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"Kenneth" <kdw### [at] gmailcom> wrote:
>
> From what I can tell, fog/turbulence appears to be a kind of sophisticated
> 'camera-projection effect', onto object SURFACES-- not actual volumetric fog,
> even though it looks very similar to media... However, turbulence behavior
> is strangely affected by the geometry of the objects; the animation examples
> show the rather odd behavior of the fog 'conforming' to the particular geometry
> its projected onto.
If the idea of fog being a 'projection effect' is true, then fog/turbulence
behavior reminds me very much of Rune's old ILLUSION.INC include file (but
possibly more sophisticated.)
AFAIU, POV-Ray uses matrices for all of it behind-the-scenes calculations of
transformations, scalings etc. Rune's file also uses a matrix for his 'illusion'
-- but with NO distortion of the 'projected' image/texture onto complex-shape
scene objects. I'm certainly no expert at matrix math, but my guess is that
fog's matrix calculations are not quite right re: the fog's splotchy shapes
bending/distorting/scaling around object contours (when they should be 'immune'
to that, IMO.)
Of course, all of this is my own supposition, as I don't really know how fog
works; but I can give a mind's-eye example of what I mean: In Part 1 of the
animation, assume that there is a nice-looking isolated 'fog cloud' shape
visible on the underside of the bridge. As the camera moves from left to right,
that cloud also appears to move, and with the proper perspective for it's
apparent position in space. So far, so good. But then a problem arises when the
cloud passes over some other complex geometry in the farther distance, or moves
'off the bridge': That part of the cloud suddenly distorts and scales down,
instead of compensating for the different distances and shapes. (For *really*
distant objects, turbulence as-is may be scaling down so small that a screen
pixel can't show the effect-- blurring the tiny clouds together. But the fog
density probably hides that anyway.) Instead, the cloud should at least retain
its own shape and size, no matter what geometry (or distance of that geometry)
it encounters. That's basically what Rune's Illusion code does.
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