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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'
![neggam.jpg](/povray.binaries.images/attachment/%3Cslrn90u8im.1km.steve%40zero-pps.localdomain%3E/neggam.jpg?preview=1)
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From: Simon Lemieux
Subject: Re: Minus Values in Assumed_Gamma = Strange :)
Date: 13 Nov 2000 00:39:03
Message: <3A0F8CB2.3C82D2B@yahoo.com>
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> 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! ;)
--
+-------------------------+----------------------------------+
| Simon Lemieux | Website : http://www.666Mhz.net |
| Email : Sin### [at] 666Mhz net | 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
--
+-------------------------+----------------------------------+
| Simon Lemieux | Website : http://www.666Mhz.net |
| Email : Sin### [at] 666Mhz net | 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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From: Mark Wagner
Subject: Re: Minus Values in Assumed_Gamma = Strange :)
Date: 13 Nov 2000 02:39:53
Message: <3a0f9ac9@news.povray.org>
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Chris Huff wrote in message ...
>In article <3a0efbd8@news.povray.org>, "Jan Walzer" <jan### [at] lzer net>
>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] gte net> 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] mac com, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tag povray org, http://tag.povray.org/
<><
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In article <3A0### [at] yahoo com>, lem### [at] yahoo com 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] mac com, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tag povray org, 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] 666Mhz net | POV-Ray, OpenGL, C++ and more... |
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From: GrimDude
Subject: Re: Minus Values in Assumed_Gamma = Strange :)
Date: 14 Nov 2000 17:52:24
Message: <3a11c228@news.povray.org>
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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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