Am 27.02.2012 19:13, schrieb Stephen:
> With the changes to the SSLT code in RC4 there is now only one
> controlling vector, translucency, instead of two, scattering and
> absorption. What if any is the relationship between translucency,
> scattering and absorption?
> Scattering and absorption made sense to me and the reference pdf gave
> values for some materials. So how do we go about using translucency?

There were various major drawbacks with the parameterization based on 
scattering and absorption.

First, while they allowed to plug in physical properties of a material 
directly, sources for such data is scarce (that one pdf is the only 
source I'm currently aware of), and tweaking those properties to mimic 
any other desired material - even just a similar material with different 
color, or a material with same color but stronger or weaker SSLT effect 
- is non-trivial to say the least.

Second, those two parameters fully determined the material's color, 
leaving the classic pigment parameter with no effect; this resulted in 
more maintenance effort if you wanted test renders without SSLT to use 
similar colors.

And last not least, with those parameters it was a major challenge - if 
not impossible - to make use of classic image textures (like, say, Poser 
skin textures) together with SSLT.


The new parameterization makes it straightforward to achieve a 
particular effective color by simply specifying that color as a pigment; 
this leaves you with only one RGB parameter to toy around if you want to 
mimic a particular material, automatically keeps SSLT and non-SSLT color 
of a material in sync, and enables you to change the material color - or 
apply standard image textures - in a pinch.

The new parameterization also imposes no added limitations: Any material 
you could model with the old parameterization can also be modeled with 
the new one.


The translucency parameter specifies the so-called "mean free distance" 
of a light ray in the material (in mm), i.e. the average distance a 
light ray travels before it is either absorbed or scattered. This can be 
more than 1.0 mm in case of very "waxy" materials, and could 
theoretically be as large as infinity (for a perfectly transparent 
material).

I've only recently heard of Poser figures coming with translucency maps, 
which obviously need to specify values in the range 0..1; it would be 
interesting to know what parameterization is used there, so we could 
adapt POV-Ray to easily make use of them as well; the SSLT syntax is 
therefore still in flow.

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