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174 Chapter 11: Space Warps and Particle Systems
Variation %—
Sets the percentage by w hich particle
size can vary. Defau lt=0.
Pivot At—Specifies the p art 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 part icles
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 Bottom.
W/H R atio—Defines the aspect (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 parameter is
not animatable. Range=0.001 to 1000. Default=1
(square p articles).
This value is not animatable.
Orientation
Use this drop-down list to choose how particles
rotate on the axes not specified by the Look At
direction. Default=Alig n 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 S pinning—To spin t he p articles, cho ose t his
option,andinthesameeventusea
Spin operator
(page 2–150)
.Inthelatter,setRotationAxisto
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 I nstance Operator
Particle View (page 2–121) > Click Shape Insta nce in an
event o r add a Shape Instance operator to the particle
system and then select it.
ShapeInstanceletsyouuseany reference object
(page 3–1096)
in the scene as par ticles. You can
effectively define only one reference object per
event, but the object can comprise any number of
sub-objects, each of w h ich Particle Flow can treat
as a separate particle. Also, using tests, you can
dividetheparticlestreamintomultiplebranches
and define a different par ticle 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 original geometry,
the part icles still appear. However, if you delete the
original geometry, the part icles are deleted as well.
Using Tra nsf or m-Ani mated R ef erence
Objects
You can create animated part icle shapes by
animating the reference object with modifiers, and
then tur ning on
Animated Shape .However,Shape
Instance ignores any rotational and positional
transformations applied directly to the reference
object; it uses only the pure scale component. For