-- Chris R.


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From: jr
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 15:00:00
Message: <web.62b4b76d3b7b3290d6bf93276cde94f1@news.povray.org>
hi,

"Chris R" <car### [at] comcastnet> wrote:
> ...
> I have created other scenes with completely transparent objects before and you
> can't see the edges of them, and they don't reflect the scene around them.  I
> just can't figure out why it's happening here.

the same page also mentions the 'no_reflection' modifier (I was too lazy, in my
head the 'no_{image,reflection,shadow}' modifiers "go together").  also, I
wonder, perhaps give the box an explicitly non-reflective finish?


regards, jr.


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From: Chris R
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 15:50:00
Message: <web.62b4c3863b7b32904ebf5035cc1b6e@news.povray.org>
"jr" <cre### [at] gmailcom> wrote:
> hi,
>
> "Chris R" <car### [at] comcastnet> wrote:
> > ...
> > I have created other scenes with completely transparent objects before and you
> > can't see the edges of them, and they don't reflect the scene around them.  I
> > just can't figure out why it's happening here.
>
> the same page also mentions the 'no_reflection' modifier (I was too lazy, in my
> head the 'no_{image,reflection,shadow}' modifiers "go together").  also, I
> wonder, perhaps give the box an explicitly non-reflective finish?
>
>
> regards, jr.

I took my main scene and eliminated everything but the transparent box, and even
added media to it and everything looked fine.  I added one scene element at a
time back and it was all fine until I added in the drip of water I had modeled
falling from the downspout.  As soon as I added that object back into the scene,
the transparent box with media became a reflective mirror instead!

The drip of water is modeled as a quartic.  It had a material to look like
water, (so transparent, ior 1.33, reflective, etc.).  When I changed the
material to just a simple gray pigment, the media box was no longer visible or
reflective, but the media also disappeared.

I wonder if there is some bug with quartics and textures?



-- Chris R.


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From: jr
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 16:35:00
Message: <web.62b4cd913b7b3290d6bf93276cde94f1@news.povray.org>
hi,

"Chris R" <car### [at] comcastnet> wrote:
> ...
> I wonder if there is some bug with quartics and textures?

media, too, may harbour a bug or two.  :-)


regards, jr.


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From: William F Pokorny
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 19:19:48
Message: <62b4f514$1@news.povray.org>
On 6/23/22 15:48, Chris R wrote:
> I took my main scene and eliminated everything but the transparent box, and even
> added media to it and everything looked fine.  I added one scene element at a
> time back and it was all fine until I added in the drip of water I had modeled
> falling from the downspout.  As soon as I added that object back into the scene,
> the transparent box with media became a reflective mirror instead!
> 
> The drip of water is modeled as a quartic.  It had a material to look like
> water, (so transparent, ior 1.33, reflective, etc.).  When I changed the
> material to just a simple gray pigment, the media box was no longer visible or
> reflective, but the media also disappeared.

Unsure...

What version of POV-Ray is being used?

Are the shapes involved part of the same union or merge?

The only obvious thing I can think of would be if the water material was 
defined by name and happened to have the same name as the one used for 
the media box - and the water material definition was seen last.

If you can get a simpler scene showing the issues I'll add it to my 
collection for debug, though no promises as to when I'd dig. I've been 
busy with other than POV-Ray of late.

--- Deeper and less likely possibility...

With some older shape types - due ill advised automatic polynomial order 
reduction code combined with the lack of tight automatic / manual 
bounding for some shapes - we sometimes end up with an additional root 
(and so another surface) being introduced(a) by the auto-order reduction 
itself.

Maybe something like this is going on but, that surface by order 
reduction would have to have a very particular position to see the 
results you posted.

A test for this fail would be to change your quartic to a simple sphere 
or similar and see if the apparent reflection propagation to the box 
problem persists?

(a) - One of the shipped scenes long has had a vertical line in the 
result due the auto-order reduction changing two roots (4th order eqn) 
to three roots (third order eqn) for one or more of the scene's shapes. 
I cannot remember which it is for sure - primitiv.pov maybe? The povr 
branch fixed this particular exposure(b) by ripping out the automatic 
order reduction code.

(b) - Order reduction introduces slight root/surface shifts in the 
intended roots even when it mostly works in a consistent way. The 
reduction code also sometimes flips 'sturm on' to fixed solvers, without 
notice due the order reduction, on a per ray->surface equation basis. 
The sturm vs fixed solver results for identical equations are often not 
themselves identical and this occasionally introduce at same surface, 
subtle, consistency issues - which are sometimes visible. And there's 
more 'solver noise' possible due the order reduction I won't detail - 
partly because I no longer remember all the possible implications!

Bill P.


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From: Alain Martel
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 19:55:23
Message: <62b4fd6b$1@news.povray.org>
Le 2022-06-23 à 13:46, Chris R a écrit :
> The attached image shows two renderings of the same scene.  The right image is
> the same as the left, except that I added a completely transparent box, whose
> purpose was to support media with some ground fog:
> 
> box {
>     -size/2, size/2
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2, 0>
> }
> 
> As you can see, the box shows up as a mirror instead of being transparent.
> 
> The scene is fairly complex, but here are a few details that might help point
> out to the experts what the problem might be:
> 
> 1.  The whole scene is inside of a mesh-based sky sphere, but I verified that
> the problem shows up even if that is turned off.
> 2.  I have enabled radiosity, but I verified that the problem shows up without
> it as well.
> 3.  The light source is a sun emulator using a parallel area_light at a far
> distance from the scene
> 
> I also tried using rgbf 1.0 as the pigment for the box, but I get the same
> problem.
> 
> Any ideas?  It's driving me crazy
> 
> -- Chris R.

That looks like it could be a coincident surface issue.
Try placing the box slightly lower so that it's bottom face is just 
under the ground or slightly higher so that it don't touch the ground.
Some options :
box {
    -size/2-0.1*y, size/2 // lower
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2, 0>
}

box {
    -size/2+0.1*y, size/2 //higher
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2, 0>
}

box {
    -size/2, size/2
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2-0.1, 0> // lower
}

box {
    -size/2, size/2
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2+0.1, 0> //higher
}


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From: Chris R
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 22:30:00
Message: <web.62b5213f3b7b32904ebf5035cc1b6e@news.povray.org>
Alain Martel <kua### [at] videotronca> wrote:

> > The attached image shows two renderings of the same scene.  The right image is
> > the same as the left, except that I added a completely transparent box, whose
> > purpose was to support media with some ground fog:
> >
> > box {
> >     -size/2, size/2
> >     texture { pigment { color rgbt 1 } }
> >     translate <0, size.y/2, 0>
> > }
> >
> > As you can see, the box shows up as a mirror instead of being transparent.
> >
> > The scene is fairly complex, but here are a few details that might help point
> > out to the experts what the problem might be:
> >
> > 1.  The whole scene is inside of a mesh-based sky sphere, but I verified that
> > the problem shows up even if that is turned off.
> > 2.  I have enabled radiosity, but I verified that the problem shows up without
> > it as well.
> > 3.  The light source is a sun emulator using a parallel area_light at a far
> > distance from the scene
> >
> > I also tried using rgbf 1.0 as the pigment for the box, but I get the same
> > problem.
> >
> > Any ideas?  It's driving me crazy
> >
> > -- Chris R.
>
> That looks like it could be a coincident surface issue.
> Try placing the box slightly lower so that it's bottom face is just
> under the ground or slightly higher so that it don't touch the ground.
> Some options :
> box {
>     -size/2-0.1*y, size/2 // lower
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2, 0>
> }
>
> box {
>     -size/2+0.1*y, size/2 //higher
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2, 0>
> }
>
> box {
>     -size/2, size/2
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2-0.1, 0> // lower
> }
>
> box {
>     -size/2, size/2
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2+0.1, 0> //higher
> }

Thanks, I thought about that too, but I simplified the scene to the point where
there were no more coincident surfaces and it still happened.

-- Chris R.


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From: Chris R
Subject: Re: Weird problem with transparent box
Date: 23 Jun 2022 22:35:00
Message: <web.62b5225c3b7b32904ebf5035cc1b6e@news.povray.org>
William F Pokorny <ano### [at] anonymousorg> wrote:
> On 6/23/22 15:48, Chris R wrote:
> > I took my main scene and eliminated everything but the transparent box, and even
> > added media to it and everything looked fine.  I added one scene element at a
> > time back and it was all fine until I added in the drip of water I had modeled
> > falling from the downspout.  As soon as I added that object back into the scene,
> > the transparent box with media became a reflective mirror instead!
> >
> > The drip of water is modeled as a quartic.  It had a material to look like
> > water, (so transparent, ior 1.33, reflective, etc.).  When I changed the
> > material to just a simple gray pigment, the media box was no longer visible or
> > reflective, but the media also disappeared.
>
> Unsure...
>
> What version of POV-Ray is being used?
>

I am using the latest 3.8 beta.

> Are the shapes involved part of the same union or merge?

My scene, as is my usual practice, is a single union of all of the individual
objects in the scene.

>
> The only obvious thing I can think of would be if the water material was
> defined by name and happened to have the same name as the one used for
> the media box - and the water material definition was seen last.
>

The material for the media box was defined inline with the box declaration, so
there could be no name collision.

> If you can get a simpler scene showing the issues I'll add it to my
> collection for debug, though no promises as to when I'd dig. I've been
> busy with other than POV-Ray of late.
>

I will play around with it and see if I can find something simple and
repeatable.  I had the scene down to just a few elements, but there are a lot of
my own include files involved, so it will take a bit to try and reproduce it.

> --- Deeper and less likely possibility...
>
> With some older shape types - due ill advised automatic polynomial order
> reduction code combined with the lack of tight automatic / manual
> bounding for some shapes - we sometimes end up with an additional root
> (and so another surface) being introduced(a) by the auto-order reduction
> itself.
>
> Maybe something like this is going on but, that surface by order
> reduction would have to have a very particular position to see the
> results you posted.
>
> A test for this fail would be to change your quartic to a simple sphere
> or similar and see if the apparent reflection propagation to the box
> problem persists?
>
> (a) - One of the shipped scenes long has had a vertical line in the
> result due the auto-order reduction changing two roots (4th order eqn)
> to three roots (third order eqn) for one or more of the scene's shapes.
> I cannot remember which it is for sure - primitiv.pov maybe? The povr
> branch fixed this particular exposure(b) by ripping out the automatic
> order reduction code.
>
> (b) - Order reduction introduces slight root/surface shifts in the
> intended roots even when it mostly works in a consistent way. The
> reduction code also sometimes flips 'sturm on' to fixed solvers, without
> notice due the order reduction, on a per ray->surface equation basis.
> The sturm vs fixed solver results for identical equations are often not
> themselves identical and this occasionally introduce at same surface,
> subtle, consistency issues - which are sometimes visible. And there's
> more 'solver noise' possible due the order reduction I won't detail -
> partly because I no longer remember all the possible implications!
>
> Bill P.

Thanks, if I can find some time to reproduce the problem with a simple example
I'll post it.  Would you prefer it in bug reports, sources, or here in the
images newsgroup?


-- Chris R.


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