In article <3aa19e29$1@news.povray.org>, "Rune" 
<run### [at] inamecom> wrote:

> When I have just gravity the particles accelerate downwards. When I 
> add wind from the side, the particles move downwards and to the side, 
> accelerating, but in a straight line. When I use less drag, they move 
> less to the side and when I use more drag they move more to the side. 
> But always in a straight line. I'm can't figure out if it's supposed 
> to behave that way or not? 
> How should the particles move when wind is used?

In my particle system, I consider particles as infinitely small points, 
with a constant wind resistance (not related to velocity). While it 
doesn't simulate any physical case, it seems to work well for the 
purposes of the particle_system object.
I first figure out the velocity of the surrounding air *relative to the 
velocity of the particle*, by subtracting the velocity of the particle 
from that of the wind. I then multiply by the drag, which is a simple 
number specified by the user, and add the result to the other forces.
If the wind is slower than the particle, air resistance will cause the 
particle to slow down toward the wind speed. Otherwise, it will speed up 
to match...in other words, it always tends toward the wind velocity, 
even if that is < 0, 0, 0>, so it is really just a simulation of air 
drag.
Again, this doesn't match any physical effect, but it's "close enough" 
for graphics and probably a lot faster than computing the actual drag, 
which you would probably have to compute first for a range of speeds, 
and put into an array or spline to look up later, in order to get a 
reasonable speed. Actually, using a spline instead of a plain float 
value might not be a bad idea...

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

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