Split POP node

Splits (emits) particles from existing particles.

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.

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 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.

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.