"[GDS|Entropy]" <gds### [at] hotmailcom> wrote:
> My understanding of renderman compliant renderers was that you can program
> shaders for nearly any behavior, is this incorrect?

Renderman is a biased renderer, meaning it is not a good way to achieve the
physical accuracy clipka seeks.  It may technically be possible to implement
it, but it's certainly not a good idea.

http://www.cgafaq.info/wiki/Bias_in_rendering

 - Ricky

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I gotcha.
Thanks for the info. ;-D

ian


"triple_r" <nomail@nomail> wrote in message 
news:web.49af4ce980e98aa063a1b7c30@news.povray.org...
> "[GDS|Entropy]" <gds### [at] hotmailcom> wrote:
>> My understanding of renderman compliant renderers was that you can 
>> program
>> shaders for nearly any behavior, is this incorrect?
>
> Renderman is a biased renderer, meaning it is not a good way to achieve 
> the
> physical accuracy clipka seeks.  It may technically be possible to 
> implement
> it, but it's certainly not a good idea.
>
> http://www.cgafaq.info/wiki/Bias_in_rendering
>
> - Ricky
>
>
>
>
>

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"clipka" <nomail@nomail> wrote:
> I'm looking for a (free) renderer that does realistic subsurface scattering,
> using physical material parameters as input (i.e. absorption and scattering
> coefficients) - or a way to do it in POV. Rendering times don't matter, even if
> it takes days to render a simple wax sphere on a checkered plane. Main
> prerequisite is high physical accuracy.
>
> I had hoped for a combination of scattering and absorbing media in POV to do the
> job, but it doesn't seem to work with real physical material properties - unless
> someone happens to know a reliable way to convert those properties to POV media
> parameters.
>
> The parameters I have available - aside from index of refraction - are
> "absorption coefficients" and "reduced scattering coefficients" for red, green
> and blue light respectively (computed from measurements under the simplifying
> assumption that the "phase function" is isotropic).
>
> Any ideas?

Clipka a Sam Benge process may be useful?:
http://news.povray.org/489cc386%40news.povray.org

Clipka, sorry if I did not fully understand your question. :-)

(
POV info:
For those who do not know, I remember that Mr. Cason is working on:
http://news.povray.org/49688faa%40news.povray.org
)

--
Carlo

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"[GDS|Entropy]" <gds### [at] hotmailcom> wrote:
> http://aqsis.org/ Renderman Compliant, so you should be able to make about
> whatever you want.
> http://www.indigorenderer.com/joomla/  Indigo rocks too.
>
http://www.indigorenderer.com/joomla/component/option,com_gallery2/Itemid,26/?g2_itemId=6441

Hum... looks like they both require a non-trivial workflow to get anything done
with them...

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"nemesis" <nam### [at] gmailcom> wrote:
> It uses a predefined material with a few parameters that don't seem to bear much
> resemblance to any accurate physical properties.
>
> Aqsis, being a Renderman system, should be even farther away from physical
> correctness than unbiased renderers like Lux and Indigo.

Thanks for the info! That about wraps it up for those renderers, I guess.

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"[GDS|Entropy]" <gds### [at] hotmailcom> wrote:
> My understanding of renderman compliant renderers was that you can program
> shaders for nearly any behavior, is this incorrect?

I don't want to *program* those shaders, I need them ready-to-use. Otherwise I'd
need a reference renderer to check that my code is actually doing what it is
supposed to do... (which, as a matter of fact, is why I'm searching for such a
renderer in the first place)

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"Carlo C." <nomail@nomail> wrote:
> Clipka a Sam Benge process may be useful?:
> http://news.povray.org/489cc386%40news.povray.org

Unfortunately not, as it is basically just a toying-around that happens to
produces a very intriguing subsurface-scattering-like effect, but is quite far
from the actual real-life physics.

> (
> POV info:
> For those who do not know, I remember that Mr. Cason is working on:
> http://news.povray.org/49688faa%40news.povray.org
> )

Yes, I know of that approach. As a matter of fact, if it was finished already,
it would be *the* perfect fit for what I'm looking for: Virtually no learning
curve (I know POV already), strictly scientific model to approximate the
real-world phenomenon (so can be expected to yield very realistic results), and
governed by parameters that by some strange coincidence happen to be exactly
those I have available here.

Unfortunately, I'll probably need a solution *before* this thing will reach any
suitable stage. Work is proceeding (though someone else took over from Chris),
but is currently stalled by problems still poorly understood. Somehow the
fitting of the math to the algorithmic part seems to be still wrong, producing
much too dark results.

Guess what - the guy currently working on it is desparately searching for a
software to produce reference shots, to get a clearer picture of what is going
wrong... >_<

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"clipka" <nomail@nomail> wrote:
> Guess what - the guy currently working on it is desparately searching for a
> software to produce reference shots, to get a clearer picture of what is going
> wrong... >_<

Are there some more scientific tests in the literature?  In the classic paper*,
they talk about and reference some more precise measurements of the phenomena.
I'd hate to go to all the trouble of setting up a test case in another program
and finding out it's not possible to make it so precise.  This probably
wouldn't be the case, but it seems I remember some funny business with lighting
intensities in Kerkythea...

 - Ricky


* http://graphics.stanford.edu/papers/bssrdf/

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Somewhat related to the Pov part of your question :
http://news.povray.org/povray.binaries.images/thread/%3C3B97ABD4.5E70568D%40engineer.com%3E/

Thibaut



> I'm looking for a (free) renderer that does realistic subsurface scattering,
> using physical material parameters as input (i.e. absorption and scattering
> coefficients) - or a way to do it in POV. Rendering times don't matter, even if
> it takes days to render a simple wax sphere on a checkered plane. Main
> prerequisite is high physical accuracy.
> 
> I had hoped for a combination of scattering and absorbing media in POV to do the
> job, but it doesn't seem to work with real physical material properties - unless
> someone happens to know a reliable way to convert those properties to POV media
> parameters.
> 
> The parameters I have available - aside from index of refraction - are
> "absorption coefficients" and "reduced scattering coefficients" for red, green
> and blue light respectively (computed from measurements under the simplifying
> assumption that the "phase function" is isotropic).
> 
> Any ideas?
> 
> 
>

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I was going through my povray output directory a few days ago and found a 
half-rendered image of this scene. Was actually wondering who made the scene 
and here you answered that question for me. Not suprised it was Kari.

Mike

"Thibaut Jonckheere" <tua### [at] MAPSONyahoofr> wrote in message 
news:49b4f265@news.povray.org...
>
> Somewhat related to the Pov part of your question :
>
http://news.povray.org/povray.binaries.images/thread/%3C3B97ABD4.5E70568D%40engineer.com%3E/
>
> Thibaut
>
>

>> I'm looking for a (free) renderer that does realistic subsurface 
>> scattering,
>> using physical material parameters as input (i.e. absorption and 
>> scattering
>> coefficients) - or a way to do it in POV. Rendering times don't matter, 
>> even if
>> it takes days to render a simple wax sphere on a checkered plane. Main
>> prerequisite is high physical accuracy.
>>
>> I had hoped for a combination of scattering and absorbing media in POV to 
>> do the
>> job, but it doesn't seem to work with real physical material properties - 
>> unless
>> someone happens to know a reliable way to convert those properties to POV 
>> media
>> parameters.
>>
>> The parameters I have available - aside from index of refraction - are
>> "absorption coefficients" and "reduced scattering coefficients" for red, 
>> green
>> and blue light respectively (computed from measurements under the 
>> simplifying
>> assumption that the "phase function" is isotropic).
>>
>> Any ideas?
>>
>>

Post a reply to this message