8

TheLifeofaParticle 107
1. Particle immediately after creation, with no speed.
2. The Speed operator sets the particle in motion.
3. The particle continues moving until acted upon by
another act ion.
Thesecondwaythatparticlesmoveislogically,
from event to event through the particle diagram,
as construc ted in
Part icle View (page 2–121) .
Each event can contain any number of operators
that can affect, in addition to motion, a particle’s
surface appearance, its shape and size, and others.
The part icles start out in the birth event, which
typically is the first event after the global event.
During a particle’s residence in an event, Particle
Flow evaluates each of t he event’s ac tions from top
to bottom, once per integration step, and makes
anyapplicablechangestotheparticle.Iftheevent
contains a test, Particle Flow determines whether
theparticletestsTrueforthetest’sparameters,
such as whether it has collided with an object in
thescene. Ifitdoes,andifthetestiswiredto
another e vent, Part icle Flow sends the par ticle to
thenextevent.Ifitdoesn’t,theparticleremains
in the current event, and may be further acted
upon by its operators and tests. Thus, each particle
exists in only one event at a time.
An action in an event can change the particle shape (1), or
the particle spin (2), or spawn new particles (3).
Actions can also apply forces to particles (1), specify collision
effects (2), a nd alter surface proper ties (3).
In this way, the particle continues to travel through
thesystem.Duetotheflexiblenatureofschematic
construction in Particle Flow, a particle may be
redirected to the same event several times. But
at some point, you might want the particle’s life
to end. For this purpose, you’d use the
Delete
operator (page 2–142)
or the Collision Spawn
test (p age 2–211)
or Spawn test (page 2–226) .
Otherwise, the part icle lives throughout the entire
animation.
Particle age can be used to kill a particle.
As a particle moves through the system, it’s
accompanied by a number of channels. For
example, each part icle has a speed channel
that defines how fast it moves, and a material
ID channel that lets P article Flow know which
sub-material to apply to it. However , the material
itself is not defined by a channel, but by a Material
operator that acts loc a lly or globally. Properties
that are defined by channels persist, unless altered
by an action. For example, the
Material Dynamic
operator (page 2–187)
can change a particle’s
material ID. In effect, by setting up a part icle
diagramandmodifyinghowparticleslookand
act during the animation, you’re deciding how
channel values change based on events and
animation keyframes.