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An increase in *mass* impedes acceleration for a given constant force.

*Drag* is a force opposing the direction of motion. Drag is velocity
sensitive: the larger the velocity, the greater the effect of drag. It’s
useful for limiting the velocity of particles.

This POP is similar to with wind force; however it does not accelerate particles to a maximum speed. The Force POP applies a constant directional force.

Tip

Use the Property POP to set "physical" attributes of particles such as mass and drag.

This operator modifies the following attributes: `accel`

.

See the Wind POP for different kind of directional force you can apply separately or in combination with this operator.

## 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.

## Force

Reference Object

Path of an object from whose transform to extract the force
direction. Overrides the **Force** parameter below.

Force

Strength of the force along each axis.

Scale

Scaling factor for the force.

Ignore Mass

Ignores any mass attributes on the input particles.

Override Mass

Replaces any mass attributes on the input
particles with the value of the **Mass** parameter below.

Mass

Mass to use when **Override mass** is on.

## Noise

The parameters on this tab add randomness to the force. This makes particles more or less affected at random, creating a less uniform effect.

Seed

Seed value for the random turbulence generator.

Turbulence

Number of iterations of fractal noise to add.

Roughness

Scale of noise added with each iteration.

Exponent

Noise attenuation exponent.

Frequency

Spatial frequency of noise field in X, Y, and Z.

Amplitude

Maximum value of noise field.

Offset

Amount to shift noise along X, Y, and Z axes.

Noise Type

Method used to generate noise. **Hermite Interpolation** and **Improved Hermite** are fastest.

Hermite Interpolation

Uses splines to interpolate values in the noise field.

Sparse Convolution

More computationally expensive, but produces noise fields free of artifacts present in the Hermite methods.

Improved Hermite

Uses a more linear interpolation than Hermite interpolation.

Alligator Noise

Provides a very different look than the other noise types.

## 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.

## Examples

The following examples include this node.

AverageSpeed Example for Vector channel node

This example shows you how to calculate the average speed for each particle over a given time interval using CHOP nodes such as Vector, Trim, and Math.

popswithrbdcollision Example for RBD Point Object dynamics node

Shows an RBD Simulation being attatched to a POP simulation to provide RBD style collisions to POPs.

ConvertMetaballs Example for Convert Meta geometry node

This example demonstrates using the Convert Meta SOP to convert metaballs to polygons.

IntersectGrid Example for Intersect VOP node

This example demonstrates how grid points can be transferred to particles on the surface of a sphere using the Intersect VOP.

See also |