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166 Chapter 11: Space Warps and Particle Systems
The user interface appears in the parameters panel,
ontherightsideoftheParticleViewdialog.
The first setting lets you choose whether the
operator controls speed once or continuously.
Set S peed Once—The operator sets the speed for
each particle once: when the particle enters the
event.
Control Speed Continuously —The operator sets
particle sp eed throughout the event. When you
choose this option, the
Continuous Speed Control
group (page 2–168)
becomes available.
Speed—Theparticlespeedinsystemunitsper
second. Default=300.
With the Control Speed Continuously option, you
canturnoffSpeed.ThiscausesParticleFlowto
use the cur rent particle speed.
Using a negative Speed value causes particles to
moveintheoppositedirectionofthateffectedbya
positive speed.
Variation—The amount by which particle speed
can vary, in system units per second. Default=0.0.
To obtain each part icle’s speed, the system
multipliestheVariationvaluebyarandomnumber
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 par ticle’s speed would
be b etween 200 and 400.
Sur face Geometr y group
Use these cont rols 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 cont rol speed and direction.
If you don’t assign any objects, the current particle
speed and direct ion are not affected.
You can assign multiple Surface Geometry objects,
butifthey’renotallemittersaswell,theresultscan
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.
Add—Adds an object to the list. C lick Add, and
thenclickanobjectintheviewport.
By List—Adds multiple objects to the list. Click By
List to open the Select Surface Objects dialog. This
works just like the Select O bjects dialog; highlight
the objects to use to control speed and direction,
and then click the Select button.
Remove—Removes an object from the list.
Highlig ht the object in the list, and then click
Remove.
Animated Shape—Turn on to allow particles to
followthesurfaceofanobjectwhoseformis
animated by morphing or with modifiers.
Subf r ame S ampl ing—When on, the operator
acquires animation of the Surface Geometry shape
on a tick basis (every 1/4,800th of a second) r ather
than a f rame basis. This provides g reater precision
in allowing the particle positions to follow
animation of the Surface Geometry object’s form.
Speed by M a ter ia l—Varies part icles’ existing speed
anddirectionbasedonpropertiesofthematerial
applied to each Surface Geometry object. For
example, if an object is assigned a black-and-white
checkered diffuse map and you choose the
Grayscale Multiplier option, particles near the
white-checked areas move faster than those from
the black-checked areas.
Note: For material-influenced speed to appear
properly in the viewports, two conditions are
required: at least one view p or t must be set to a
shaded display mode, and the material or map