Hi Chris,

I'm making my way through your particle include file and was hoping I could
ask you a few questions regarding using it.

The end goal is a welding sparks animation, so there would be a point
source, with a dozen or so particles proceeding mostly upward and outward at
random shallow angles from that source. I think the particles would need to
be expressed as short lines (cylinders or linear sphere sweeps perhaps) so
there
might be the necessity to have a head and a tail to the particle, both of
which follow the same path under the influence of gravity and which are then
considered as a single object for rendering. Variations in the instantaneous
speed of head and tail particle pairs might create some nice changes in
particle length during their lifetimes (i.e. shorter streaks near the top of
an arc, longer ones as the particles accelerated downwards under gravity).

In the following hopefully stripped-down example, I was trying to send ten
particles straight up from <0,0,0>. They seem, however, to be spiraling in
the +y direction around the reference cylinder (which represents the +y
axis). Admittedly I've just started unravelling your include, but I thought
I had turned off or checked any defaults that might have imparted a rotation
to the creation of the particles. Could you tell me where the rotation is
coming from?

camera {
   location <0, 1, -60>
   look_at <0, 20, 0>
}

light_source {<0, 0, -50> rgb 1}

plane { y, 0
   pigment {color rgb <.2, .5, .8>}
}

cylinder { <0, 0, 0>, <0, 100, 0>, .25
   pigment { White }
}

// SPRAY OPTIONS
#declare spray_loop = true;
#declare particle_dist = 0;
#declare particle_count = 10;
#declare particle_size = 1;
#declare spray_color_map = color_map {
   [0 rgb <1, 1, 1>]
   [1 rgb <1, 1, 1>]
}
#declare particle_life = .5;
#declare spray_strength = 100;
#declare nozzle_angle = 0;
#declare nozzle_spin = 0;
#declare spray_direction = <0, 1, 0>     // DO NOT give this a value of <0,
0, 0> or crash!

#include "Spray.inc"

Also, given my description of the "welding sparks" goal, could I ask you for
any comments on whether your include could be modified to create the
head-tail type of particles I'm hoping to use here? There's an animation
with sparks of the type I'm going for on the www.xl5.com site, under
"Motion", example 4.

Thanks very much for your considerations,

Ian J.

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