I sometimes use transmitting surfaces for special effects.
For example you can make a surface which inverts colors this way:

pigment {color rgb 0.5 transmit -1}
finish {ambient 1 diffuse 0}

By looking at the output from POV-Ray I have figured out that a the color
you see when looking at a surface is calculated this way (or at least I
think so):

 transmit_value * color_behind_the_surface
+ ( 1 - transmit_value ) * surface_color
= result

So if the color behind an inverting surface is <0.2,0.5,0.7>, then this
would be the result:

-1 * <0.2,0.5,0.7> + ( 1 - (-1) ) * <0.5,0.5,0.5>
= <-0.2,-0.5,-0.7> + <1.0,1.0,1.0>
= <0.8,0.5,0.3>

The color behind the surface has been inverted by the surface.

The reason I wrote this message is that it seems that it doesn't always work
this way.

I think that when the transmit value is above 1, the color of the surface is
ignored. I don't think the surface color should be ignored. It may not be
intuitive to have the surface color multiplied with negative values, but
then, users who use transmit values above 1 shouldn't expect intuitive
results.

I wanted to make a surface which makes the contrast bigger:

pigment {color rgb 0.5 transmit 2}
finish {ambient 1 diffuse 0}

I had expected that it would make color vales below 0.5 lower and color
values above 0.5 higher, but it didn't work that way, and I think it is
because the surface color is ignored when the transmit value is above 1.

What do you think of this?

Can and should we correct it?

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In article <39118632@news.povray.org> , "Rune" <run### [at] inamecom> 
wrote:

> I sometimes use transmitting surfaces for special effects.
> For example you can make a surface which inverts colors this way:

Well, it is more of a documentation problem:

"Each of the five components of a color are float values which are normally
in the range between 0.0 and 1.0. However any values, even negatives may be
used."

it should probably read:

"Each of the five components of a color are float values which are normally
in the range between 0.0 and 1.0. However any values, even negatives may be
used but proper operation can only be expected for values in the range
between 0.0 and 1.0. All other values do not model physical effects and
results may vary."

This is simply because what is a surface that inverts light? It does not
exist, there isn't a surface that increases the strength of light while it
is passing through it.
So there is no universal rule for such cases, and it really depends on the
implementation.  Most renderers will simply not allow you to specify such
values - POV-Ray does to give you greater artistic freedom.


     Thorsten

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"Thorsten Froehlich" wrote:
> This is simply because what is a
> surface that inverts light?

It is not realistic, but I was able to mathematically predict the result,
and it worked.

> It does not exist, there isn't a
> surface that increases the strength
> of light while it is passing through
> it. So there is no universal rule for
> such cases, and it really depends on
> the implementation.

When the transmit value is below 1 (even when it is below 0) POV-Ray uses a
certain formula to calculate which color is seen. All I'm asking is that
that same formula is used when the transmit value is above 1. That way,
although the result wouldn't be realistic, you should still be able to
mathematically predict the result.

To those who are able to look in the source code of POV-Ray:
Does it make sense what I'm saying?

Greetings,

Rune

---
Updated April 25: http://rsj.mobilixnet.dk
Containing 3D images, stereograms, tutorials,
The POV Desktop Theme, 350+ raytracing jokes,
miscellaneous other things, and a lot of fun!

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Thorsten Froehlich wrote:
> 
> In article <39118632@news.povray.org> , "Rune" <run### [at] inamecom>
> wrote:
> 
> > I sometimes use transmitting surfaces for special effects.
> > For example you can make a surface which inverts colors this way:

> 
> This is simply because what is a surface that inverts light? It does not
> exist, there isn't a surface that increases the strength of light while it
> is passing through it.

Ok, I'm nit-picking here, and it's not really a surface but a media, but there
is such material that increase the strength of light while it is passing 
through it (and/or even change it radically).

That's what the first laser were made with: a cristal was excited by a strong
flash lamp and a resonating cavity made of a 100% reflective mirror and 
a 99.99 % one was used to amplify a spontaneous photon emission.

There is also this strange medical laser media: you have a classical laser
beam as input via optical fiber and the output is also a laser beam, but
the frequency is different (halfed or doubled, I do not remember, but
the effect is that on one side you have a red beam and on the other side a 
blue beam (because the blue is 0.35 whereas red is 0.7 in the relevant unit :-)

There is also some protective material against infra-red laser beam used
in labs and industrial environment, which, when hit by the beam double
the light frequency, turning the invisible beam into an orange spot.
 (the material may be destroyed by high energy laser and need replacement, 
but at least you and your eyes were a minimum protected from the loose beam).

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J. Grimbert <jgr### [at] atos-groupcom> wrote:
: Ok, I'm nit-picking here, and it's not really a surface but a media, but there
: is such material that increase the strength of light while it is passing 
: through it (and/or even change it radically).

  Changing the frequency of the light is not the same thing as changing it's
strength (ie. amplitude).

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Warp wrote:
> 
> J. Grimbert <jgr### [at] atos-groupcom> wrote:
> : Ok, I'm nit-picking here, and it's not really a surface but a media, but there
> : is such material that increase the strength of light while it is passing
> : through it (and/or even change it radically).
> 
>   Changing the frequency of the light is not the same thing as changing it's
> strength (ie. amplitude).
> 

And what is "optical pumping" in classical laser if it is not
increasing the number of photons (--> strength) ?

I may have misleaded you with the example of frequency change, but the
initial question was more generic than just increase of strength.
 (He gave an example of reverting the color...)

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On 10 May 2000 10:09:54 -0400, Warp <war### [at] tagpovrayorg> wrote:

>  Changing the frequency of the light is not the same thing as changing it's
>strength (ie. amplitude).

J. Grimbert gave examples of two totally different concepts. In the
*first* example, he described Light Amplification by Stimulated
Emission of Radiation, the classic laser concept. This certainly *is*
an example of changing amplitude.

What I'd like for someone to invent are light-intensifying window
panes. You could look outside your home or car at night, and be able
to see as well as in full daylight - and in full color as well. Before
that happens, reverse-engineered UFO technology will probably become
open-source.    :)

Later,
Glen Berry

( Remove the "7" from 7no### [at] ezwvcom to email me. )

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