Hi,

To do an experiment, I want to render a reflective parabolic surface (a
newtonian reflective lens, for the astronomy buffs). My problem is that
the surface stops being reflective if I make it too "flat". I have
renedered a sample here, where the object in the centre is supposed to be
a parabolic mirror pointing at the camera. You can view the image and the
source at:

  http://ketil.froyn.name/lens.tga
  http://ketil.froyn.name/lens.pov

The odd thing (to me, anyway) is that if I change the y-parameter from
f_paraboloid(x/2.1,y/0.5,z/2.1,1)
to
f_paraboloid(x/2.1,y/0.7,z/2.1,1)
then the lens looks fine. 

Any hints on how to avoid the brown bands are appreciated, I've done some
experimenting with different parameters (as I'm sure you can see from the
messy .pov file) but haven't understood why they are there yet.

If I can get this solved, I'll be interested in finding out how I can
calculate exactly the depth of a parabolic surface. As an example, if I
would like to render a parabolic surface that is 200mm wide and 0.5mm deep
(let's say my scale is in millimeters), what would my f_paraboloid look
like?

Thanks.

Ketil Froyn
ket### [at] froynname
http://ketil.froyn.name/

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"Ketil Froyn" <ket### [at] froynname> wrote in message
news:Pin### [at] lexxinfelineorg...
>
> the surface stops being reflective if I make it too "flat". I have
> renedered a sample here, where the object in the centre is supposed to be
> a parabolic mirror pointing at the camera. You can view the image and the
> source at:
>
>   http://ketil.froyn.name/lens.tga
>   http://ketil.froyn.name/lens.pov   (I had to prefix with "www" for
these)
>
> Any hints on how to avoid the brown bands are appreciated

It's the max_gradient being a little too low. Suggested number was above 2
when I rendered the scene and you had it at only 1.5. It turned out okay
then.

> If I can get this solved, I'll be interested in finding out how I can
> calculate exactly the depth of a parabolic surface. As an example, if I
> would like to render a parabolic surface that is 200mm wide and 0.5mm deep
> (let's say my scale is in millimeters), what would my f_paraboloid look
> like?

Egads... I have no idea, sorry.

-- 
Bob H.
http://www.3digitaleyes.com

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In article <Pin### [at] lexxinfelineorg>,
 Ketil Froyn <ket### [at] froynname> wrote:

> To do an experiment, I want to render a reflective parabolic surface (a
> newtonian reflective lens, for the astronomy buffs). My problem is that
> the surface stops being reflective if I make it too "flat". I have
> renedered a sample here, where the object in the centre is supposed to be
> a parabolic mirror pointing at the camera. You can view the image and the
> source at:

Probably a precision issue. Try using smaller values for accuracy and/or 
increasing max_gradient. Also, you might want to look at using a 
polynomial shape for the parabola rather than an isosurface...it will 
likely give more accurate results, possibly faster as well. Look in the 
shapes_old.inc include file, specifically for Paraboloid_X and its 
relatives.

BTW, you have an interesting name...I guess I'm not the only one who 
thinks so, seeing that it's the first item on your web site. I guess you 
don't have much trouble getting mixed up with other people...I sat next 
to another Chris in Spanish class, and the teacher loved to turn in our 
general direction and just say "Chris! You're next!".

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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On Mon, 23 Feb 2004 23:04:03 -0500, Christopher James Huff
<cja### [at] earthlinknet> wrote:

>BTW, you have an interesting name...I guess I'm not the only one who 
>thinks so, seeing that it's the first item on your web site. I guess you 
>don't have much trouble getting mixed up with other people...I sat next 
>to another Chris in Spanish class, and the teacher loved to turn in our 
>general direction and just say "Chris! You're next!".

You should have seen our class in high-school... there were 7 guys
named Nikolai, two of them having the same family name and on top of
that their surnames had the same initials. Not to mention that the
stem of the surname (i.e. his father's first name) of one of them was
actually a nickname/diminutive for a very common Bulgarian name which
happened to be the name of the other guy's father :)

Something like "John Richard Doe" sitting next "John Richie Doe" :)

One of our English teachers found out a solution for it - he just
named them Nick 1st to 7th and the Nick-names (sorry, couldn't resist
:) ) just stuck.


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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Among other things, Peter Popov wrote:

> You should have seen our class in high-school... there were 7 guys
> named Nikolai, two of them having the same family name and on top of
> that their surnames had the same initials. Not to mention that the
> stem of the surname (i.e. his father's first name) of one of them was
> actually a nickname/diminutive for a very common Bulgarian name which
> happened to be the name of the other guy's father :)
> 
> Something like "John Richard Doe" sitting next "John Richie Doe" :)

In my school class there were two guys with the *same* name. They both have 
a compound name, the same (it's a common compound name, however). In Spain 
we have two surnames, one comes from our father, the other from our mother. 
They both have the same surnames (the first one is not that common). They 
used to have some arguments about who was 2nd and who was 3rd in the class 
list (alphabetically). They reached a consensus: one of them would be 2nd 
for a year, the other one would be 2nd next year... Both wore glasses 
too... (and they are not relatives).

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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