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156 Chapter 11: Space Warps and Particle Systems
See also
Speed By Icon Operator (page 2–158)
Keep Apar t Operator (page 2–168)
Procedure
Example: To change particles’ speed:
The Speed operator works on an instantaneous
basis: It sets each particle’s speed once only, when
it enters the event. Even if you animate the Speed
value, each particle moves at a constant rate of
speed, defined by the value at the time it enters the
event. This procedure demonstrates a trick you
can use to change part icle speed with an animated
Speed value, thanks to Particle Flow’s looping
ability.
1. Create a default particle system, and position it
at the top of the Perspective viewport.
2. Play the animation.
Theparticlesfalldownwardatthedefaultrate:
300 units per second.
3. AddaSendOuttestatthebottomofEvent01.
4. Add a Speed operator to an empty area of the
event display.
This creates a new event.
5. Tur n on Auto Key, a nd move the time
slider to f rame 30.
6. In Particle View, click the new Speed 02
operator, and then in the Particle View
parameters panel, set Speed to 0.
This animates the Speed value from 300 at
frame 0 to 0 at frame 30.
7. Turn off Auto Ke y.
8. Wire the Send Out test in Event 01 to Event 02.
9. Play the animation.
The par ticles born later move slower, but all
still move at a constant rate of speed.
10. Add a Send Out test at the end of Event 02.
11. Create a new event using an Age Test. Click the
Age Test to display its parameters, and then set
the followi ng:
•EventAge
•TestValue=1
•Variation=0
12. Wire the Send Out test in Event 02 to Event 03.
13. Wire the Age Test in Event 03 to Event 02.
14. Play the animation.
Alltheparticlesslowdownsimultaneouslyand
eventually come to a stop.
Her e’s how it works: As each particle enters
Event 02, its speed is set to the current Speed
value in the Speed operator. Particle Flow
then sends the part icle immediately to Event
03, where it sits for one frame. Event 03 then
returns the particle to Event 02, whose Speed
value is now lower. P article Flow perceives the
returned par ticle as newly entering the event,
so it changes its speed to the current Speed
value. Thus, the particles continually return to
Event 02 one frame later than before, and are
assigned a pro gressively lower speed.
Ifyouwantedtheparticlestodosomethingelse
after they stop, you could add a Speed Test to
Event 02, above the Send O ut test, set Test True
If Particle Value to Is Less Than Test Value, and
set Test Value to a very low va lue, such as 0.01.
Then wire the Speed Test to a different event.