Le Thu, 05 Jun 2008 02:00:52 +0200, Alain <ele### [at] netscapenet> a  


> Bruno Cabasson nous illumina en ce 2008-06-03 09:50 -->
>> Hi there!
>>  Still working on atmospheres and cloudscapes for TerraPOV, I discoverd  
>> recently
>> something that puzzles me unexpectedly. Two media-filled objects that I  
>> was
>> expecting to render the same do not. Run the following fragment of  
>> code. You
>> can notice that the spheres on the right (UnionedSpheres) are  
>> significantly
>> brighter than those on the left (DistinctSpheres).
>>  In UnionedSpheres, there is a single interior with two  
>> spherical-shaped medias.
>> In DistinctSpheres, each sphere has its own interior media.
>>  I had in mind that the two definitions were equivalent.
>>  I cannot figure out where the problem is. Could it be related to the  
>> container
>> artifact?
>>       Bruno
>>
> Even with the density of the bottom sphere set to zero, the blue media  
> still looks dencer.
> The two media are supposed to add together, but a value of zero in one  
> media should leave the other media unchanged.
>
> Not related to your problem.
> This line cause your rendering speed to drop significantly:
>
>>             intervals 3 samples 3 method 3
> This rewriten line give you more samples AND is faster:
> samples 12
>
> You don't need to specify method 3, it's the default.
> With this method, you should always use only one intervals (the new  
> default) and increase the number of samples as needed, and never set it  
> to less than 3.
>
>

Hello Alain.

Concerning the values for the media, I am aware of your advises, thanks  
anyway for enlighting.

The fact is that, with the study on clouds I am currently conducting, I  
never got acceptable results with only one interval, especially for rays  
that approach the horizon, and more especially when the sun is at low  
elevation. This is due to the occupation of the 'high' density media  
within the container (a spherical shell), and also due to the turbulent  
nature of the media for clouds. The integration must not miss the  
turbulenced media, otherwise you get awful splotchy results and nothing  
that resembles a cloud. So I always use such values as a reflex, as I am  
focused on the subject since quite a while. Articles coming soon, I hope.

In this regard, 3 intervals and 3 samples yield better results that 1  
interval and 9 or 12 samples. It is slower, but one conclusion of my study  
is that you can't expect realistic wolumetric media clouds and a quick  
render. Some of my experiments require several hours on an overclocked  
Q6600.

However, for spheres, default values are OK and I could have used them for  
my example. Sorry if it was confusing.

And, when coding, I generally prefer be explicit rather than use implicit  
default values. Depends on cases, and mood ...

Trying to understand what happens with this media artifact, origin of the  
thread, using several medias, I ended up with the code that renders  
something like a nebula that I quoted in my reply to Kenneth earlier in  
the thread (see  
http://news.povray.org/*/message/%3Cweb.4846bf2b9a2aa192e8ba46670%40news.povray.org%3E/#%3Cweb.4846bf2b9a2aa192e8ba46670%40news.povray.org%3E).
 
I am thankful to him that he got interested and is willing to dig.

So far, I tend to believe that the problem is related to containers,  
because it seems it is the only difference I see between the two cases.

For my amusement, I improved the nebula a little in order to have a more  
artistic result. The media is highly turbulenced, and I had to specify 10  
intervals (you read well...), and 3 samples, leaving the adaptive method 3  
do the job within the intervals. Render time 51 minutes on a Q6600@3.2 GHz.


     Bruno.

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