"clipka" <nomail@nomail> wrote:
> "MessyBlob" <nomail@nomail> wrote:
> > 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.
> [...] 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.
> Try the "subsurface.pov" sample scene. [...]

Yes, some of those subtleties had escaped me at first. I guess you've just
written a part of the new manual in this thread, so now I don't feel quite so
guilty asking the questions. :o)

I like the effect: I can 'look into' the surface, and 'see the illuminated
volume', and where the light comes from. In fact, the RNG patterns are helping
me to see which effects are attributable to SSLT. The patterns look like a
consequence of choosing sampling points, intervals and directions; I'm only
guessing there, but it seems that when sufficient randomness is implemented,
there can be a sliding scale of 'quick-and-dirty' to 'smooth-and-accurate'.

I'm seeing some (RNG) SMP blocking artefacts on the effect, most noticeable
around the specular highlight on the candle, and near the horizon line on the
(reflected) sphere when looking into the floor. I'm sure shared cacheing would
help with this. Would memory be a problem? I imagine SSLT cacheing would need
more memory than radiosity for applicable surfaces - process limits are already
a problem for most (W32) users on simple radiosity scenes.

Standards-based parameters are good, in that parameters are not arbitrary, so
people are less likely to be adversely surprised when the algorithm improves:
given that you're already most of the way there, any change would not be very
significant.

So I have a question on the back of this. I could answer it myself by testing,
but I expect other folk would like to know the answer too: Is this presently
calculated against light sources, or from the light calculated at the surface?
I'm asking because the former wouldn't work with radiosity scenes (having no
light sources), and the latter would.

Caustics/photons would look especially good and be efficient, provided the
algorithm is adaptive. Would it the light have a 'momentum vector' into the
subsurface, to give a volumetric diffusion, e.g. for a laser-thin shaft of
light would the sub-surface continue to show the shaft to some extent, or would
it just be a surface glow observed in the locality of the sub-surface? (I expect
the latter, given the isotropy assumptions, and that the unscattered light is
omitted).

IOR seems to be well-handled (re. exit angle from the sub-surface of the floor),
but I haven't thought about that in any detail.

All round, I think a good decision to include it, and a brave decision to try
it! It's unfortunate that you'll perhaps need to justify the decision (and I
hope this thread has helped). I'm sure many enthusiasts will love to play with
this 'new toy' to produce demo scenes that have an extra edge to their reality:
cue the skin models, the milk glasses, the acrylic dice, the cloudy glass
sculpture, the marble chess set and anything else that has ever been carved
from marble ... They will all help make your case for SSLT in POV-Ray.

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