Houdini 20.5 Nodes Dynamics nodes

Slider Constraint dynamics node

Constrains an object to rotate and translate on a single axis, and limits the rotation and translation on that axis.

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Since 12.1

This involves constraining some location on the RBD Object to a goal location derived from another simulation object or from a position in world space.

Parameters describing how the constraint is applied are broken up into two sets, found under the Hinge and Orthogonal tabs of the Slider pane. Parameters under the Hinge tab affect how the constraint is applied when the current rotation or translation on the hinge axis have exceeded their defined limits. Parameters under the Orthogonal tab affect how the constraint is applied when rotation or translation has occurred on axes other than the hinge axis.

RBD Slider Constraint is a digital asset.

This constraint type is currently only supported by the Bullet solver.

Using RBD Slider Constraints

  1. Click the RBD Slider Constraint tool on the Rigid Bodies tab.

  2. Select the object to constrain and press Enter to confirm your selection.

  3. Select the position for the slider constraint and press Enter to confirm your selection.

    Note

    You can hold Alt to detach the constraint from the construction plane.

  4. Set the Goal Hinge Axis and Goal Up Axis on the Slider tab in the parameter editor. To restrict the range of motion, modify Max Rotation and Slide Range.

Parameters

Constraint

Constrained Object

Identifies the RBD Object to be constrained.

Goal Object

Identifies an RBD Object used to determine the goal position. If this parameter is left blank, the objects will be constrained to a world space position.

Constrained Location

Specifies a location in world space used to initialize the local object space position of the constraint.

Goal Location

Specifies a location in world space used to initialize the local object space position of the constraint in the goal object.

Constraint Iterations

If greater than zero, overrides the number of iterations performed by the constraint solver for this constraint. If some groups of constraints require more iterations than others, this parameter can be used instead of globally increasing the number of iterations on the solver.

Disable Collisions

Disables collision detection between the constrained pair of objects.

Slider

Max Rotation

The maximum rotation about the hinge axis in degrees.

Slide Range

The range in which the constrained object may slide.

Axes

Goal Hinge Axis

The goal direction the constraint may slide or rotate on. Defaults to the X axis.

Goal Up Axis

The goal direction of the up axis. Defaults to the Y axis. This should be perpendicular to the hinge axis. The out axis is calculated as the cross product of the hinge and up axes.

Goal Rotation Offset

This parameter rotates the Goal Up Axis around the Goal Hinge Axis. Specified in degrees.

Constrained Hinge Axis

The initial hinge axis of the constrained object.

Constrained Up Axis

The initial up axis of the constrained object. This should be perpendicular to the constrained hinge axis.

Constrained Rotation Offset

This parameter rotates the Constrained Up Axis around the Constrained Hinge Axis. Specified in degrees.

Hinge

Position Softness

Increase this to soften the bounds on position along the hinge axis. This specifies the rate at which the constraint corrects positional errors along the hinge axis.

Position Damping

Increase this to cause the constrained object to bounce once it has slid to the end of its range. When the constrained object reaches a positional limit along the hinge axis, a higher damping value increases the velocity in the direction opposite the current direction along the hinge axis.

Position Cfm

Increase this to loosen the bounds on position along the hinge axis, and potentially increase the stability of the simulation. The positional constraint along the hinge axis may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Angle Softness

Increase this to soften the bounds on rotation about the hinge axis. This specifies the rate at which the constraint corrects rotational errors around the hinge axis.

Angle Damping

Increase this to cause the constrained object to bounce once it has rotated to the end of its range. When the constrained object reaches an angular limit around the hinge axis, a higher damping value increases the angular velocity in the direction opposite the current direction around the hinge axis.

Angle Cfm

Increase this to loosen the bounds on rotation about the hinge axis, and potentially increase the stability of the simulation. The angular constraint about the hinge axis may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Ortho

Position Softness

Increase this to the soften the alignment of the position of the goal and constrained hinge axes. This specifies the rate at which the constraint corrects positional errors around axes other than the hinge axis.

Position Cfm

Increase this to loosen the alignment of the position of the goal and constrained hinge axes, and potentially increase the stability of the simulation. The positional constraint along axes other than the hinge axis may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Angle Softness

Increase this to soften the alignment of the rotation of the goal and constrained hinge axes. This specifies the rate at which the constraint corrects rotational errors around axes other than the hinge axis.

Angle Cfm

Increase this to loosen the alignment of the rotation of the goal and constrained hinge axes, and potentially increase the stability of the simulation. The angular constraint along axes other than the hinge axis may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Guide Options

Show Guide Geometry

Enable the display of guide geometry.

Color

The color of the primary guide geometry.

Secondary Color

The color of the guides indicating the current rotation around the hinge axis.

Guide Size

Scales the guide geometry.

Show Object Link

This parameter controls the display of guide geometry connecting the constraint to the constrained object.

Activation

Determines if this node should do anything on a given timestep and for a particular object. If this parameter is an expression, it is evaluated for each object (even if data sharing is turned on).

If it evaluates to a non-zero value, then the data is attached to that object. If it evaluates to zero, no data is attached, and data previously attached by this node is removed.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.

All Other Inputs

If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.

The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

Outputs

First Output

The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).

If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.

Locals

ST

The simulation time for which the node is being evaluated.

Depending on the settings of the DOP Network Offset Time and Scale Time parameters, this value may not be equal to the current Houdini time represented by the variable T.

ST is guaranteed to have a value of zero at the start of a simulation, so when testing for the first timestep of a simulation, it is best to use a test like $ST == 0, rather than $T == 0 or $FF == 1.

SF

The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.

Depending on the settings of the DOP Network parameters, this value may not be equal to the current Houdini frame number represented by the variable F. Instead, it is equal to the simulation time (ST) divided by the simulation timestep size (TIMESTEP).

TIMESTEP

The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.

SFPS

The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.

SNOBJ

The number of objects in the simulation. For nodes that create objects such as the Empty Object DOP, SNOBJ increases for each object that is evaluated.

A good way to guarantee unique object names is to use an expression like object_$SNOBJ.

NOBJ

The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.

NOBJ may return 0 if the node does not process each object sequentially (such as the Group DOP).

OBJ

The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies the object’s position in the current order of processing.

This value is useful for generating a random number for each object, or simply splitting the objects into two or more groups to be processed in different ways. This value is -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

The unique identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.

This value is also the best way to look up information on an object using the dopfield expression function.

OBJID is -1 if the node does not process objects sequentially (such as the Group DOP).

ALLOBJIDS

This string contains a space-separated list of the unique object identifiers for every object being processed by the current node.

ALLOBJNAMES

This string contains a space-separated list of the names of every object being processed by the current node.

OBJCT

The simulation time (see variable ST) at which the current object was created.

To check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

A string value containing the name of the object being processed.

Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.

The object name can also be used to treat a number of similar objects (with the same name) as a virtual group. If there are 20 objects named “myobject”, specifying strcmp($OBJNAME, "myobject") == 0 in the activation field of a DOP will cause that DOP to operate on only those 20 objects.

This value is the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

A string value containing the full path of the current DOP network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP network that contains the node.

Note

Most dynamics nodes have local variables with the same names as the node’s parameters. For example, in a Position DOP, you could write the expression:

$tx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

Examples

GravitySlideExample Example for Slider Constraint dynamics node

This sample creates a box which can only slide and rotate on one axis, using the Slider Constraint.

Dynamics nodes