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 ligh
> 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/#%3Cwe
> 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

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