Alain <aze### [at] qwertyorg> wrote:

> > Alain<aze### [at] qwertyorg>  wrote:

> >>> 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
> >
> > Hello Alain, thanks for the reply, it solved most of my issues. there is only
> > little problem left, when i scaale up, the bounding sphere appears in a light
> > shadowy grey even if a specify correct transmit value, i do not know if it is
> > linked to the fog, sky sphere above plan cloud etc...
> > if you could shed some light on the issue a would appreciate
> >
> >
>
> The use of fog is the most porbable cause of what you describe.
>
> With fog, you start evaluating the fog between the camera and the
> container sphere, this gives you a value for the fog effect. You then
> evaluate the fog inside the hollow sphere giving you another value. And
> finaly, you evaluate the fog for the rest of the ray after the sphere up
> untill you hit something else or the background, giving you a third
> value. All those values are added toggether and don't summ up to the
> same as the fog alone when you never encounter the container. This
> especialy true when you reatch the background or a background feature as
> a sky_sphere.
>
> The cure is usualy to use actual media instead of fog... With the
> obvious effect on the render time.
>
>
> Alain

Hi again! i appreciate very much your help.
the culprit in fact is a sky sphere to render a dawn sky.
i tried to replace it with a media, but the smoke container is still visible,
if you like i can send you my scene (or your friends) (it is quite nice) and
maybe you could see what could be done, sorry to bother you but pov ray has
quite a difficult learning curve :)
my mail is rodv92 at the gmail.com provider.

Rodrigo,

ps: are you french ?

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