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RBD Angular Spring Constraint dynamics node

Constrains an RBD object to have a certain orientation, but with a set amount of springiness.

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This involves constraining the orientation of the RBD Object to a "goal" orientation derived from another simulation object or from an orientation in world space.

RBD Angular Spring Constraint is a digital asset.

Using RBD Angular Spring Constraints

  1. Click the RBD Angular Spring 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 angular spring constraint and press Enter to confirm your selection.

    Note

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

  4. Set the Strength and Dampening on the Spring tab in the parameter editor.

Note

To parent to another object, use the Parent Constraint tool.

Parameters

Constraint

Constrained Object

Identifies the RBD object to be constrained.

Goal Object

Identifies an RBD object used to determine the goal orientation. If this parameter is left blank then the objects will be constrained to a world space orientation.

Constrained Location

Specifies a position 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 goal position for the constraint.

Goal Rotation

Specifies a rotation in world space used to initialize the goal rotation for the constraint.

Mirror Constraint

If enabled, all objects involved in the constraint will be made mutual affectors.

Spring

Strength

Controls how strongly the spring constraint acts on the constrained object to return the anchors to the same orientation.

Damping

Controls the amount of damping in the spring relationship. As damping increases, the spring constraint acts more slowly and oscillates less.

Rest Rotation

Sets the rest angle between the object and goal. The force applied on a constrained object by a solver will tend to keep the object rotated by this amount relative to the goal object.

Limit Torque

If this is set, the constraint will be disabled if the torque applied to satisfy this constraint exceeds the maximum specified by the "Maximum Torque" parameter.

Maximum Torque

Sets a threshold for disabling the constraint. If the torque applied to satisfy this constraint exceeds this threshold, the constraint will be disabled.

Limit Rotation

If this is set, the constraint will be disabled if the angle between the two constrained objects exceeds the maximum specified by the "Maximum Rotation" parameter.

Maximum Rotation

Sets a threshold for disabling the constraint. If the angle between the two constrained objects exceeds this threshold, the constraint will be disabled.

Bullet Data

The following parameters are used by the Bullet Solver.

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 objects.

Guide Options

Show Guide Geometry

Turning on this option causes guide geometry to be displayed in the viewport representing this constraint.

Radius

Controls the radius of the spheres drawn in the viewport as guide geometry for this constraint.

Color

This parameter controls the color of 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

This value is the simulation time for which the node is being evaluated.

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

This value 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

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

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

TIMESTEP

This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.

SFPS

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

SNOBJ

This is the number of objects in the simulation. For nodes that create objects such as the Empty Object node, this value will increase for each object that is evaluated.

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

NOBJ

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

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

OBJ

This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just 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 will be -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

This is the unique object 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.

The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example.

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

This value is the simulation time (see variable ST) at which the current object was created.

Therefore, to check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

This value is the simulation frame (see variable SF) at which the current object was created.

This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

This is 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 only on those 20 objects. This value will be the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

This is 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 node, you could write the expression:

$tx + 0.1

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

Dynamics nodes