# Property particle node

Manually set physical attributes.

 All Parameters Local variables Example files

This operator does not affect particle behavior independently. Instead, you use it to set “physical” attributes such as mass, drag, and charge that control how other operators, such affect the particles.

You can also use this operator to set the `follow` attribute, which controls which leader a particle follows in the Follow POP, which attractor point affects a particle in the Attractor POP, and which center to orbit in the Orbit POP

This operator modifies the following attributes: `attract`, `charge`, `drag`, `follow`, `mass`, `oaxis`, `ocenter`, `ocntrid`, `oradius`, `ospeed` and `pscale`.

## 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.

### Physical

The parameters on this tab set attributes corresponding to real world physical properties on the input particles. Other operators use these attributes to simulate the effect of forces on the particles.

 Mass Sets the mass of the input particles.Changing the mass of a particle scales forces on the particle according to the following formula:F = M * Awhere F is force, M is mass, and A is acceleration. So, particles with high mass require more force (F) to change their motion. Center of Mass Sets the relative position of the center of mass for input particles.This is a displacement in object space from the origin (0,0,0) of the object. The rigid body particle representing an object is always placed at the origin of the object, which may not be the real center of the geometry. In this case you would specify a center of mass so that the particle behaves as if its center of mass is at the center of the geometry. Bounce Sets the bounce coefficient of the input particles.When two objects collide, the product of the bounce coefficients of the objects determines the energy lost to the collision (and so the velocities of the object after the collision). Dynamic Friction Sets the attribute controlling friction between two moving rigid body particles.When two objects are in contact and sliding across each other, the product of their dynamic friction values determines the force of friction acting against the sliding motion. Static Friction Sets the attribute controlling friction between two stationary rigid body particlesWhen two objects are in contact and relatively at rest, the product of their static friction values determines the amount of force required to induce a relative (sliding) motion of the objects.

### Orbit

The parameters on this tab control how the input particles orbit a center in the Orbit POP.

 Axis The axis around which the particle rotates (a vector normal to the plane of rotation). Center 3D point to orbit. The interpretation of this value is controlled by the Orbit POP. Radius Radius of the orbit in units. Speed Speed of the orbit in RPM. Orbit Index ID of the point in the “Center Group” to be used as the center of orbit. The interpretation of this value is controlled by the Orbit POP.

### Misc

The parameters on this tab set attributes used by the Attractor POP, Follow POP, Interact POP, and other operators.

 Follow Index The ID of the particle to follow. The interpretation of this value is controlled by the Follow POP. Attractor Point The point that attracts/repels the particle in the Attractor POP. Charge The positive or negative “electrical charge” of the particle in the Interact POP. Particles with opposite charges attract, and particles with like charges repel. Uniform Scale Sets a scaling factor for the input particles. This may be used by the Interact POP to determine the size of the particle’s field of influence on other particles, and by the Render POP to control the rendered size of particles. Scale Sets non-uniform scaling factors for the input particles. Drag Sets the drag attribute of the input particles. Other operators use this attribute to control how forces affect the particles. Cling Sets the cling factor, used when the particles are sliding. Note Cling is implemented as a force that acts to counter forces pulling the particle away from the surface. Cling is not applied if there is no force to counter. Sliding particles apply friction to act against the tangential motion of the particle.

## 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.

 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.

 CA Point or vertex alpha value. CR CG CB Diffuse point or vertex color.

 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.

 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.

 ROTA Rotation angle. ROTX ROTY ROTZ Rotation axis.

 GEN Generation. ID ID number, which always remains constant. ORIGIN Original Source point was birthed from. PARENT Parent’s ID Number.

 SPEEDMAX Maximum speed. SPEEDMIN Minimum speed.

 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.

 PVX PVY PVZ Previous velocity. UPX UPY UPZ Up vector.

Added by Location, Source, Softbody, Split POPs

 SPEED Absolute speed of particle.

## Example files

### Swarm

`\$HFS/houdini/help/examples/nodes/pop/property/Swarm.cmd`

This network utilizes the Property POP to give particles certain Orbit attributes which in turn are referenced by the Orbit POP. A Mass value, for example, is one of the attributes that can be assigned by the Property POP. Here, one particle is given a much greater mass hence affecting the orbiting motion of all the other lighter particles.

The greater the mass of a particle, the less other forces (including those from other particles) can influence it.

There are two particle systems in this example:

Particle system 1

The source1 POP births several particles, with a random forward velocity. The property POP gives the particles orbit attributes oaxis, oradius and ospeed. These attributes are only useful in conjunction with the Orbit POP. The orbit axis is oriented along the z-axis, with some variance. The expression rand(\$ID) is used to generate a random number that is unique per particle, but remains constant throughout the animation. The orbit radius also uses this expression to assign a radius between 1 and 1.5 units. The orbit speed is proportional to the orbit radius. The group1 POP groups this particle system as “light”.

Particle system 2

The source2 POP births a single particle, with a random forward velocity. The property1 POP is used to assign a mass of 20 to this particle. This is done so that when this system is combined with the first system, the center of mass will be closer to this particle. The property2 POP gives the particle an orbit axis oriented along the z-axis, an orbit radius of 1 unit, and an orbit speed of 10 RPM. The group2 POP groups this particle system as “heavy”.

The two systems are combined with the collect1 POP. The orbit1 POP makes both particle systems orbit about their combined center of mass. None of the overrides (for axis, radius and speed) are set so that the individual attributes can be used for each particle.

As the animation plays, change the mass of the heavy particle in the property1 POP to see the effect on the center of the orbit. Also try changing the Center Type setting in the orbit1 POP to Spatial or Density Center to see the effect.

### Usages in other examples

Example name Example for

Orbit particle node

Rotation particle node