"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.)

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}
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 ;-)

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