Split
particle node
Splits (emits) particles from existing particles.
See also: Location, Source, Stream
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.
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
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Activation |
Turns this node on and off. The operator is only active if this value is greater than 0. This is useful to control the effect of this node with an expression. |
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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
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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.
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Sample Threshold |
See the Source POP’s Sample Threshold parameter. |
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Density Threshold |
See the Source POP’s Density Threshold parameter. |
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Density Minimum |
See the Source POP’s Density Minimum parameter. |
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Use Metaball Density |
See the Source POP’s Use metaball density parameter. |
Birth
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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 |
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Birth Group |
Name of a group to put the new points into. |
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Preserve Group |
If the Birth group already exists, append the new particles to the group instead of replacing its contents. |
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Life Expectancy |
How long the particle will live (in seconds). |
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Life Variance |
Particles will live the number of seconds in Life expectancy, plus or minus this number of seconds. Use 0 for no variance. |
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Kill Original Particle |
Kill the source particle after splitting off new particles. |
Attributes
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Inherit Attributes |
A list of names/patterns of attributes to inherit from the source geometry. |
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Initial Velocity |
How to set the initial velocity of the emitted particles.
Note
Acceleration is inherited independently, if applicable. |
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Inherit Velocity |
(When Initial velocity is Use inherited velocity or Add to inherited velocity) The proportion of the inherited velocity to use. Use |
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Velocity |
Set or add to velocity attribute. |
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Variance |
Variance to velocity set above. The node will add +/- from 0 to this number along each axis to the Velocity parameter. |
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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. |
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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.
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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.
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BBX BBY BBZ |
The point’s relative position in the bounding box. |
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CA |
Point or vertex alpha value. |
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CR CG CB |
Diffuse point or vertex color. |
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DIST |
Distance from particle to last collision. |
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DRAG |
Point drag. |
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ITER |
Processing iteration number. |
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MAPU MAPV MAPW |
Texture coordinates. |
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MASS |
Point mass. |
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NPART |
Number of particles. |
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NPT |
Total number of points. |
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NPRIM |
Total number of primitives. |
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NGRP |
Total number of points in source group. |
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NX NY NZ |
Normal vector. |
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P |
Source point. |
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PSCALE |
Particle Scale. |
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SCALEX SCALEY SCALEZ |
Non-uniform scale. |
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SPEED |
Absolute speed of particle. |
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SPRINGK |
Elasticity of a point. |
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TENSION |
Spring tension. |
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TIMEINC |
Time increment. |
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TX TY TZ |
Point position. |
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UPX UPY UPZ |
Up vector. |
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U V |
Surface UV values. |
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VX VY VZ |
Velocity direction. |
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WEIGHT |
Point spline weight. |
Example files
FireworksSetup
SplitSelf
$HFS/houdini/help/examples/nodes/pop/split/SplitSelf.cmd
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.