Houdini 16.5 Nodes Particle nodes

Follow POP node

Makes particles follow a leader.

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This operator calculates and applies forces necessary to make particles chase a leader.

You can use the second input to specify the leader(s), or use the parameters on the Leader tab to choose a group in the first input as the leader(s).

Note

The Follow POP merges the particle streams from the first and second input.

The Leader parameter on the Leader tab let you choose how the leader works:

This force is similar to the Attractor POP. This tool should be used when you want to attract particles to a moving object, whereas the attractor tool should be used when you want to attract particles to a static object.

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.

Follow

Behavior

Controls how the node affects the follower particles.

Normal Follow

For particles of unit mass, the velocity for the next frame will be towards the specified position with a speed equal to that of the leader.

Accelerating Follow

Applies unit force towards the leader. This will cause the particle speed to continually increase if the leader does not turn.

Minimum Distance

Stop particles closer than this distance to the leader.

Maximum Distance

If you enter a value greater than 0, particles further than this distance from the leader will NOT follow.

Ambient Speed

If the speed of the leader falls below this value, this is used as the target speed instead.

Attract Distance

If the distance to the leader is less than this value, the force is repulsive. Otherwise, it is attractive. To make this parameter available, you must set the Behavior to Accelerating Follow.

Predict Intercept

Particles will try to predict the leader’s direction on the next frame and head it off.

Stop At Leader

Particles come to a complete stop when they reach the leader.

Scale

Scaling factor for the force applied to the particles.

Speed Scale

Scaling factor for the target speed.

Leader

Leader Group

Subset of points from the first input to act as leaders. You can use this parameter instead of connecting the second (Leader) input.

Leader

Specifies what the particles follow.

Particle Spatial Center

The center of the box bounding the leader particles.

Particle Center of Mass

The center of mass of the leader particles.

Particle Density Center

The center of mass of the leader particles, with all particles having equal mass.

Individual Particles

Each following particle follows a leader particle. If you have follow attributes on the follower particles, they will use the attribute to decide which leader to follow (using the Follow index parameter below). Otherwise, Houdini will automatically assign followers to leaders.

Follow Index

This parameter is available when Leader is "Individual Particles". It controls how the node interprets the follow attribute on particles to assign followers to leaders.

Particles that do not have follow attributes are assigned to leaders automatically, using particle order to refer to a leader.

Particle ID

The follow attribute on the follower particles refers to the particle ID of the leader they should follow.

Particle Order

The follow attribute on the follower particles refers to the position of the leader particle in the list of particles. In a particle system where leader particles are dying and points are being reused, this could mean the leaders change as particles die (even if they are not in the leader stream).

Defaults

Ignore Mass

Ignores any mass attributes on the input particles.

Override Mass

Replaces any mass attributes on the input particles with the value of the Mass parameter below.

Mass

Mass to use when Override mass is on.

Inputs

Follow

The particles that will be made to follow the leader.

Leader

The particles that define the leader(s) to chase. If you do not connect this input, you must set the Leader Group parameter on the Leader tab.

Locals

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.

Particle nodes