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At each frame, each particle in the input (or in the group specified by
the **Source Group** parameter) will birth a random number of new particles
between the minimum and maximum specified in the **Birth Probability**
parameter (on the **Birth** tab). You can make this a true split by killing
the original particle (turn on **Kill Original Particle** on the **Birth**
tab).

The parameters on the **Attributes** tab let you control which attributes
the new particles inherit from the birthing particle, as well as set the
velocity of the newly split particles. If you leave the **Initial velocity**
as **Use inherited velocity**, the new particles will be traveling in the
exact same direction and speed as the original, making them indistinguishable
in the viewer unless you group them and apply different rendering to the
group.

More commonly, you will set **Initial Velocity** to **Add to inherited velocity**
(to define the particle’s new velocity relative to the birthing particle) or
**Set initial velocity** (to set the particle’s new velocity without reference to
the birthing particle). The **Velocity** parameter lets you set the new particle’s
velocity, with a random **Variance** added for each particle (set **Variance**
to `0, 0, 0`

for no random variance.

This node is useful for increasing the number of particles as time goes on. It is also useful in combination with groups to have a group of "main" particles that throw off differently-rendered "secondary" particles (by splitting them into a different group). For example, an explosion effect where white-hot central exploding parts throw off orange and red colored secondary parts.

Another classic example of a split is fireworks: use the **Activation**
parameter to only activate the split at a certain frame, split off the
"explosion" points from the "rocket" point, and kill the original
"rocket" point.

Note

The original particles will split new particles *at every frame*. This
can quickly lead to a very large number of particles if the maximum
birth probability value is high or if the **Life Expectancy** of the
new particles is long.

Tip

If you turn on the Bypass flag the color changes made below will not appear because the Split POP only takes reference inputs. Bypassing this node will create a break in the workflow.

## Parameters

Activation

Turns this node on and off. The node is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.

Note that this is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.

Source Group

Only affect a group of points (created with, for example, a Group POP or Collision POP) out of all the points in the input.

## Source

Emission Type

Where to emit the new particles from.

Except for **From particle**, these options are the same as in
the Source POP's **Emission Type** parameter, though
where the Source POP emits particles from a piece of
referenced geometry, these options emit the particles from
the particle’s *instanced* geometry.

From particle

Emit the new particles from the source particle’s position.

Points (ordered) of Instance

See the Source POP’s **Points (ordered)** option.

Points (random) of Instance

See the Source POP’s **Points (random)** option.

Prim. center (ordered) of Instance

See the Source POP’s **Prim center (ordered)** option.

Prim. center (random) of Instance

See the Source POP’s **Prim center (random)** option.

Edges (ordered) of Instance

See the Source POP’s **Edges (ordered)** option.

Edges (random) of Instance

See the Source POP’s **Edges (random)** option.

Surfaces (ordered) of Instance

See the Source POP’s **Surfaces (ordered)** option.

Surfaces (ordered) of Instance

See the Source POP’s **Surfaces (random)** option.

Volume of Instance

See the Source POP’s **Volume** option.

Metaballs of Instance

See the Source POP’s **Metaballs** option.

Sample Threshold

See the Source POP’s **Sample Threshold** parameter.

Density Threshold

See the Source POP’s **Density Threshold** parameter.

Density Minimum

See the Source POP’s **Density Minimum** parameter.

Use Metaball Density

See the Source POP’s **Use metaball density** parameter.

## Birth

Birth Probability

How many particles to emit from each source point. This not a probability in the sense of "x chances out of y." The first number is the minimum number of particles that might be born each frame, and the second number is the maximum number.

For example, values `0, 1`

means a 50% chance of splitting off
a particle each frame (a minimum of 0 new particles and a maximum
of 1 new particle). Values `5, 10`

means a random number between
5 and 10 (inclusive) of particles will be split off each frame.

Birth Group

Name of a group to put the new points into.

Preserve Group

If the **Birth group** already exists, append the new particles
to the group instead of replacing its contents.

Life Expectancy

How long the particle will live (in seconds).

Life Variance

Particles will live the number of seconds in **Life
expectancy**, plus or minus this number of seconds. Use 0 for no
variance.

Kill Original Particle

Kill the source particle after splitting off new particles.

## Attributes

Inherit Attributes

A list of names/patterns of attributes to inherit from the source geometry.

Initial Velocity

How to set the initial velocity of the emitted particles.

Use inherited velocity

Use the inherited velocity attribute as the initial velocity of the particles.

Add to inherited velocity

Add the inherited velocity attribute to the values from the
**Velocity** and **Variance** parameters below.

Set initial velocity

Set the initial value of the velocity attribute using the
**Velocity** and **Variance** parameters below.

Note

Acceleration is inherited independently, if applicable.

Inherit Velocity

(When **Initial velocity** is **Use inherited velocity** or **Add
to inherited velocity**) The proportion of the inherited velocity
to use. Use `1`

for the full inherited value, `0.5`

to half the
inherited value, and so on.

Velocity

Set or add to velocity attribute.

Variance

Variance to velocity set above. The node will add +/- from 0 to
this number along each axis to the **Velocity** parameter.

Ellipsoid Distribution

By default, the variance (if any) is distributed in a box, the
size of which is determined by the **Variance** parameter. When
this option is on, the variance is distributed in an ellipsoid
instead.

Use as Origin

Controls what Houdini sets as the value of a particle’s origin attribute (the identity of the geometry that emitted the particle). This lets you distinguish the origin of a particle later.

index

Set the origin to the value of the **Origin index**
parameter below.

index + geo num

Sets the hit ID to the value of the **Origin index**
parameter, *added to* the number of the geometry operator
the particle was emitted from.

Typically, you would use large steps for the **Origin
index** parameter (hundreds or thousands) so you can still
distinguish different origins after the geometry number is
added.

For example, you could have two Source POPs with origin index settings of 100 and 200. When the Source POP adds the geometry number to the origin index, you would get origins like 101, 102, 205, 210, and so on. As long as there is no geometry number larger than 99, you can still distinguish the different POPs.

Origin Index

Number to use as the origin of emitted particles. This lets you distinguish different origins.

## Locals

These variables refer to the *source particle’s* properties. They
do not refer to the properties of the instanced geometry when
birthing from instanced geometry.

BBX BBY BBZ

The point’s relative position in the bounding box.

CA

Point or vertex alpha value.

CR CG CB

Diffuse point or vertex color.

DIST

Distance from particle to last collision.

DRAG

Point drag.

ITER

Processing iteration number.

MAPU MAPV MAPW

Texture coordinates.

MASS

Point mass.

NPART

Number of particles.

NPT

Total number of points.

NPRIM

Total number of primitives.

NGRP

Total number of points in source group.

NX NY NZ

Normal vector.

P

Source point.

PSCALE

Particle Scale.

SCALEX SCALEY SCALEZ

Non-uniform scale.

SPEED

Absolute speed of particle.

SPRINGK

Elasticity of a point.

TENSION

Spring tension.

TIMEINC

Time increment.

TX TY TZ

Point position.

UPX UPY UPZ

Up vector.

U V

Surface UV values.

VX VY VZ

Velocity direction.

WEIGHT

Point spline weight.

See also |