8

Birth Script Operator 141
Note:
The Emit Start and Emit Stop values are tied
to the system frame rate. If you change the frame
rate, Particle Flow automatically adjusts the Emit
values according ly. For example, if you set Emit
Start to 120 and Emit Stop to 300 using the default
NTSCframerate(30fps),andthenswitchtoPAL
(25 fps) using the Time Configuration dialog ,
you’vereducedtheframerateto5/6oftheoriginal
value. Thus, Particle Flow automatically uses the
same ratio to adjust the Emit settings, resulting in
an Emit Start value of 100 and an Emit Stop value
of 250. This allows the particle system to retain
the timing you specif y, no matter which frame rate
you use.
Note: With Subframe Sampling off, such
adjustments are rounded off to the nearest integer
frame number. With Subframe Sampling on,
fractional frame values that result from such
adjustments will be used, but not displayed.
Amount—To specify the total number of par ticles
emitted by the operator, choose A mount, and then
set the quantity of part icles.
Using the Amount option, the first particle is
alwaysemittedattheEmitStartframe,andthelast
particle is always emitted at the Emit Sto p frame.
Par ticles emitted between these endpoints appear
at equal intervals within the emission period.
Forexample,ifyousetAmountto3,thesecond
particle is emitted halfway through the emission
period.
To determine the number of particles emitted per
framewhenusingAmount,dividetheAmount
value by the number of emission frames (Emit
Stop-Emit Start+1).
Rate—To specify the number of par ticles emitted
per second , choose Rate, and then set the value.
The operator emits this number of part icles per
second starting at the Emit Start frame and ending
at the Emit Stop frame.
If you specif y a birth rate value that isn’t an integer
multiple of the system f r a mes-per-second value
(set in the Time Configuration dialog), Part icle
Flow uses interpolation to determine when to
emit particles. For example, if you use the system
default rate of 30 frames per second, and set the
birthrateto4,thesystemwouldemiteachparticle
at intervals of seven or eight frames if Subframe
Sampling is off, or at intervals of 7.5 frames if
Subframe Sampling is on.
Total—The calculated t otal number of particles
emitted by the operator.
Subframe Sampling—Turningthisonhelpsavoid
par ticle "puffing" by emitt ing part icles at a much
higher subframe resolution (that is, throughout
each frame), rather than using the relatively coarse
frame resolution. Default=on.
"Puffing" is the effect of emitting separate "puffs"
or clusters of particles, rather than a continuous
stream. This effect is especially noticeable when
the emitter is animated.
Tu rn off Subframe Sampling to cause particles to
be emitted exactly at frame times. This makes it
easier to sort particles by their age later.
Tip: If, when using the Collision (page 2–208) or
Collision Spaw n test (page 2–211) ,youexperience
an irregular particle stream, try turning off
Subframe Sampling.
Birth Script Operator
Particle View (pa g e 2–121) > Click Birth Script in an event
or add a Birth Script o perator to the particle system and
then select it.
The Birth Script operator enables creation of
particles within the Particle Flow system using a
MAXScript script. The script can use any program
functionality available to MAXScript.