Houdini 16.5 Nodes Particle nodes

Interact POP node

Make particles attract or repel other particles.

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Whether particles attract or repel each other is controlled by their charge attribute. Particles with opposite (that is, positive and negative) charges attract, and like charges repel.

When two particles interact, their charges are multiplied to scale the applied forces. By default, a positive force is a repulsive force.

If you are using Particle Intersects Effect, one particle may be within the effect radius of another, but not vice versa. If both particles do happen to be within the effect radius of each other, then one particle’s influence on the other is the same as the relative force applied back to it.

To help visualize the effect of this operator, turn on the node’s template flag to view the acceleration guides.

This operator modifies the following attributes: accel.

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.

Influence

Particle Radius

The radius of the particle. The applied force will be constant in this region.

Use Particle Size/Instance Size

Radius of the particle is equal to a sphere big enough to enclose the instanced geometry object (multiplied by the absolute value of the Scale parameter below).

Use Supplied Radius

Use the value in the Radius parameter below.

Scale

When Particle Radius is Use Particle Size/Instance Size, this value lets you scale the computed radius.

Radius

When Particle radius is Use supplied radius, this is the value that is used.

Effect Radius

The extent of the effect from the particle’s "surface" (that is, out beyond the radius specified above).

The applied force decreases slowly up close, quickly in the middle, and then slowly at the outside edge. (Mathematically, it is a half cosine.)

Use particle radius

The effect radius is the same as the particle’s radius set above. That is, the particle has no effect on other particles beyond its "surface".

Use particle scale/instance size

Radius of the effect is equal to a sphere big enough to enclose the instanced geometry object (multiplied by the absolute value of the Scale parameter below).

Use supplied radius

Use the value in the Radius parameter below.

Scale

When Effect radius is "Use particle size/instance size", this value lets you scale the computed radius.

Radius

When Effect radius is "Use supplied radius", this is the radius to use.

Behavior

Overlap Behavior

Controls what happens when the radii of two particles overlap.

No force

The two particles do not interact.

Random direction

The calculated force is applied in a random direction.

Influence Type

Controls how the effect radius works.

Particle Intersects Effect

The force is applied when a particle enters the effect radius of another particle.

Effect Intersects Effect

The force is applied when one particles effect radius overlaps another particle’s effect radius.

Charge Behavior

Controls how to use the charge attributes of two interacting particles to calculate the force.

Use both charges

Use the charges of both particles to scale the force.

Use the charge of affecting particle

Use the charge of the particle whose effect radius the other particle entered.

Use the charge of particle being affected

Use the charge of the particle that entered the effect radius of the other particle.

Rolloff Exponent

Exponent of the roll-off function. Smaller values increase the effect at any given distance with the radius.

Force Multiplier

Multipliers for each component of the force. This lets you make the force stronger or weaker in a certain direction.

Defaults

Ignore Charge

Ignores any charge attributes on the input particles.

Override Charge

Replaces any charge attributes on the input particles with the value of the Charge parameter below.

Charge

Charge to use when Override charge is on.

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.

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