Houdini 21.0 Nodes Dynamics nodes

POP Steer Path dynamics node

Applies forces to agents/particles according to directions from a path curve.

Since 14.0

This is a crowd behavior node, but you can also use it on regular particles (see the Output Attribute parameter).

Tip

Once agents reach the end of the curve, they will continue to alternately move along the direction at the last point and be pulled back by the Path Variance, making them bounce around the end of the curve. You can avoid this by using a bounding box trigger at the end of the curve to put agents that reach the end into a new state.

Parameters

Activation

When turned off, this node has no effect. You can animate this to turn the behavior on or off at different times.

Group

Only modify certain groups out of all the agents/particles. You can name groups of agents using the Group Name parameter on the Crowd Source geometry node.

Guide

When turned on, shows the guide geometry for this node.

Note

Even if the guide geometry is turned on here, it can be turned off by using the hidden flag on the DOP node.

Output Attribute

Controls whether the node modifies the agent’s steerforce attribute (used by the crowd solver) or the particle’s force attribute (used by the POP solver). If you are working with agents, set this parameter to Crowds steerforce. If you are working with particles, set this parameter to POP force.

Weight

When Output Attribute is set to Crowd steerforce, this value is used by the crowd solver to normalize and scale multiple forces that apply to an agent.

Parameters

Source

Specifies the geometry to use.

SOP

Use the SOP specified in the SOP Path parameter.

DOP Data

Use the DOP specified in the DOP Object parameter.

First Context Geometry

Use the SOP connected to the DOP network’s first input.

Second Context Geometry

Use the SOP connected to the DOP network’s second input.

Third Context Geometry

Use the SOP connected to the DOP network’s third input.

Fourth Context Geometry

Use the SOP connected to the DOP network’s fourth input.

SOP Path

When Source is set to SOP, this is the path to an object or geometry node containing the path curve (for example, /obj/geo1/curve1).

DOP Object

When Source is set to DOP Data, this is the name of the DOP object.

DOP Data

When Source is set to DOP Data, this is the data attached to the DOP Object.

Method

Specifies how the node uses the curve to influence the agent’s movement.

Curve Lookup

Each agent finds the closest point on the curve and takes the forward tangent direction at that point. A force is applied to the agent in that direction.

Curve Point Walk

When an agent first “locks on” to the path, it finds the closest control point on the curve and remembers the control point’s point number in an attribute. From then on, it uses the curve direction at the control point and increases the point number when near the next control point on the curve (for example, it uses the curve tangent direction at point 2 until the agent is closer to point 3). This method allows you to use self-intersecting paths.

Resample Path

When Method is set to Curve Point Walk, turning this on uses a set of regularly spaced control points along the curve instead of using the actual control points from the curve geometry. This is usually the best option unless you've specifically placed the control points of the curve with path following in mind.

Segment Length

When Method is set to Curve Point Walk, and Resample Path is turned on, this is the spacing of the points along the curve to use.

Reverse Direction

When turned on, acts as if the direction of the curve is reversed when computing directions. (You can “actually” reverse a curve using the Reverse geometry node.)

Force Scale

The strength of the path direction force.

Constraint Force Scale

The amount of force applied to make the agents/particles maintain their initial distance to the curve.

Search Radius

The maximum distance (in meters) that agents look for the path.

Anticipation

How far ahead to look for path changes. The higher the number, the earlier agents/particles begin turning in response to upcoming bends in the curve.

Path Variance

The maximum distance (in meters) the agents/particles are allowed to be from the initial distance before the node begins to apply the Constraint Force Scale to move them back toward the curve.

Ignore Mass

When turned on, ignores any mass on the input particles.

Since forces are stored as force rather than accel (acceleration), this is done by multiplying the force by the mass attribute. This is then canceled out by the solver.

airresist is also similarly multiplied.

Ignoring mass ensures that small pieces of an RBD object move at the same speed as big pieces. This makes for a more controllable simulation.

Use VEXpressions

When turned on, uses a local expression to override specific parameters. Select from the drop-down menu on the right to see the variables that you can modify.

Bindings

Geometry

The name of the simulation data to apply the POP node to. This is commonly Geometry, but POP networks can be designed to apply to different geometry if desired.

Evaluation Node Path

For nodes with local expressions, this controls where ch() style expressions in VEX are evaluated with respect to. By setting this to “.”, you can ensure that relative references work. It is important to promote this if you are embedding a node inside an HDA if you are also exporting the local expressions.

See also

Dynamics nodes