> HEllo group, and pov ray gurus i am currently stugggling at making a realistic
> smoke, but whenever i' scale it up, smoke appears completely black and white and
> let's no light throught it, i played with scaling options of emissions but my
> objective it to provide some real transluscent smoke like in realist chimney
> that pump quite low emissions that when we've seen in the 19 century industrial
> chimenys !
>
> here is the code that is not working when scaling thanhs to lowmuller pages !
> when i cale it to about 150 all i get is completly black and white smoke that
> not let light through... it now that emission directive shoud de scalled down
> (divided by the bontainer scale, but no way)
>

The aspect of media depends heavily on the lenght of the ray as it 
travel through it. Scaling up means that the travel lenght increases by 
the scaling amount. To compensate, you must reduce the density by the 
same value.

If you scale the container up by x150, then you must divide the density 
of the media by the same amount:
media{emission 0.6/150...
scattering{1, 1*3/150...
Altrernatively, you can divide the values in the color_map by that value 
instead of dividing the main density of the media:
     [0.00 rgb 0]
     [0.05 rgb 0]
     [0.20 rgb 0.2/150]
     [0.30 rgb 0.6/150]
     [0.40 rgb 1/150]
     [1.00 rgb 1/150]


Normaly, extinction should remain at 1, it's default value.
Also, normaly, smoke don't emit any light unless it's overheated to some 
ridiculous temperature. I'd replace the emission by some actual fading 
light_source near the top of the chemeny if you realy need some glow up 
there.

Some complementary notes about media sampling:

The default method is method 3

samples only take a single integer value. You can add a second value, 
but it will be ignored. method 1 and 2 DO use 2 values.

confidence is not needed and not used. Only usefull when using method 1 
ou 2.

You use samples 1. It will take only one sample ether as the ray enter 
the container or at the middle point. Both will give you the wrong 
result. The minimum value for samples should be 3 and defaults to 10.

NEVER EVER use intervals! It defaults to 1 and should be left at that 
value. It must only be used with method 1 or method 2.
"samples 30" is faster than "samples 5 intervals 3" for twice the total 
samples count. I know, I've done some testing and found that using ONLY 
samples you may have about 10 times as many samples as you get using 
samples and intervals for about the same render time.
A media with samples 300 rendered in about the same time as the same 
media with samples 5 intervals 5
intervals can also cause artefacts to appear.



Alain

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