Houdini 16.5 Nodes Particle nodes

Limit POP node

Creates collisions when particles reach a limit boundary.

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This operator is very similar to the Collision POP, but instead of detecting collisions with geometry, this operator detects particles colliding with a limit boundary.

Use the parameters on the Attribute tab to store information in the colliding particles' attributes about the collision. With the exception of the distance attribute, these attributes are not set unless a collision has occurred.

Tip

Turn on the node’s template flag to display the limit boundary as guide geometry in the viewer.

This operator modifies the following attributes: accel, dist, hitCd, hitN, hitid, hitpos, hitposuv, hittime, hituv, numhit.

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.

Limits

The parameters on this tab define the shape and size of the limit boundary. Use the Behavior tab to define what happens when particles hit this boundary.

Type

Shape of the limit boundary.

Box

Rectangular limit, the corners of which are defined by the +Limit and -Limit parameters below.

Sphere

Spherical limit. The sphere is still defined by a bounding rectangle with the limit parameters below.

+Limit

One corner of the limit’s bounding box. This value should be in the positive quadrant (that is, the X, Y, and Z components are all positive).

-Limit

The other corner of the limit’s bounding box. This value should be in the negative quadrant (that is, the X, Y, and Z components are all negative).

Behavior

Behavior

Controls how the motion of particles is affected by the collision.

Die on collision

Particle dies.

Bounce on collision

Particle reflects off the surface.

Stop on collision

Particle stops.

Stick on collision

Particle stops and attaches itself to the surface.

Slide on collision

Particle slides along the surface.

Continue on course

Particle motion does not change.

Collision Event

Generate this event when any point collides with the target geometry.

Collision Group

The name of a group that will be created to contain the particles that collided. Subsequent operators can use this group to select the particles to affect.

Preserve Group

If the group named in the Collision group parameter exists, add the collision particles to the group instead of replacing its contents.

Use as Hit ID

Controls what Houdini sets as the "hit ID" (the identity of the geometry the particle collided with). This lets you distinguish a particle that collided in this POP later.

index

Sets the hit ID to the value of the Hit index parameter below.

index + primitive number

Sets the hit ID to the value of the Hit index parameter below, added to the primitive number of the primitive the particle collided with.

Typically, you would use large steps for the Hit index parameter (hundreds or thousands) so you can still distinguish different collisions after the primitive number is added.

For example, you could have two Collision POPs with hit index settings of 100 and 200. When the Collision POP adds the primitive number to the hit index, you would get hit IDs like 101, 102, 205, 210, and so on. As long as there is no primitive number larger than 99, you can still distinguish the different POPs.

Hit Index

Number to set in the "hit ID" of collision particles. This lets you distinguish different types of collisions.

Oversampling

Available when Hint (on the Collision tab) is Translating Geometry. Oversamples the motion of the geometry for more accurate collisions.

Bounce Accuracy

Available when Behavior is Bounce on collision. The maximum number of inter-frame bounces a particle will perform.

Gain Tangent

Energy loss tangent to collision.

Gain Normal

Energy loss normal to collision.

Final Behavior

You can optionally choose a different behavior to use when a particle’s impulse falls below a minimum. For example, if you want particles to stop instead of traveling imperceptibly slowly, set Final behavior to Stop and Minimum impulse to a low number.

Minimum impulse

Available when Final behavior is not Bounce on collision. The impulse value below which a particle will switch to using Final behavior parameter.

Attributes

This tab contains checkboxes for the different attributes you can have set on particles when they collide with the target geometry.

Add Num Hit Attribute

Number of times collided ($NUMHIT).

Add Hit ID Attribute

Hit ID ($HITID).

Add Hit Time Attribute

Time of collision ($HITTIME).

Add Hit Position Attribute

Position in space where collision occurred ($HT{XYZ}).

Add Hit Position UV Attribute

Parametric position on surface where collision occurred ($H{UV}).

Add Hit Normal Attribute

Normal where collision occurred ($HN{XYZ}).

Add Hit Diffuse Color Attribute

Diffuse color at collision ($HC{RGB}).

Add Hit Texture UV Attribute

Texture uv at collision ($HMAP{UVW}).

Add Distance Attribute

Distance from current position to collision ($DISTANCE).

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.

See also

Particle nodes