remember that smoke is (IIRC) mostly particulate matter in suspension.
- so when they form a vortex ring like that, they are more dense near the
outside of the object (I think in that particular case it has something to
do with rotational intertia). Thus, when you are looking at the edge, the
light
must travel through/bounce off of more particles than when you look through
the center.  A spherical density gradient in your media would have a similar
look - but this effect only occurs in some forms of smoke. A smoke ring is
a special case. If you really want good smoke simulation, then, well, a
simulation is what you are going to want - or at least a few pictures of
real smoke to attempt to duplicate by hand. Fluid dynamics is, um, hard ;)

--
-s
5TF!

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