"MessyBlob" <nomail@nomail> wrote:
> SSLT: A handy and compact SDL feature!
>
> ....but does it bring any functionality beyond using media? On initial trials,
> I'm thinking media scattering is more versatile (but no good with ambient
> lighting with radiosity scenes), and can do quite good milk, wax, moulded
> plastics, semi-opaque glass, particulate/impure water, atmosphere, etc.
>
> I couldn't find rationale for SSLT, so can I ask (of the dev team) what it was?
> (speed, accuracy, simplicity?)

All of these, basically.

To begin with, media was designed with low to moderately scattering / absorbing
/ emitting substance in mind; the algorithm is a fair and efficient
approximation of reality only under the assumption that scattering happens so
infrequently that double (or triple or multiple) scattering can be neglected.

The SSLT approach used by POV is designed for just the opposite, with highly
scattering substances in mind; the algorithm is a fair approximation of reality
only under the assumption that scattering happens so frequently that unscattered
light can be neglected, and the scattering function can be modeled as isotropic.

So what can SSLT do that media cannot?

Try the "subsurface.pov" sample scene. Look at the shadow cast onto the
checkered plane by the candle and chrome sphere: Do you see the light
"bleeding" into the shadow cast on the bright slab?

This is not area lights. This is not light filtering through the candle. This is
light transported below the slab's surface - genuine Subsurface Light Transport.

*This* is what SSLT can do that media cannot model.


A few other factors that might be worth mentioning:


Speed:

I never tried modelling a pint of milk using media; however, I fancy that it
takes an awful lot longer to render than the same done with SSLT.

Morover, the SSLT implementation in POV is just at the very beginning; there's a
whole lot of opportunities for optimization waiting to be exploited. Right now,
no caching is done for SSLT whatsoever.


Accuracy:

As mentioned, there are aspects of SSLT that media just can't capture. I also
expect that even those aspects that can be approximated with media are still
somewhat inaccurate.

As of now, SSLT's accuracy still suffers from RNG patterns, but these will
certainly be eliminated in future versions.


Simplicity:

Setting up media for subsurface scattering approximation is quite a hassle: It
doesn't work with the physical material coefficients, and tweaking is
non-trivial.

As of now, tweaking SSLT materials is just as non-trivial, or maybe even worse;
however, it *can* operate on physical material coefficients (like those
presented in the Jensen et al paper); plus, mathematically sound work has been
done (again by Jensen et al - though this time some other al - in a follow-up)
to re-parameterize the algorithm in a different, fairly intuitive manner (I
recently posted a macro that implements this approach).


Flexibility:

You mentioned radiosity. As a matter of fact, there is a clear path of how to
make the SSLT algorithm radiosity-aware, and it has been high on the agenda
even before I got my fingers dirty with the code. Once it's accomplished, this
will be another "can't do with media".

Similarly, making the algorithm photon-aware should be not a too big deal
either.


So to sum it all up, I guess these are reason enough to add SSLT.

Post a reply to this message