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178 Chapter 11: Space Warps and Particle Systems
Variation % —
Sets the percent age by which particle
size can vary. D efault=0.
Pivot At—Specifies the part of the particle around
which rotation is performed when maintaining
the facing direction. The choices, available from
the drop-down list, are Top, Center, and Bottom.
Default=Center.
With Top and Bottom, the center of the
corresponding side is used as the particle center
for rotation.
This option is useful, for example, when particles
are lying on a surface, and each particle is an
explosion. In this situation, you’d probably want
theentireparticlerectangletoappearabovethe
surface, so you’d set Pivot At to B ottom.
W/H R atio—Defines the aspec t (width-to-height)
ratio of the shape rectangle.
Adjustthisratiotothatoftheparticle-map
imagery. For the commonly used TV aspect ratio
of 4:3, set W/H Ratio=1.33. The par ameter is
not animatable. Range=0.001 to 1000. Default=1
(square particles).
This value is not animatable.
Orientation
Use this drop-down list to choose how p articles
rota te on the axes not specified by the Look At
direction. D efault=Align to Horizon.
Align to Horizon—Keeps the top edge aligned with
the horizon (the world XY plane).
Align to Speed Follow—Uses the right side of each
particle as the leading edge, so the top and bottom
edges are aligned with the direction of particle
motion.
Random—Orients the top edge at random.
Allow Spi nning—To spin the particles, choose this
option,andinthesameeventuseaSpin operator
(page 2–154). In the latter, set Rotation Axis to
ParticleSpaceandusethedefaultaxisvalues:X=0,
Y=0 , Z=1.
Uniqueness group
The Uniqueness setting enables changing the
randomization of the size/width variation.
Seed—Specifies a randomization value.
New—Calculates a new seed using a randomization
formula.
Sha pe Inst ance Oper ator
Particle View (page 2–125) > Click Shape Instance in an
event or add a Shape Instance operator to the particle
system and then select it.
ShapeInstanceletsyouuseanyreference object
(page 3–1002) in the scene as particles. You can
effectively define only one reference object per
event, but the object can comprise any number of
sub-objects, each of w hich Part icle Flow c an treat
as a separate particle. Also, using tests, you can
dividetheparticlestreamintomultiplebranches
and define a different p article shape for each.
Asthenameoftheoperatorindicates,reference
geometry is instanced into the particle system.
Thus, any physical changes you make to the
original geometry are reflected instantly in the
particle system. If you hide the origina l geometry,
the particles still appear. However, if you delete the
original geometr y, the particles are deleted as well.
Using Tr ans for m-Anima ted R ef erence
Objects
You can create animated particle shapes by
animating the reference object with modifiers, and
then turn ing on Animated Shape.However,Shape
Instance ignores an y rotational and positional
transformations applied directly to the reference
object; it uses only the pure scale component. For