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170 Chapter 11: Space Warps and Particle Systems
The user interface appears in the parameters panel,
ontherightsideoftheParticleViewdialog.
The first setting lets you choose w hether the
operator controls speed once or contin uously.
Set S peed Once—The operator sets the speed for
each particle once: when the particle enters the
event.
Control Speed Continuousl y—The operator sets
particle speed throughout the event . When you
choose t his option, the Continuous Speed Control
group (page 2–172) becomes available.
Speed—Theparticlespeedinsystemunitsper
second. Default=300.
With the Control Speed Continuously option, you
canturnoffSpeed.ThiscausesParticleFlowto
use the current particle speed.
Using a negative Speed value causes particles to
moveintheoppositedirectionofthateffectedbya
positive speed.
Variation—The amount by w hich 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 va lue. For example, if Speed=300 and
Variation=100, then each particle’s speed would
be b etween 200 and 400.
Sur face G eometry group
Use these cont rols to assig n 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 direct ion.
If you don’t assign any objects, the cur rent p article
speed and direction 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 t he 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 Sele ct Surface Objects dialog.
This works just like the Select Objects dialog (page
1–78); high light 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 mor phing 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) rather
than a frame basis. This provides greater precision
in allowing the particle positions to follow
animation of the Surface Geometry object’s form.
Speed by M ateria l—Varies par t 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 f rom
the black-checked areas.
Note: For material-influenced speed to appear
properly in the viewports, two conditions are
required: at least one viewpor t must be set to a
shaded display mode, and the material or map