Birth Operator

The Birth operator enables creation of particles within the Particle Flow system using a set of simple parameters. In general, use Birth as the first operator in any event connected directly to a global event; this is called the birth event.

• Particle View > Click Birth in an event or add a Birth operator to the particle system and then select it.

You can specify a total number of particles, or a rate of particles born per second. You can also tell the system when to begin emitting particles, and when to stop.

Note: The Birth operator must always come at the beginning of a particle stream; the system doesn't let you position it elsewhere. You can place a Birth operator in an isolated event, but you can't then wire that event in series with a stream that already uses a Birth operator. However, you can wire multiple Birth operators, each in its own event, into a particle stream with an existing Birth operator, in parallel . The following procedure illustrates this. If you need to create particles midstream, use the Spawn Test or Collision Spawn Test.

Procedures

To use the Birth operator:

This procedure demonstrates the impossibility of using multiple Birth operators in series, and shows how to use multiple Birth operators in parallel.

1. Start or reset 3ds Max, and add a new PF Source object to the scene.
2. Press 6 to open Particle View.

   The default particle system contains a Birth operator as the first action in the birth event.

3. Try to drag the Birth operator elsewhere in Event 01.

   You can't. As you drag over the different actions in Event 01, a red line appears at the top of the event, showing that the Birth operator will be placed here, no matter where you release the mouse button.

4. Try to drag a new Birth operator from the depot to Event 01.

   As in the previous step, the only place you can drop the Birth operator is at the top of the event, replacing the existing Birth operator.

5. From the depot, drag the Birth operator to an empty area in the event display.

   Particle Flow creates a new birth event, Event 02, containing the Birth operator and a Display operator.

6. Wire the output of the global event, PF Source 01, to the event input of Event 02.

   Each birth event must be associated with a global event to be able to generate particles.

7. From the depot, add a Send Out test at the end of both Event 01 and Event 02.
8. Try to wire the test output of Event 02 to the event input of Event 01.

   Particle Flow doesn't let you, because this would result in two Birth operators in series.

9. Try to wire the test output of Event 01 to the event input of Event 02.

   Again, Particle Flow doesn't let you, because this would result in two Birth operators in series.

10. Drag a Speed operator to an empty area of the event display.

    Particle Flow creates a new event, Event 03.

11. Wire the test output of Event 01 to the event input of Event 03.
12. Wire the test output of Event 02 to the event input of Event 03.

    There's no problem wiring the two birth events to a single, third event. The birth events exist in the particle stream in parallel, each generating particles independently and then feeding its particle stream into a common event, where the two streams are combined.

    If the second birth event had its own global event, you could, at any point further downstream, separate the streams back out according to their origin using the Split Source test. To do this, delete the wire from PF Source 01 to Event 02, add an Empty Flow to the system, and then wire it to Event 02.

Interface

The user interface appears in the parameters panel, on the right side of the Particle View dialog.

Emit Start

The frame number at which the operator begins emitting particles.

Emit Stop

The frame number at which the operator stops emitting particles.

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 accordingly. For example, if you set Emit Start to 120 and Emit Stop to 300 using the default NTSC frame rate (30 fps), and then switch to PAL (25 fps) using the Time Configuration dialog, you've reduced the frame rate to 5/6 of the original 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 specify, 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 particles emitted by the operator, choose Amount, and then set the quantity of particles.

Using the Amount option, the first particle is always emitted at the Emit Start frame, and the last particle is always emitted at the Emit Stop frame. Particles emitted between these endpoints appear at equal intervals within the emission period. For example, if you set Amount to 3, the second particle is emitted halfway through the emission period.

To determine the number of particles emitted per frame when using Amount, divide the Amount value by the number of emission frames (Emit Stop-Emit Start+1).

Rate

To specify the number of particles emitted per second, choose Rate, and then set the value. The operator emits this number of particles per second starting at the Emit Start frame and ending at the Emit Stop frame.

If you specify a birth rate value that isn’t an integer multiple of the system frames-per-second value (set in the Time Configuration dialog), Particle 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 birth rate to 4, the system would emit each particle 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 total number of particles emitted by the operator.

Subframe Sampling

Turning this on helps avoid particle "puffing" by emitting particles 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.

Turn 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 or Collision Spawn Test, you experience an irregular particle stream, try turning off Subframe Sampling.