Is BSSRDF method implemented in POV-Ray? If not,
is it possible to simulate on POV-Ray BSSRDF? Are
similar methods implemented?

In particular should be useful for me to apply
this method for marble translucency...

Best regards.

Antonio Ferrari

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Antonio Ferrari wrote:
> Is BSSRDF method implemented in POV-Ray? If not,
> is it possible to simulate on POV-Ray BSSRDF? Are
> similar methods implemented?

You can do subsurface scattering in POV-Ray using media.  In p.b.i. past 
postings you will find quite a lot of postings showing examples for this 
(just like other faked scattering techniques).

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

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In article <ch4150$2ij$1@chho.imagico.de>, chr### [at] gmxde says...
> Antonio Ferrari wrote:
> > Is BSSRDF method implemented in POV-Ray? If not,
> > is it possible to simulate on POV-Ray BSSRDF? Are
> > similar methods implemented?
> 
> You can do subsurface scattering in POV-Ray using media.  In p.b.i. past 
> postings you will find quite a lot of postings showing examples for this 
> (just like other faked scattering techniques).
> 
Not to complain, but media is confusing and complicated, while a 
subsurface scattering algorithm would just do what it was designed for 
and would thus likely be less complicated to use. Also, while I have 
heard lots of claims about how it could be done with media, I am not sure 
there is any examples of it being used with organics or other similar 
situations where the effect needs to be fairly depth limited and 
specific, but the shape of the object doesn't lend itself well to just 
stuffing it full of media and hoping it doesn't have any leaks. Something 
less container specific would be better for such models, if for no other 
reason than to save time in designing and debugging. I am not sure you 
can really call it a useless or redundant feature for these reasons, but 
that is just my opinion.

-- 
void main () {

    call functional_code()
  else
    call crash_windows();
}

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Patrick Elliott <sha### [at] hotmailcom> wrote:

> Not to complain, but media is confusing and complicated, while a
> subsurface scattering algorithm would just do what it was designed for
> and would thus likely be less complicated to use. Also, while I have
> heard lots of claims about how it could be done with media, I am not sure
> there is any examples of it being used with organics or other similar
> situations where the effect needs to be fairly depth limited and
> specific, but the shape of the object doesn't lend itself well to just
> stuffing it full of media and hoping it doesn't have any leaks. Something
> less container specific would be better for such models, if for no other
> reason than to save time in designing and debugging. I am not sure you
> can really call it a useless or redundant feature for these reasons, but
> that is just my opinion.

I've tried different examples with different configurations, made by me and
also taken from the past messages.

I can say the following:

- the results that can be reached seems very poor, and in any case are only
a rough approximation of BSSRDF real results;

- results can be easily approximated for simple / not_to_much_complicated
objects, but for complicated objects it seems difficult to attach a proper
media (for example using a density based on extrusion with negative offset
of object itslef that must produce sub-surface scattering... this only an
idea, but I don't know if it's appliable);

- a simpler interior description specific for BSSRDF, or similar effects /
algorithms is desiderable.

For these reasons I agree with Patrick. But these are only my opinions...
Maybe I've simply been unable to find an efficent and perfect way to
implement BSSRDF through povray.

Antonio Ferrari

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Antonio Ferrari wrote:
> [...]
> 
> - the results that can be reached seems very poor, and in any case are only
> a rough approximation of BSSRDF real results;

To be frank this argument shown quite well the misconception you have 
about this effect.  POV-Ray scattering media is much more 'real' than 
any commonly used subsurface scattering trick you find advertised in 
other programs.  The point is of course these tricks can be much faster 
than a full media simulation but the above sentence as you wrote it is 
simply an inversion of reality.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

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In article <chsckk$r7i$1@chho.imagico.de>, chr### [at] gmxde says...
> Antonio Ferrari wrote:
> > [...]
> > 
> > - the results that can be reached seems very poor, and in any case are only
> > a rough approximation of BSSRDF real results;
> 
> To be frank this argument shown quite well the misconception you have 
> about this effect.  POV-Ray scattering media is much more 'real' than 
> any commonly used subsurface scattering trick you find advertised in 
> other programs.  The point is of course these tricks can be much faster 
> than a full media simulation but the above sentence as you wrote it is 
> simply an inversion of reality.
> 
Speaking of reality.. There is 'the right way' and there is the 
'practical way'. Sometimes it simply isn't practical to do things the 
'right' way. A good example would be using an organic model design by 
someone else. There is no 'practical' way to make sure that the model is 
solid, especially if you used it for animation, where each frame runs the 
risk of added unexpected holes in the object. Media cannot function with 
such uncertainty to produce subsurface scattering. Not without applying 
an insane amount of impractical work into testing the media on every 
frame, then tweaking each frame of that model to 'fix' the problem. There 
may even be circumstances where you 'intend' to produce cutaways or other 
effects that may make the use of media nearly impossible for that 
application. You would want sss on the parts that are not cut away, but 
media and sss absent from the part you cut into. You can't do that in any 
practical sense with media. I don't think that for some specific cases 
'fast' with respect to the algorithm is the point, so much as that using 
media in some circumstances is simply far more complicated and time 
consuming to use than stimulating the effect.

This may be a hobbyist program, but some people would like to use it for 
more practical applications. That isn't feasible when you adhere strictly 
to 'do it the correct way or not at all' ideas. For most uses of media 
that philosophy is perfectly reasonable, for this specific use.... it is 
questionable imho. For it to be practical requires a dependence on 
everyone else making a product to 'get it right' as well and they have no 
incentive to cater to POV-Ray's limitations, even if we see it the other 
way around. I think it would make a simpler and easier option for some 
applications. Those that want to do it 'right' can still do so and good 
for them. Anyone else can save time and aggravation by doing it the other 
way.

-- 
void main () {

    call functional_code()
  else
    call crash_windows();
}

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Patrick Elliott wrote:
> 
> Speaking of reality.. There is 'the right way' and there is the 
> 'practical way'. Sometimes it simply isn't practical to do things the 
> 'right' way. A good example would be using an organic model design by 
> someone else. There is no 'practical' way to make sure that the model is 
> solid, especially if you used it for animation, where each frame runs the 
> risk of added unexpected holes in the object. Media cannot function with 
> such uncertainty to produce subsurface scattering. [...]

What makes you think that a dedicated subsurface scattering feature 
(which in any case is an approximation of scattering media simulation) 
would lead to good results where media does not?

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

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In article <chvhs4$vp1$1@chho.imagico.de>, chr### [at] gmxde says...
> Patrick Elliott wrote:
> > 
> > Speaking of reality.. There is 'the right way' and there is the 
> > 'practical way'. Sometimes it simply isn't practical to do things the 
> > 'right' way. A good example would be using an organic model design by 
> > someone else. There is no 'practical' way to make sure that the model is 
> > solid, especially if you used it for animation, where each frame runs the 
> > risk of added unexpected holes in the object. Media cannot function with 
> > such uncertainty to produce subsurface scattering. [...]
> 
> What makes you think that a dedicated subsurface scattering feature 
> (which in any case is an approximation of scattering media simulation) 
> would lead to good results where media does not?
> 
I am just looking at it logically. Since media must be completely 
contained to prevent it from spilling into parts of the scene that it 
does not belong in, the probability of a flaw completely screwing it up 
in complex meshes is quite high. There is also the issue where you want 
the scattering to take into account a second 'surface', such as a 
simulation of blood vessels, etc. under skin. This could be accomplished 
by placing a texture or map at an arbitrary 'depth' where the scattering 
is supposed to happen using BSSRDF, or you have to apply it as a texture 
to a smaller version of the same mesh, inside the original. The problem 
is that there 'may' be cases where simply scaling the model by some small 
factor to produce such a layer wouldn't work. The scaling might cause 
unintended overlaps or other complications. Yes, if everything works 100% 
perfectly, then media is just as good for both cases, but life and CG is 
never perfect. In the cases of something like skin, the scattering needed 
is a lot easier to fake with certainty that it should work, than try to 
implement using media, which can go wrong if the mesh has defects. It is 
as simple as that.

I am not saying it doesn't work, just that for some applications it seems 
like it would prove more complicated and failure prone, which makes it a 
bit less practical to try to get working correctly.

-- 
void main () {

    call functional_code()
  else
    call crash_windows();
}

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Unfortunately I haven't got the book "Realistic Image Synthesis Using Photon
Mapping" of Henrik Wann Jensen... Something must be written in chapter 10
or 11... Have anyone read it?

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