2009

The user interface appears in the parameters panel, on the right side of the
Particle View dialog.
The first setting lets you choose whether the operator controls speed once or
continuously.
Set Speed Once The operator sets the speed for each particle once: when the
particle enters the event.
Control Speed Continuously The operator sets particle speed throughout
the event. When you choose this option, the
Continuous Speed Control group
on page 2893 becomes available.
Speed The particle speed in system units per second. Default=300.
With the Control Speed Continuously option, you can turn off Speed. This
causes Particle Flow to use the current particle speed.
Using a negative Speed value causes particles to move in the opposite direction
of that effected by a positive speed.
Variation The amount by which particle speed can vary, in system units per
second. Default=0.0.
To obtain each particle's speed, the system multiplies the Variation value by
a random number between -1.0 and 1.0, and then adds the result to the Speed
value. For example, if Speed=300 and Variation=100, then each particle's speed
would be between 200 and 400.
Surface Geometry group
Use these controls to assign objects to affect particle speed and direction. The
list in this group shows the objects, or reference geometry, that the operator
uses to control speed and direction.
If you don't assign any objects, the current particle speed and direction are
not affected.
You can assign multiple Surface Geometry objects, but if they're not all emitters
as well, the results can be difficult to interpret. In general, assign the same
objects as Surface Geometry that your system uses as emitters.
TIP For optimal performance, when moving particles with respect to high-polygon
objects, create a low-poly proxy version to use as surface geometry, link it as a
child of the high-poly object if necessary, and then hide the low-poly proxy.
Use the Add and Remove buttons to edit this list.
2890 | Chapter 14 Space Warps and Particle Systems