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2 Aug 2021 19:06:17 EDT (-0400)
  Possible bug with blobs (Message 1 to 10 of 10)  
From: Hugo
Subject: Possible bug with blobs
Date: 12 Jan 2002 13:05:07
Message: <3c407ad3$1@news.povray.org>
Hello,

I hope you don't mind another blob question. I need to ask, before I can say
it's a bug.

I can provide you with a scene file that shows two spheres of the same size,
one of them is a blob. On the left side of them, I have traced a lot of
small blobs randomly connected with each other, to form a layer of paint or
snow. Here the problem arises: If I add a transmit or filter value to the
snow, there will be a lot of odd shadows (they are actually holes) on the
blob sphere, but not on the other sphere, or on other kind of objects. It
only happens on the blob sphere, although it's an independant object from
the snow.

I have cut down the scene file to a minimum, and posted it on p.b.s-f.. It
is a problem with all POV versions. Please let me know what you think.

Regards,
Hugo Asm.


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From: Thorsten Froehlich
Subject: Re: Possible bug with blobs
Date: 12 Jan 2002 13:31:46
Message: <3c408112@news.povray.org>
In article <3c407ad3$1@news.povray.org> , "Hugo" <hua### [at] post3teledk> wrote:

> (they are actually holes)

You mean block dots?  Diod you try to include max_trace_level?

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

Visit POV-Ray on the web: http://mac.povray.org


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From: Christopher James Huff
Subject: Re: Possible bug with blobs
Date: 12 Jan 2002 14:07:39
Message: <chrishuff-4D3C1B.14082612012002@netplex.aussie.org>
I haven't had a chance to look at the code, but this sounds like an 
accuracy problem...
Did you try using "sturm" for either the blob object or the snow?

-- 
 -- 
Christopher James Huff <chr### [at] maccom>


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From: Hugo
Subject: Re: Possible bug with blobs
Date: 12 Jan 2002 14:14:27
Message: <3c408b13@news.povray.org>
> You mean block dots?  Diod you try to include max_trace_level?

I tried to increase max_trace_level and max_intersections, but it doesn't
change the look, even with extremely high values. What's really strange is
that the problem only appears on the blob and not on other objects.

The holes are fairly sized, and located in the shadow of the snow. I found
out they are actually holes, because I used no_shadow and could see the
inside luminate.

Thanks for your time!
Hugo


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From: Hugo
Subject: Re: Possible bug with blobs
Date: 12 Jan 2002 14:25:40
Message: <3c408db4$1@news.povray.org>
> I haven't had a chance to look at the code, but this sounds like an
> accuracy problem...
> Did you try using "sturm" for either the blob object or the snow?

I didn't try until now actually. But it doesn't change the problem.

An accuracy problem?  Hmm.. Caused by what?  If it's the snow, why would it
affect the blob sphere (an independant object) but not other kind of
objects? If it's the blob sphere (a single sphere object), why would the
holes appear in the shadow of the snow?

Thanks for your input!
Hugo


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From: Mike Williams
Subject: Re: Possible bug with blobs
Date: 12 Jan 2002 14:42:25
Message: <4S+5rCAwEJQ8Ewfx@econym.demon.co.uk>
Wasn't it Hugo who wrote:
>Hello,
>
>I hope you don't mind another blob question. I need to ask, before I can say
>it's a bug.
>
>I can provide you with a scene file that shows two spheres of the same size,
>one of them is a blob. On the left side of them, I have traced a lot of
>small blobs randomly connected with each other, to form a layer of paint or
>snow. Here the problem arises: If I add a transmit or filter value to the
>snow, there will be a lot of odd shadows (they are actually holes) on the
>blob sphere, but not on the other sphere, or on other kind of objects. It
>only happens on the blob sphere, although it's an independant object from
>the snow.

I confirm that there is a problem. 

It happens in POV 3.1g, POV 3.5 beta 9 and MegaPOV.

Here's a much simpler scene with larger holes. All there is here is a
spherical blob and a transparent sphere. A hole appears in the blob when
the surface of the transparent sphere is close to the surface of the
blob, and between the camera and the blob. I've set a ridiculously high
max_trace_level just to show that it's nothing to do with a shortage of
that.


global_settings {
  max_trace_level 50
  assumed_gamma 1
  }

background {rgb z*0.3}

camera { location <0,0,-3> look_at 0 }
light_source {<-100,200,-300>, 1 }

//The Blob
blob { threshold 0.1 sphere { <0,.02,0>,1.2 ,1 } 
        pigment { rgb x }
        //interior_texture{pigment {rgb y}}
}

//The invisible sphere
sphere {0,1 pigment {rgbt <1,1,1,1>}}


-- 
Mike Williams
Gentleman of Leisure


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From: Mike Williams
Subject: Re: Possible bug with blobs
Date: 13 Jan 2002 03:59:06
Message: <SPdjcCA8uUQ8EwKX@econym.demon.co.uk>
Wasn't it Myself who wrote:

> A hole appears in the blob when the surface of the transparent sphere
> is close to the surface of the blob, and between the camera and the 
> blob.

I did a bit more thinking about the conditions under which the effect
appears, and came up with the idea that perhaps the algorithm that finds
where a ray intersects the blob surface is failing when the ray starts
very close to the surface. [The logic being that the first traced ray
stops at the invisible surface, then a second ray might start from that
invisible surface going in the same direction.]

This hypothesis leads to the prediction that it might be possible to
observe the same effect by reflecting the rays off a mirror that's very
close to the surface. In this case a ray restarts at the surface of the
mirror, and the hypothesis suggests that there would be a region where
the ray would sneak through the front surface and hit the interior.

This does, in fact, turn out to be the case. The interior texture of
this red blob is bright green, and a sliver of green can be seen where
the blob touches the mirror.

global_settings {
  max_trace_level 50
  assumed_gamma 1
  }

background {rgb z*0.3}

camera { location <0,1.5,-3> look_at <0,-0.6,0> angle 40 }
light_source {<-100,200,-300>, 1 }

blob { threshold 0.1 sphere { <0,0,0>,1.2 ,1 } 
        pigment { rgb x }
        interior_texture{pigment {rgb y*4}}
}

plane {y,-0.5
 finish {reflection 1}
 pigment {rgb 0}
}


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From: Micha Riser
Subject: Re: Possible bug with blobs
Date: 13 Jan 2002 14:51:49
Message: <3c41e555@news.povray.org>
This is the same problem as I reported in my posting 
http://news.povray.org/povray.beta-test/18087/ to p.b-t. It is still on the 
know bug listing.

Your scene works without problems with my modified megapov-version.

- Micha


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From: Hugo
Subject: Re: Possible bug with blobs
Date: 13 Jan 2002 19:19:59
Message: <3c42242f$1@news.povray.org>
> Your scene works without problems with my modified megapov-version.
>
> - Micha
>

I'm glad to hear. I'm also glad that Mike took his time to analyse this
problem. I didn't recognize the similarity between the known bugreports and
the bug I discovered.

This probably means it's not necessary to "bug" the people in the beta-test
group.. I will leave the subject here unless there is more to add. Thanks
again for your help!


Hugo Asm.


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From: Tom York
Subject: Re: Possible bug with blobs
Date: 4 Feb 2005 07:20:00
Message: <web.42036782e8c0faad2ff34a90@news.povray.org>
"Hugo" <hua### [at] post3teledk> wrote:
> > Your scene works without problems with my modified megapov-version.
> >
> > - Micha
> >
>
> I'm glad to hear. I'm also glad that Mike took his time to analyse this
> problem. I didn't recognize the similarity between the known bugreports and
> the bug I discovered.
>
> This probably means it's not necessary to "bug" the people in the beta-test
> group.. I will leave the subject here unless there is more to add. Thanks
> again for your help!
>
>
> Hugo Asm.

I seem to have run into this problem recently. I'm using POVRay for Windows
version 3.6.1.icl8.win32. I was going to post a small demonstration scene,
but then I found this thread. Is there any sort of simple workaround for
this behaviour?

Thanks,

Tom York


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