And lo on 7 Jul 2004 10:57:30 -0400, Warp <war### [at] tagpovrayorg> did spake,  
saying:

> Phil Cook <phi### [at] nospamdeckingdealscouk> wrote:
>> >   Media does not need light sources to show up.
>
>> So what does it use to sample along if their are no light_source's?
>
>   I didn't say light sources do not affect media. I just said light
> sources are not needed for media to show up (that is, they are not
> the only thing that affect them).

Might be bad phrasing on my part I'm not arguing with you I'm am genuinely  
trying to understand this, the ambient light_source has no point source  
how does Povray calculate the angle of the light ray from the surface in  
order to sample along it? lets put it another way. I shine a parallel  
light into a scattering media box that has a hole cut in it, the result is  
a 'light beam' through the media, I now substitute the parallel  
light_source for an ambient 1 plane will this still produce the 'light  
beam' with radiosity enabled. Hmm just tried it,seems to be a no.

>   In fact, light sources do not have any effect whatsoever on emitting
> media.

Yup

> In absorptive media light sources only affect the shadow of
> the media.

Yup

> Scattering media fully takes into account light sources,
> but can still be seen even if there are none.

Ah here we are, what are they seen with? All I can assume is that with  
radiosity a ray is being produced as a reflection from the surface from  
the ambient light_source to any surrounding surfaces and that the media  
affects this or uses this to sample along. I saw this when I altered a  
scattering media in front of an ambient 1 object and saw the apparent  
brightness drop, however the media was not illuminated around the object.  
In the code previously the media was red scattering with an white ambient,  
if the radiosity was being affected by the media why did the object  
continue to appear white when it should have been red?

>   If you are lighting your scene using emitting media and radiosity,
> light sources will not affect the brightness of the media.

I'm not sure what you mean by the brightness of the media, the only way I  
can take this is that the media affects the ambient/emission but the media  
*itself* is not affected which is why it does not show up. Ah I sort of  
see, perhaps, if you have a media with a density/colour map then an  
ambient object will shine through those areas that are 'clear' but be  
blocked by the 'dense' parts and show up as black rather than as a colour.  
Have I got that right?

--
Phil Cook

-- 
All thoughts and comments are my own unless otherwise stated and I am  
happy to be proven wrong.

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