On Sun, 12 Nov 2000 21:14:20 +0100, Jan Walzer wrote:

>So this is what I originaly ment ... If you use the
>gamma-correction(with negativ values) on an antialiased image ,
>you don't have any longer the effect of antialiasing. The GC
>could produce again (unexpected) stairsteps.
>
>But: AFAIK Gamma is only defined for values > 0 ...
>
>So does it make sense to ask if the current model is correct ???
>Of course, we often have other functions in POV, working with
>unrealistic values (negative pigments or lights ...[New question:
>what is with negative IOR ???]), but they are or could be quite
>predictable, but what do you expect from a negative gamma value
>??? How would define the result ???
>
>BTW: has someone here have the code for the gamma function ???

Don't think I understand most of your questions but this should
answer some.  The attached image was gamma corrected in a graphics
package, to -1.6, and I get exactly the same results as using
negative gamma in pov.

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Attachments:
Download 'neggam.jpg' (29 KB)

Preview of image 'neggam.jpg'

> Negative ior may not be unrealistic. The ior of a bubble of air in water
> is lower than the ior of water, it just makes the light bend the other
> way. An ior of 1 usually means no effect, because POV sets the ior of
> "space" to 1. It is the change in ior that causes light to bend...

If I can just correct this... space is void and there is no matter, so there is
no IOR, but there is in the air.. the air's IOR is ~1 and so the light passing
from air to space will be refracted with an indice of 1, 0 means no refraction
(ie: passing from space to space), common glass is at 2.2 i think...  And again,
look at the glass on earth, it looks nice, look at it in space and it looks
different, because the IOR passes from 1 to 2.2 and in space from 0 to 2.2...

A negative IOR should simply result in the opposite effect, thus making a
magnifying-glass(?) into a fisheye-lens and making a fisheye-lens into a
magnifier...

Something pretty cool about IOR is when it's animated, ie relative to the clock,
that gives a very nice effect!

Just my two cents! ;) 

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| Simon Lemieux           | Website : http://www.666Mhz.net  |
| Email : Sin### [at] 666Mhznet | POV-Ray, OpenGL, C++ and more... |
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> Don't think I understand most of your questions but this should
> answer some.  The attached image was gamma corrected in a graphics
> package, to -1.6, and I get exactly the same results as using
> negative gamma in pov.

I don't think I understand the term of "gamma" but I guess this is much like a
inversion of the colors, the image look like a negative (negative? hehe)

Try to open the image with a normal gamma with a photo editor (gimp, photoshop)
and invert the colors and then compare with the negative gamma rendered one...

Simon

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| Email : Sin### [at] 666Mhznet | POV-Ray, OpenGL, C++ and more... |
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Jan Walzer wrote:
> 
> I assume it won't work with AA ...

For the record this was rendered with an AA of .01 so I would
say your assumptions are correct.

-- 
Ken Tyler - 1400+ POV-Ray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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Simon Lemieux wrote:
>
> If I can just correct this... space is void and there is no matter, so there is
> no IOR, but there is in the air.. the air's IOR is ~1 and so the light passing
> from air to space will be refracted with an indice of 1, 0 means no refraction
> (ie: passing from space to space), common glass is at 2.2 i think...  And again,
> look at the glass on earth, it looks nice, look at it in space and it looks
> different, because the IOR passes from 1 to 2.2 and in space from 0 to 2.2...

Back to shool for you! Vacuum ior=1.0.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/refrn/u14l1d.html

______________________________________________________________________
Kari Kivisalo                                  http://www.kivisalo.net

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Chris Huff wrote in message ...
>In article <3a0efbd8@news.povray.org>, "Jan Walzer" <jan### [at] lzernet>
>wrote:
>> [New question:
>> what is with negative IOR
>
>Negative ior may not be unrealistic. The ior of a bubble of air in water
>is lower than the ior of water, it just makes the light bend the other
>way. An ior of 1 usually means no effect, because POV sets the ior of
>"space" to 1. It is the change in ior that causes light to bend...


Wrong.  IOR is defined as the *ratio* of the speed of light in a given
material to a reference material (usually, the reference material is a hard
vacuum).  Consequently, the IOR of a bubble of air in water is a value less
than 1 but greater than 0, if water is given an IOR of 1.  If an object has
a negative IOR, it implies that light entering the object will have a
negative velocity -- it will enter the object, stop, turn around, and head
back the other direction!

--
Mark

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In article <3a0f9ac9@news.povray.org>, "Mark Wagner" 
<mar### [at] gtenet> wrote:

> Wrong.  IOR is defined as the *ratio* of the speed of light in a 
> given material to a reference material (usually, the reference 
> material is a hard vacuum).

Ok, thanks for the info...I wasn't certain about there being a lower 
limit to ior. I dimly remember reading about the exact meaning of ior, 
but not reliably enough to claim only positive values are allowed.


> If an object has a negative IOR, it implies that light entering the 
> object will have a negative velocity -- it will enter the object, 
> stop, turn around, and head back the other direction!

Which would probably look like a reflection in most cases...possibly a 
useful effect.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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In article <3A0### [at] yahoocom>, lem### [at] yahoocom wrote:

> If I can just correct this... space is void and there is no matter, 
> so there is no IOR, but there is in the air..

I wasn't talking about outer space(which actually has an ior of 1 as I 
recall), I was talking about the "environment" in POV-Ray, which is set 
to an ior of 1.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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> Back to shool for you! Vacuum ior=1.0.
> http://www.glenbrook.k12.il.us/gbssci/phys/Class/refrn/u14l1d.html

Uh... true!  My mistake, I actually had in mind the difference between air and
vacuum which is  near 0 and so I assumed vacuum's ior was 0, sorry!  Also for
the crown glass at 1.54 ( i actually thought it was about 2.2...)...  But I
learned this 2 years ago in a general physic course...

Thanks for the tips!

-- 
+-------------------------+----------------------------------+
| Simon Lemieux           | Website : http://www.666Mhz.net  |
| Email : Sin### [at] 666Mhznet | POV-Ray, OpenGL, C++ and more... |
+-------------------------+----------------------------------+

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I saw something today that reminded me of your image here. X-Ray images of
distant galaxies.

Grim

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