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216 Chapter 11: Space Warps and Particle Systems
Control By ...—
Thedrop-downlistatthetopof
the parameters panel lets you choose whether to
send part icles to a target by specifying the speed
and acceleration, or by specifying the amount of
time they should take. Alternatively, by choosing
No Control, you can test particles’ distance from
atarget.
•
Co ntrol By Speed—Specif y speed and
acceleration settings for the par ticles to follow
whiletravelingtothetarget.UsetheControlBy
Speed g roup to set the parameters.
•
Co ntrol By Time—Specify time-based settings
for the particles to follow while trav eling to the
target. Use the Control By Time g roup to set
the parameters.
•
No Control—With this option, Find Target
functionsonlyasaproximitytest. Whena
particle comes within the specified distance
of the target, the particles become eligible for
redirection to the next event.
Note: When you choose No Control, the Find
Target test does not affect particle speed or
direction.
Test True If Distance To group
These settings let you choose which distance
the Find Target test measures and specify the
measured distance. You can choose Target Pivot
or Target Point, and specify a distance w ith the Is
Less Than setting.
Target Pivot—Measures the distance between the
particle and the target pivot.
Iftheparticlesaredirectedtotheoutsideofthe
target and t he Is Less Than value is small, t his
condition might never be satisfied.
Target Point—Measures the distance between the
particle and the
target point .
Is Less Than—When particles are closer than this
distance from target pivot or point, they test True
andbecomeeligibleforredirectiontothenext
event. Measured in scene units.
Note: If you set Is Less Than to 0, the particles
might never test True. This might be desirable for
an animation of, for example, bees buzzing around
a flower but not landing on it. In this case, you
mightwanttousealowvalueforAccelLimitso
thebeesdon’tcircletooclosetotheflower.
Control By Speed group
Use these settings to specify speed and acceleration
parameters when using Control By Speed or Speed
Then Time. This g roup is available only when
using Control By Speed.
UseCruiseSpeed—When on, the software gives
you explicit control over particle speed and speed
variation. When off, the software calculates
part icle speed automatically using the Accel Limit
value. Default=on.
Speed—Part icle speed in scene units per second.
Default=300.0.
Variation—The amount by which the actual Speed
value can vary randomly. Default=0.0.
To obtain the actual speed for each par ticle, the
system multiplies the Variation value by a random
number between -1.0 and 1.0, and then applies
the result as a pe rcentage of the Speed setting.
For example, if Speed=200 and Variation=10,
then the actual speed for each par ticle would vary
randomly between 190 and 210.
Accel Limit—Sets the acceleration limit. This va lue
impacts the inertia and speed of the particles.
Default=1000.0.
The default acceleration limit value is based on
the default Speed va lue of 300.0. If you change
the Speed value, it is recommended that you also
change the Accel Limit value proportionately.
Tip: Use a lower Accel Limit value for smooth
motion, and a higher value when greater accuracy