Le 17-09-02 à 17:46, Kenneth a écrit :
> "Kenneth" <kdw### [at] gmailcom> wrote:
>> "omniverse" <omn### [at] charternet> wrote:
>>>
>>> Where I get most confused is that factoring in of background colors, which I
>>> think always remain additive (emitting) or subtractive (absorbing)
>>> regardless of the media itself.
>>
>> I think that's true, when using a SINGLE media. But when two types of media are
>> used (like emission + absorption), it gets a bit trickier-- and seems to depend
>> on their own respective colors.
> 
> Actually, I'm starting to come around to your idea ;-)-- that at least emission
> and absorption media effects depend (solely??) on what the background colors
> are, for their final 'filtered' media-color. The use of a pure-color
> media-- with one or more zeros in the color vector-- seems to confirm this. And,
> that using multiple medias (well, emission + absorption) of COMPLEMENTARY pure
> colors serves to filter *all* of the background color to some extent-- because
> there are no longer any zeros in the 'combined' color-filtering vector-- with
> the result *looking like* actual opacity.
> 
> This is a paradigm shift for me: I used to think that volumetric media was a
> 'thing unto itself', more or less, with its effects only modifying the colors
> of objects WITHIN the media object-- and having nothing at all to do with
> filtering the background and *its* colors. I guess I never really noticed the
> background-color effects, because I've only lately tried creating a PURE-color
> media (where there's a zero in one or more of the components, showing the
> obvious filtering that's going on-- and showing NO so-called 'opacity' for those
> colors.)  My previous uses of a single media never had a zero in the color
> vector-- so I took the resulting 'all-color filtering' to mean 'opacity.'
> 
> This is my latest theory, anyway ;-)
> 
> HOWEVER... Scattering media might be a different animal (or not?) The current
> way that I think about scattering (and its 'extinction' value) is that it's
> basically emission and absorption media combined (while also showing effects
> from lighting, of course.) That's probably a too-simplistic description, but it
> will do for now.
> 
> But PURE-color scattering used as a SINGLE media also shows
> the background filtering (no 'opacity' or filtering for certain colors) , even
> with a very high extinction value. For example,
>          scattering{1, <1,0,0> extinction 300}
> Using this in your laser code, it completely extinguishes the red background
> hexagons (i.e., makes them black)-- but leaves the green and blue hexagons
> unaltered.  So extinction 300 is actually   300*<1,0,0>   in this case (or can
> be thought of that way) -- the SAME color as the media color itself-- but
> 'extinguishes' that red color. So scattering media-- when used with extinction--
> is a 'complementary-color' filter for the background, and for the impinging
> light source. (Interestingly, scattering with extinction 0-- and no light
> source--shows NO media effect at all, as if the media wasn't there.)

Something is fishy here. Extinction is supposed to anly affect the 
absorbtion of incoming light and the shadowing effect. In a way, 
extinction 0 should be similar to no_shadow.
Maybe the media is intercepting your light.

> 
> Currently, scattering's extinction allows just a single float value. I have a
> dim and fuzzy memory, from v3.6xx days, that extinction could actually take a
> color vector (but I might be confusing that with an added absorption media.) It
> would be a nice feature addition to allow a color there-- so that a
> 'complementary' color could be used for the color extinction. For example,
>          scattering{1, <.2,1,.2>, extinction 1}

In version 2.5 and 3.6, that's exactly how it worked. It still work that 
way now.

> produces a green-ish cloud-- but the 'complementary' color is filtered out of
> the incoming light, resulting in purple self-shadowing. With extinction
> <.8,0,.8>, the self-shadowing color would match the main green media, and the
> cloud would look nice and green throughout. That's not physically accurate, of
> course, but it would be more visually appealing ;-)

For that effect, you need to use extinction 0 for the scattering media 
and absorbing media of the complementary colour to get colour matching 
shadow.

> 
> 

Emissive and absorbing medias are not filtering.
Emissive media ADD to whatever is behind. In a radiosity scene, with 
media on, it also illuminate it's surounding. They can't be seen against 
a white background. Well, if you save as high dynamic range image (HDR 
or EXR), you can see it as brighter than white if you reduce the 
exposure in your viewing application.
Absorbing media SUBSTRACT from whatever is behind, clipping to zero for 
any negative results. It also cast shadows. They can't be seen against a 
black background.

If you have a red emissive media and a cyant absorbing edia you have this:
Against a white background, the emissive media is invisible and the 
absorbing media remove the green and blue, leaving only the red.

Against a black background, the absorbing media is invisible and you 
only see the red one.

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