Thomas de Groot <tho### [at] degrootorg> wrote:
> Op 12/12/2019 om 15:25 schreef Bruno Cabasson:
> > "And" <49341109@ntnu.edu.tw> wrote:
> >> "Bruno Cabasson" <bru### [at] cabassoncom> wrote:
> >>>
> >>> Besides, real atmosphere is a very complex thing, not easy to model, and
> >>> real clouds are also quite complex and there are many kinds of them, each one
having a specific behaviour with li
gh
> > t.
> >>
> >>
> >> In fact, for a fully automatic volumn(cloud) renderer, it is not so different
> >> between different kind of cloud. The main difference of them is the height, and
> >> the shape(density map), and the average density(because a dense cloud needs more
> >> multiple scattering)
> >>
> >> ice droplets and liquid water droplets have different phase function when
> >> scatters light. but for a dense media(if light from sun light source collide
> >> many times in the cloud) the difference of the final appearance is small.
> >> And because water droplets almost never absorb light, represent the albedo of
> >> cloud almost 1.0( between visible wavelength range), so it is seldom eliminated
> >> in cloud, a light beam can collide hundred of times in a heavy cloud. You can
> >> see this:
> >>
http://news.povray.org/povray.binaries.utilities/message/%3Cweb.5bec15ae87bb2a51c8edf6b30%40news.povray.org%3E/#%3
Cwe
> > b.
> >> 5bec15ae87bb2a51c8edf6b30%40news.povray.org%3E
> >>
> >> This is the power(sp-radiance) at different level scattering output quantity.
> >> Maybe you can say the most of it still concentrated at first level. But this is
> >> an image contains a white cloud on the center, and the blue sky the other. It is
> >>   the average sp-radiance on the whole image. If look at the cloud separately,
> >> the multiple scattering part is the dominate.
> >>
> >> And I attach a comparison. The image contains three images I rendered one year
> >> ago but should be the same (or similar) condition. They are 1 level, 3 level,
> >> and 27 level scattering renders.
> >> (up to down.)
> >>
> >>
> >>
> >>
> >>
> >>
> >> "Bruno Cabasson" <bru### [at] cabassoncom> wrote:
> >>

> >>> the rest is the same values, except the sun power and the parmeters for the fog
> >>> media. Colors are obtained automatically thanks to media, mainly the atmosphere
> >>> with rayleigh scattering.
> >>>
> >>> Regards
> >>
> >> When I watch this I guess you use scattering "extinction" not 1, because if you
> >> use correct extinction 1 and lacks of multiple scattering, your clouds
> >> is impossible so white. finally you use 0.25. When I use POV-Ray rendering
> >> cloud, I always find I need use filled lights or additional emission.
> >
> > Well... The fact is that I used exinction 0.25 for the cloud media for the
> > renders I posted in this thread. But a value of 1 is supposed to the the only
> > one realistic. I'll play with this, but I think it will not be dramatic.
> >
> > AKAIK, POV-Ray allows you to define several media within the same container
> > (they add together), and several density functions for each of them (they
> > multiply together). Therefore, you can complexify and enrich the model that way,
> > at the expense of render time (and carbon footprint). I'll also play with this.
> >
> > interior
> > {
> >      // Media #1
> >      media
> >      {
> >          <media1 parameters>
> >          density
> >          {
> >              <media1/density1 function>
> >          }
> >          density
> >          {
> >              <media1/density2 function>
> >          }
> >
> >          .../...
> >
> >          // Densities are multipled together for the current media
> >      }
> >
> >      // Media # 2
> >      media
> >      {
> >          <media2 parameters>
> >          density
> >          {
> >              <media2/density1 function>
> >          }
> >          density
> >          {
> >              <media2/density2 function>
> >          }
> >
> >          .../...
> >
> >          // Densities are multipled together for the current media
> >      }
> >
> >      // More media
> >
> >      .../...
> >
> >      // Media contributions are added together
> >
> > }
> >
> > B.
> >
> >
>
> Something that I do fairly often is to use separate, "layered", media
> containers, separated by a minimal amount of space. This can be useful
> e.g. in the case of different cloud layers. This way it is easy to
> create (1) a water media, over which there is (2) an atmospheric media
> (fog) with (3 etc) cloud layers above. The fact that the containers do
> not overlap makes the render faster and avoids mutual media "contamination".
>
> --
> Thomas

Thats'a good thing, to my opinion. I don't really unerstand what you mean by
media "contamination". Do you mean that there are some issues if the containers
overlap? For my cloudscape experimental scenes, containers do overlap :
atmosphere container includes all other layers. BUT, and I agree with you, I
make these layers not overlap by means of appropriate values for heights and
thicknesses.

B.

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