Warp napsal(a):
> Fa3ien <fab### [at] yourshoesskynetbe> wrote:
>> If being able to take double_illuminate into account in radiosity (it
>> seems that it makes sense, anyway) helps getting more realistic
>> pictures, it should be envisioned.
> 
>   double_illuminate is not realistic. It doesn't correspond to any
> physical phenomenon.
> 
It does correspond to any surface that is thin enough to be translucent 
but thick enough not to be transparent. A good example is a paper 
(although the light path translute-diffuse reflect-translute is much 
weaker than the direct reflection the double_illumination effect is very 
strong if the paper is backlit).
Try taking two pieces of paper, the bottom one having been written on. 
You only see the top one. Then backlight them - you can see clearly what 
is written on the bottom side, which means paper is either a bit 
transparent or a bit double_illuminated. Now lift the top one. You will 
see the bottom paper's projection blur srongly, which means that paper 
is translucent, not transparent. And that's what double_illuminate is for.
However, I do see a bit of unrealism:
- It's an object property. It should belong to a texture.
- It should be possible to regulare the amount of double_illumination. 
Right now it is 1:1.
   My idea is a back_diffuse keyword in the finish statement. 
[diffuse]*[direct lighting*]+[back_diffuse]*[backlight*]=total 
illumination. back_diffuse defaults to diffuse if double_illuminate is 
set and 0 if unset. It causes a warning if both appear.
*including radiosity
- Povray should consider every texture layer, not just the top one.
-- Also I think that the default inside texture shoud be the outside one 
*in the reverse order* (imagine gluing a few stickers onto a glass. 
Which one do you see from the back side, the top one?). Or even forbid 
interior_texture completely (it is more realistic but it reduces 
artistic possibilities). But that's another chapter.

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