Limit particle node

Creates collisions when particles reach a limit boundary.

All

Parameters

Local variables

Example files

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 operator is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.
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

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).

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

Limit_DONE

$HFS/houdini/help

Load | Launch

This files demonstrates a basic use of the Limit POP.

The Limit POP places a boundary around the particles, either in the shape of a box or a sphere. The size of these boundaries can also be animated.

An attribute, such as color in this example, and the grouping information provided by the Limit POP can be used by Color and Split POPs to affect the look and the behavior of the particles.

Usages in other examples

Example name	Example for