Hi,

when I put a camera in the focal point of a prabolic mirror it looks like there
is a singularity. But this constilation is exactly what I need/want. Is there a
posibillity to change the way of calculation from povray or something else what
I can do?

thx,
  Tim

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Am 18.01.2013 13:08, schrieb timer:

> when I put a camera in the focal point of a prabolic mirror it looks like there
> is a singularity. But this constilation is exactly what I need/want. Is there a
> posibillity to change the way of calculation from povray or something else what
> I can do?

Well, what do you expect?

At the focal point of a parabolic mirror, by its very nature, it doesn't 
matter where you look - you'll always see whatever is placed right in 
the optical axis of the mirror.

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clipka <ano### [at] anonymousorg> wrote:
>
> Well, what do you expect?
>
> At the focal point of a parabolic mirror, by its very nature, it doesn't
> matter where you look - you'll always see whatever is placed right in
> the optical axis of the mirror.

That would be awesome because this is what I want. But unfortunately it's get
black when I move the camera in the focal point. A little but over and under
that focal point it looked nice, but this is, for me, the wrong view point.

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Am 18.01.2013 15:24, schrieb timer:
> clipka <ano### [at] anonymousorg> wrote:
>>
>> Well, what do you expect?
>>
>> At the focal point of a parabolic mirror, by its very nature, it doesn't
>> matter where you look - you'll always see whatever is placed right in
>> the optical axis of the mirror.
>
> That would be awesome because this is what I want. But unfortunately it's get
> black when I move the camera in the focal point. A little but over and under
> that focal point it looked nice, but this is, for me, the wrong view point.

That's surprising to me. Can you give a sample scene?

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clipka <ano### [at] anonymousorg> wrote:
>
> That's surprising to me. Can you give a sample scene?


During generating a sample scene I realize that this work. So I worked reverse
to localize my real problem.

It look like a combination of two paraobolic mirrors generate this singularity.

##### SAMPLE CODE #####

global_settings {
        max_trace_level 5
}

camera {
//      location <300,250,-50>  // sideview
        location <0,100,-1>     // worked
//      location <0,100,0>      // doesn't worked
//      location <0,100,1>      // worked
        look_at <0,100,20>
}

light_source {
        <0,2000,0>
        color rgb <1,1,1>
}

plane {
        y, 0
        pigment {
                checker color rgb<0,0,0> color rgb<1,1,1>
        }
        scale <100,100,100>
}
plane {
        y, 10000
        pigment { color rgb<0,0,1> }
        finish {
                ambient 0.6
        }
}

// Parabel 1
intersection {
        quadric {
                <-5/132,-5/132,0>
                <0,0,0>
                <0,0,-1>
                33/5
        }
        box {
                <-4,-4,4>
                <4,4,8>
        }
        scale <10,10,10>
        finish {
                metallic
                reflection 1
        }
        translate <0,100,0>
}

// Parabel 2
quadric {
        <5/26,5/26,0>
        <0,0,0>
        <0,0,1>
        7/10
        scale <10,10,10>
        finish {
                metallic
                reflection 1
        }
        translate <0,100,0>
}

##### END SAMPLE CODE #####

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Am 18.01.2013 19:49, schrieb timer:
> clipka <ano### [at] anonymousorg> wrote:
>>
>> That's surprising to me. Can you give a sample scene?
>
>
> During generating a sample scene I realize that this work. So I worked reverse
> to localize my real problem.
>
> It look like a combination of two paraobolic mirrors generate this singularity.

It appears to me that the quadric solver goes berserk on the second 
parabola whenever a ray is exactly parallel to the parabola's axis 
(which happens when the camera is exactly in the first quadric's focal 
point, but I suspect it would also happen with an orthographic camera).

At present my only idea would be to try modeling the second parabola 
with some other primitive type (isosurface for instance) - and to file a 
bug report on http://bugs.povray.org.

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Am 18.01.2013 20:48, schrieb clipka:

> It appears to me that the quadric solver goes berserk on the second
> parabola whenever a ray is exactly parallel to the parabola's axis

Turns out that this is not /precisely/ true. Actually the problem occurs 
when the ray is /almost/ parallel to the parabola's axis (which happens 
because when reflected earlier at the other parabola there are ever so 
slight rounding errors). In such a case the algorithm gags on precision 
issues.

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Le 18/01/2013 21:26, clipka nous fit lire :
> Am 18.01.2013 20:48, schrieb clipka:
> 
>> It appears to me that the quadric solver goes berserk on the second
>> parabola whenever a ray is exactly parallel to the parabola's axis
> 
> Turns out that this is not /precisely/ true. Actually the problem occurs
> when the ray is /almost/ parallel to the parabola's axis (which happens
> because when reflected earlier at the other parabola there are ever so
> slight rounding errors). In such a case the algorithm gags on precision
> issues.
> 

would sturm help ?
(quadric does not seem to be documented to like sturm, but a change of
syntax for a poly ? hmm not sure)

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Am 18.01.2013 22:10, schrieb Le_Forgeron:
> Le 18/01/2013 21:26, clipka nous fit lire :
>> Am 18.01.2013 20:48, schrieb clipka:
>>
>>> It appears to me that the quadric solver goes berserk on the second
>>> parabola whenever a ray is exactly parallel to the parabola's axis
>>
>> Turns out that this is not /precisely/ true. Actually the problem occurs
>> when the ray is /almost/ parallel to the parabola's axis (which happens
>> because when reflected earlier at the other parabola there are ever so
>> slight rounding errors). In such a case the algorithm gags on precision
>> issues.
>>
>
> would sturm help ?
> (quadric does not seem to be documented to like sturm, but a change of
> syntax for a poly ? hmm not sure)

Quadrics use their own specialized root solving code.

But the code can apparently be fixed by replacing a "(a != 0.0)" test 
with "(fabs(a) >= EPSILON)".

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Now I made the second parabolic with poly and it worked very well. With and
without sturm.

Thank you very much.

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