Split particle node

Splits (emits) particles from existing particles.

All

Parameters

Local variables

Example files

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.

Using Split

Create a particle system.
Click Split on the Create Particles tab.

The particle system will attach itself to the particle system you have selected.
Click play on the playbar to see the particles.

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.

Local variables

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.

Example files

FireworksSetup

$HFS/houdini/help/examples/nodes/pop/split/FireworksSetup.cmd

Load | Launch

This example demonstrates how to create a firework effect using the Split POP.

It also demonstrates how a POP network can inherit attributes, such as color and velocity, from the SOP network by using the Source POP.

SplitSelf

$HFS/houdini/help/examples/nodes/pop/split/SplitSelf.cmd

Load | Launch

This example demonstrates how the Split SOP can be used to birth new particles based on the death of other particles.

Particles are birthed from a source Circle and are given a velocity in the positive-y direction. As they die, the particles are placed in a group that the Split SOP accesses, called “dead”. Each of these “dead” particles gives birth to three new particles.

The new particles are given a color based on their lifespan.

Usages in other examples

Example name	Example for
AddParticleCurves	Particle surface node	Load \| Launch
InstanceCoils	Instance particle node	Load \| Launch
Limit	Limit particle node	Load \| Launch
BounceSplit	Collision particle node	Load \| Launch
Meteor	Collision particle node	Load \| Launch