Orbit particle node

Makes particles orbit a center-point.

All

Parameters

Inputs

Local variables

Example files

This operator calculates and applies forces necessary to make particles orbit around a center-point.

You can use the second input to specify the center particles, or use the parameters on the Center tab to choose a group in the first input that will specify the center.

The Center Type parameter on the Center tab let you choose how the center works:

You can specify the center with 3D coordinates.
The “center” can be a computed point based on the center particles (the spatial center of the leader particles, the center of mass, or the center of density).
Each particle in the center stream can be the center for individual particles from the satellite stream. Houdini can automatically assign satellites to centers, or you can set a orbit attribute on particles to determine which center they will orbit. Use the Property POP to set the orbit-related attributes on particles.

Orbit works similarly to the Follow POP, which makes particles follow a leader or leaders.

Tip

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

This operator modifies the following attribute: accel.

Using Orbit

Select the particle system you want to be affected.
Click the Orbit tool on the Drive Particles tab.
Specify the node settings in the operation controls toolbar or parameter editor.

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.

Orbit

Override Axis	Use the value of the Axis parameter below instead of the orbit axis attribute if it exists on a particle.
Axis	A vector to use as the orbit axis, when Override axis is on.
Override Radius	Use the value of the Radius parameter below instead of the orbit radius attribute if it exists on a particle.
Radius	Radius of orbit (in units), when Override Radius is on.
Override Speed	Use the value of the Speed parameter below instead of the orbit speed attribute if it exists on a particle.
Speed	Speed of the orbit (in RPM), when Override Speed is on.
Scale	Scaling factor that controls how fast particles move into their proper orbit from their initial position.

Center

Center Group

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

Center Type

Specifies what the particles orbit around.

Particle Spatial Center	The center of the box bounding the center particles.
Particle Center of Mass	The center of mass of the center particles.
Particle Density Center	The center of mass, with all particles having equal mass.
Center Point	Use the coordinates in each particle’s orbit center attribute (or the coordinates in the Center Point parameter below if Override Center Point is on).
Individual Particles	Each orbiting particle orbits a center particle. If you have `orbit` attributes on the follower particles, they will use the attribute to decide which leader to follow (using the Orbit index parameter below). Otherwise, Houdini will automatically assign followers to leaders.

Override Center Point

Use the value of the Center Point parameter below instead of the orbit center attribute if it exists on a particle, when Center Type is “Center point”.

Center Point

Point to orbit around, when Center type is “Center point” and Override Center Point is on.

Orbit Index

(Available when Center type is “Individual Particles”) Controls how the node interprets the orbit attribute on particles to assign satellites to centers.

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

Particle ID	The `orbit` attribute on the satellite particles refers to the particle ID of the center they should orbit.
Particle Order	The `orbit` attribute on the follower particles refers to the position of the center particle in the list of particles. In a particle system where leader particles are dying and points are being reused, this could mean the centers change as particles die (even if they are not in the center stream).

Inputs

Orbit	The particles that will be made to orbit the center or centers.
Center	The particles that define the center(s) to orbit. If you do not connect this input, you must set the Center Group parameter on the Center tab.

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

MultiAxisOrbiting

$HFS/houdini/help/examples/nodes/pop/orbit/MultiAxisOrbiting.cmd

Load | Launch

This network demonstrates how the Group POP can be used in conjunction with the Orbit POP to create a multi-axis orbiting effect.

The result resembles the classic depiction of orbitting electrons around a nucleus.

The Group POP creates 7 groups each of which allows particles to orbit around specific axes designated by the Orbit POP.

Orbit

$HFS/houdini/help/examples/nodes/pop/orbit/Orbit.cmd

Load | Launch

This example demonstrates how the Orbit POP can be used to make a particle system orbit another particle system.

Particle System 1

The source_particle_system_1 POP births a single particle. The orbit1 POP causes this particle to orbit the origin in the XZ-plane. The group1 POP then groups the particle system as “level1”.

Particle System 2

The source_particle_system_2 POP also births a single particle. The group2 POP then groups the particle system as “level2”.

These particle systems are collected by the collect1 POP. The orbit2 POP causes level2 to orbit the center of mass of the level1 particle system.

Particle System 3

The source_particle_system_3 POP also births a single particle. The group3 POP then groups the particle system as “level3”.

This particle system, as well as the combined systems 1 and 2, are collected by the collect2 POP. Finally, the orbit3 POP causes level3 to orbit the center of mass of the level2 particle system.

OrbitAlternate

$HFS/houdini/help/examples/nodes/pop/orbit/OrbitAlternate.cmd

Load | Launch

This example demonstrates how the Orbit POP can alter the acceleration of particles to rotate about a single or multiple centers.

It also shows how a Null OBJ can be used to create animation to be referenced at the POP level to create a dynamic network.

SimpleOrbit

$HFS/houdini/help/examples/nodes/pop/orbit/SimpleOrbit.cmd

Load | Launch

This is a simple network using the Orbit POP with a sphere as the particle source.

The particles rotate around a specified axis designated by the Orbit POP.

Usages in other examples

Example name	Example for
SplitAndStick	Follow particle node	Load \| Launch
Swarm	Property particle node	Load \| Launch