Group particle node

Creates and/or combines named groups of particles.

All

Parameters

Local variables

Example files

This operator creates groups of particles which you can then reference in subsequent nodes, allowing you to apply node effects to a subset of the entire particle system.

Grouping rules

Unlike other types (such as surfaces), particle grouping using the Combine tab applies to all active particles, not just particles in the current branch of the operator hierarchy.

In the example network, initial_group defines an initial group (using the rule $LIFE < .4), and a firsttrail group using the Combine tab to specify a second group containing any particles not in the initial group (using the rule firsttrail = !initial).

Because this branch is eventually merged with another branch (at collect1), the Combine rule will apply to all particles in both branches, and so the firsttrail group will also include particles generated by source2, which is probably not what you wanted.

To work around this, use two separate Group operators to create the two groups. In this example, the initial_group node creates the initial group with rule $LIFE < .4 and the initial_group2 node creates the firsttrail group with rule $LIFE >= .4 (that is, the logical inverse of the rule for initial).

Parameters

Activation

Turns the POP on and off.

Create

Source Group	Subset of points to act on.
Group Name	Name of the new group to create from the points.
Preserve Group	Append particles to the group if it already exists, instead of replacing its contents.

Rule

Enable

Turning on this checkbox puts particles in the new group based on an expression.

Predefined rules

Use the items in this menu to fill in the Rule field with a predefined, commonly-used rule.

Note

The list of predefined rules is specified by $HFS/houdini/POPGroupRules.

Particle is dead	The particle is dead.
Particle is stopped	The particle is flagged as being stopped. This is not the same as a particle having zero velocity.
Particle just collided	The particle collided in this timestep. See the definition of the `$JUSTHIT` local variable.
Particle has collided	An input particle has collided at any point in the past.

Rule

An expression to evaluate for each input particle. If the expression evaluates to true (that is, non-zero) for a particle, the operator puts the particle in the new group.

Bounding

Enable

Turning on this checkbox puts particles in the new group if they are inside a bounding volume.

Bounding Type

The shape of the bounding volume.

Bounding box	Group particles inside the box defined by the Center and Size parameters below.
Bounding sphere	Group particles inside the ovoid defined by the Center and Size parameters below.
Bounding object	Group particles inside the volume of the surface specified in the SOP parameter below. The behavior of this option is undefined if the surface is not closed.
Bounding metaball	Group particles inside the volume of the metaball specified in the SOP parameter below, at densities at or above the Density minimum parameter below.

Center

Center of bounding box/sphere, when Bounding type is “Bounding box” or “Bounding sphere”.

Size

Size of bounding box/sphere, when Bounding type is “Bounding box” or “Bounding sphere”.

SOP

Sop containing geometry to use as bounding volume, when Bounding type is “Bounding object” or “Bounding metaball”.

Ignore Transform Object

When this option is off, the geometry will automatically be transformed into the space of the object using this POP network. When this option is on, the geometry is not transformed.

Density Minimum

When Bounding type is “Bounding metaball”, only consider the point to be inside the metaball volume if the density is at or above this value.

Generator

Enable	Turning on this checkbox puts particles created by certain generators in the new group.
Generators	Group particles that were originally generated by these generators. This field can contain a space separated list of paths and/or patterns specifying generator nodes.

Random

Enable

Turning on this checkbox sorts particles into groups.

# of Groups

Number of groups to create. The groups will use the name in the Group name parameter with a number on the end.

For example, if the Group name is “Flock” and 5 groups are requested, the operator will create groups Flock1, Flock2, Flock3, Flock4 and Flock5 and the particles will be randomly distributed into the 5 new groups.

Group Index

Sets the group the particle will be placed into. Normally, particles are sorted into groups in order (the first particle goes into the first group, etc.). You can use a custom expression here to manually determine which group number each particle goes into. If the expression evaluates to a number greater than the value in # of Groups, the number wraps around (that is, the value of the expression is modulo the number of groups).

This has several advantages:

You can use some criterion other than particle order to sort the particles into groups, so particles dying will not shift the group membership of other particles.
You can use the rand function to sort the particles into groups pseudo-randomly.

Instances

Enable	Turning on this checkbox puts particles in the new group based on the value of their `instance` attributes.
Instances	Groups particles whose `instance` attribute matches a value in this list.

Combine

The parameters on this tab let you create a group that is a combination of existing groups, based on boolean operators and combination methods.

Enter the name of the new group to create in the left-hand field on the first row, and the name of the group from which to take particles in the right-hand field (or select Equals All But if you want to start with particles not in the right-hand group).

Once you set up the top row, additional rows become available to further combine and/or filter the contents of the new group.

(top row)

On the left-hand side, enter the name of the new, combined/filtered group to create.

On the right-hand side, enter the name of the group from which to take particles (or select Equals All But if you want to start with particles not in the right-hand group).

(additional rows)

On the right-hand side, enter the name of another group of particles you want to combine with or filter from the new group.

Use the menu on the left side to choose how to combine/filter the groups.

Select With All But if you want to use the complement of the specified group in the operation.

None	Do not use this row.
Union (Or)	Add this group to the new group.
Intersect (And)	The new group will only contain particles that are also in this group.
Exclusive Or	Add this group to the new group, but eliminate particles that are in both groups.
Subtraction	The new group will not contain any particles that are also in this group.

Select

Select group in detail	Select the particles in the group specified by the Select Group parameter below. This highlights the particles in the viewer.
Select Group	Select this group, when Select group in detail is on.

Local variables

Standard POP local variables

AGE	The seconds a particle in the template has been alive.
AX AY AZ	Acceleration of the particle.
BBX BBY BBZ	The point’s relative position in the bounding box.
DEAD	Point is dead.
ITER	Processing iteration number.
JUSTHIT	A collision for this particle was detected (for example, by the Collision POP) during the processing of this timestep (that is, this iteration of the particle simulation). This variable is cleared at the beginning of each timestep. Note that the collision POP actually detects any collisions which would have occurred the during the previous frame.
LIFE	Percent of total life used (from 0 to 1).
LIFESPAN	Expected lifetime of particle.
MAPU MAPV MAPW	Point or vertex texture coordinates.
NPT	Total number of points.
NGRP	Total number of points in source group.
NX NY NZ	Normal vector.
PT	The point number of the currently processed point. The `PT` is not constant like `ID`; it changes based on the number of points.
RESTX RESTY RESTZ	The rest position.
SLIDING	The sliding state of the particle.
SPRINGK	Elasticity of a point.
STOPPED	Point is stopped.
STUCK	1 if particle is stuck to a collision object.
TENSION	Spring tension.
TIMEINC	Time increment.
TX TY TZ	Point position.
U V	Surface UV values.
VX VY VZ	Velocity direction.
WEIGHT	Point spline weight.

Added by Collision POP/Limit POP

DIST	Distance from particle to last collision.
HCR HCG HCB	Diffuse color at the collision point on the surface the particle collided with.
HITID	ID for last collision. You can control how this attribute is set in the Collision or Limit POP to help distinguish types of collisions.
HITTIME	The time at which the last collision occurred.
HMAPU HMAPV	The texture map UV coordinates for the surface location where the last collision occurred.
HNX HNY HNZ	The normal at the surface location where the last collision occurred.
HTX HTY HTZ	World space position of the last collision.
HU HV	The UV coordinates for the surface location where the last collision occurred.
NUMHIT	Number of times the particle has collided.

Added by Color POP

CA	Point or vertex alpha value.
CR CG CB	Diffuse point or vertex color.

Added by Property POP

ATTRACT	Attractor point.
CHARGE	Charge of the particle.
CLING	Point is clinging to geometry.
DRAG	Point drag.
FOLLOW	Leader to follow.
MASS	Point mass.
PSCALE	Particle Scale.
SCALEX SCALEY SCALEZ	Non-uniform scale.

Added by Proximity POP

NEAREST	Either the point number or id of the particle nearest to this one.
NEARESTDIST	The distance to the nearest particle.
NUMPROXIMITY	The number of particles within a specified proximity to this particle.

Added by Rotation POP

ROTA	Rotation angle.
ROTX ROTY ROTZ	Rotation axis.

Added by Source POP

GEN	Generation.
ID	ID number, which always remains constant.
ORIGIN	Original Source point was birthed from.
PARENT	Parent’s ID Number.

Added by Speed Limit POP

SPEEDMAX	Maximum speed.
SPEEDMIN	Minimum speed.

Added by Sprite POP

SROT	Sprite rotation around view axis (in degrees).
STEXU STEXV	Texture coordinate of sprite’s lower-left corner.
STEXW STEXH	Size of sprite in texture space.
SX SY	Sprite scale.

Controlled by Suppress Rule POP

SUPPPOS	Suppress default position rule.
SUPPVEL	Suppress default velocity rule.
SUPPUP	Suppress default up-vector rule.
SUPPAGE	Suppress default aging rule.
SUPPROT	1 if particle is suppressing its default rotation rule.
SUPPANGVEL	1 if particle is suppressing its default angular velocity rule.

Added by Up Vector POP

PVX PVY PVZ	Previous velocity.
UPX UPY UPZ	Up vector.

Added by Location, Source, Softbody, Split POPs

SPEED

Absolute speed of particle.

Example files

GroupColor

$HFS/houdini/help/examples/nodes/pop/group/GroupColor.cmd

Load | Launch

This example demonstrates how the Group POP can group particles by different criteria. These criteria include Rule, Generator, Bounding Box, and Random.

Particles are colored using the Color POP to visualize the designated criteria.

Usages in other examples

Example name	Example for
SplitSelf	Split particle node	Load \| Launch
FireworksSetup	Split particle node	Load \| Launch
NewGroupiesRavage	Follow particle node	Load \| Launch
Lemmings	Follow particle node	Load \| Launch
FlockOnPath	Follow particle node	Load \| Launch
FollowInteract	Follow particle node	Load \| Launch
AttractToRing	Attractor particle node	Load \| Launch
SoftBodySheet	Soft Body particle node	Load \| Launch
Swarm	Property particle node	Load \| Launch
MultiAxisOrbiting	Orbit particle node	Load \| Launch
Orbit	Orbit particle node	Load \| Launch
InstanceCoils	Instance particle node	Load \| Launch
Instance	Instance particle node	Load \| Launch
InstanceBounce	Instance particle node	Load \| Launch
Capture	Collision particle node	Load \| Launch
Meteor	Collision particle node	Load \| Launch
DrainExample	Particle Fluid Emitter dynamics node	Load \| Launch
ParticlesAndCloth	Particle Fluid Solver dynamics node	Load \| Launch
TankDrain	Particle Fluid Sink dynamics node	Load \| Launch
DrainExample	Particle Fluid Sink dynamics node	Load \| Launch