Jan Dvorak <jan### [at] centrumcz> wrote:
> >   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.

  No, it doesn't. Just take any material you want and compare its
illumination on the side facing the light and on the opposite side.
The illumination is totally different. With double_illuminate the
illumination on both sides will be *identical*. This corresponds to
nothing in nature.

  The only reason why double_illuminate exists in povray is because it's
so laughably easy to implement (it's a 1-liner: 1 single 'if' statement;
or more precisely, one additional condition in an existing 'if' statement),
and it can be used in certain situations to eg. alleviate some rendering
artifacts, as well as other purposes. It's not even intended to be a
physically accurate feature.

> A good example is a paper 

  Emulating the illumination on the backside of a paper in a physically
accurate way would be quite laborious and slow.

  double_illuminate might be a somewhat acceptable approximation in some
situations, assuming you are not seeing both sides at the same time (in
which case it would be very unrealistic).

-- 
                                                          - Warp

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Warp napsal(a):
> Jan Dvorak <jan### [at] centrumcz> wrote:
>>>   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.
> 
>   No, it doesn't. Just take any material you want and compare its
> illumination on the side facing the light and on the opposite side.
> The illumination is totally different. With double_illuminate the
> illumination on both sides will be *identical*. This corresponds to
> nothing in nature.
> 
>   The only reason why double_illuminate exists in povray is because it's
> so laughably easy to implement (it's a 1-liner: 1 single 'if' statement;
> or more precisely, one additional condition in an existing 'if' statement),
> and it can be used in certain situations to eg. alleviate some rendering
> artifacts, as well as other purposes. It's not even intended to be a
> physically accurate feature.
> 
>> A good example is a paper 
> 
>   Emulating the illumination on the backside of a paper in a physically
> accurate way would be quite laborious and slow.
> 
>   double_illuminate might be a somewhat acceptable approximation in some
> situations, assuming you are not seeing both sides at the same time (in
> which case it would be very unrealistic).
> 
Read the "However, I do see a bit of unrealism" part.

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