Event - Houdini online help

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.

Event Name

Name of the event to generate

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/POPEventRules.

Any particle is dead	An input particle is dead.
Any particle is stopped	An input particle is flagged as being stopped. This is not the same as a particle having zero velocity.
Any particle just collided	An input particle collided in this timestep. See the definition of the `$JUSTHIT` local variable.
Any 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 generates the event specified in the Event Name parameter.

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

Splatter

$HFS/houdini/help/examples/nodes/pop/event/Splatter.cmd

Load | Launch

This example shows how to apply different behaviors when events happen. The particles fall from the sky onto the sphere and stick to the collision object. Only when a second particle strikes the sphere do the particles then fall off the sphere.

This effect is accomplished by having different POPs applied to the particles over time. Initially particles are birthed from the grid and a gravitation force is applied. Let’s take a look at the input selection rule in the Switch POP.

----- popevent(“release”) || popevent(“switch”) -----

The popevent() function returns a 1 when the event is occurring and 0 otherwise. So, this statement says that whenever the event “release” or “switch” occur, use input 1, otherwise use input 0. To start, none of these events are occurring, so the first branch is cooked.

So, to start, the particles are processed by collision1 which collides them against sphere2 and stops them on contact.

Now, let’s look at source2 and collision3. At frame 100, a single particle is birthed which travels towards the sphere. In the Collision POP, an event, “release” will be generated when the particle hits the sphere. The generation of this event will cause the switch to start evaluating the other branch.

Instead of being processed by collision1, the original particles will be processed by state1, collision2, and event1. The State POP will “unstick” the particles so that they will continue to move.

The Collision POP now collides them against the sphere again, this time bouncing them with no gain normal. This will cause the particles to slide off the sphere.

Finally, an Event POP is used to generate the “switch” event, which will cause this branch to continue to be evaluated.

Note that there are actually two collision spheres, sphere1 and sphere2. sphere1 is just a little bit bigger and is the first object to test for collision. This is so that when they are released, they will bounce off the slightly smaller sphere2. Otherwise, the particles might continue to stick even after they have been unstuck because they are right on the boundary of the collision object. By making the second sphere slightly smaller, this numerical accuracy issue is relieved.

Usages in other examples

Example name	Example for
BounceSplit	Collision particle node	Load \| Launch
BirthRing	Collision particle node	Load \| Launch

Event particle node

Parameters

Local variables

Example files

Splatter

Usages in other examples