Houdini 20.5 Nodes Dynamics nodes

Cone Twist Constraint dynamics node

Constrains an object to remain a certain distance from the constraint, and limits the object’s rotation.

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Since 12.5

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.

RBD Cone Twist Constraint is a digital asset.

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

Using RBD Cone Twist Constraints

  1. Click the RBD Cone Twist 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 cone twist 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 Twist Axis and Goal Up Axis on the Cone Twist tab in the parameter editor. To restrict the range of motion, modify Max Up Rotation, Max Out Rotation and Max Twist.

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.

Cone Twist

Max Up Rotation

The maximum rotation up or down in degrees.

Max Out Rotation

The maximum rotation from side to side in degrees.

Max Twist

The maximum twist in degrees.

Softness

Once an angle is greater than softness * the maximum angle, the constraint begins to take effect. Lowering the value of softness softens the constraint boundaries.

Allow Initial Violation of Limits

If the rotation limits are initially violated, the limits will not be enforced but further rotation will be prevented. This allows the objects to naturally move back within the rotation limits, instead of introducing sudden motion at the beginning of the simulation.

Enable Soft Constraint

When enabled, the position limits, rotation limits, and motor target are treated as soft constraints with individual Stiffness and Damping Ratio parameters. This is primarily useful for following an animated Target Rotation or Target Position in a spring-like manner (e.g. for a ragdoll with target animation), but also allows the position or rotation limits to behave as softer boundaries by decreasing their stiffness.

Constraint Force Mixing

Increase this to make the constraint spongier, and potentially increase the stability of the simulation. The angular component of the constraint may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Bias Factor

The rate at which the constraint corrects errors in orientation. A value of 1 will ensure that the constraint is always obeyed. It is recommended to keep bias between 0.2 and 0.5.

Relaxation Factor

The rate at which the angular velocity is changed by the constraint. A low value means the constraint will modify the velocities slowly, leaving the boundaries appearing softer.

Position CFM

Increase this to make the constraint spongier, and potentially increase the stability of the simulation. The position component of the constraint may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.

Position ERP

Specifies what proportion of the position error will be fixed during the next simulation step. A value between 0.1 and 0.8 is recommended for most simulation.

Position Limit Stiffness

Specifies the strength of the force that attempts to enforce position limits. This value is equivalent to the frequency of a spring.

Position Limit Damping Ratio

Specifies how much damping is applied to the motion when enforcing position limits. This value is equivalent to the damping ratio of a spring. A value of 0 specifies no damping, and a value of 1 provides just enough damping to prevent oscillation. Values between 0 and 1 allow oscillation (with some damping), and values greater than 1 provide increasingly damped motion that has no oscillation.

Angular Limit Stiffness

Specifies the strength of the force that attempts to enforce rotation limits. This value is equivalent to the frequency of a spring.

Angular Limit Damping Ratio

Specifies how much damping is applied to the motion when enforcing rotation limits. This value is equivalent to the damping ratio of a spring. A value of 0 specifies no damping, and a value of 1 provides just enough damping to prevent oscillation. Values between 0 and 1 allow oscillation (with some damping), and values greater than 1 provide increasingly damped motion that has no oscillation.

Goal Twist Axis

The goal direction of the cone. 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 twist axis. The out axis is calculated as the cross product of the twist and up axes.

Goal Twist Offset

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

Constrained Twist Axis

The initial twist axis of the constrained object.

Constrained Up Axis

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

Constrained Twist Offset

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

Enable Motor

If enabled, the constraint will attempt to also guide the constrained object to a target orientation and position within the rotation limits.

Target Current Pose

The target position and orientation will continually be set to the current relative transform, similar to plasticity. This can be used to add resistance to changes in the relative orientation (controlled by the Target Angular Stiffness or Max Impulse) when there isn’t a specific target.

Target Rotation

Specifies the target orientation (relative to the goal anchor) that the motor should attempt to achieve.

Use Previous Target

Optionally specifies the motor target at the beginning of the timestep. The solver will interpolate the motor target at each substep for more accurate behavior when the motor target is animated.

Initial Target Rotation

Specifies the Target Rotation at the beginning of the timestep.

Ignore Mass

Factors out the mass of the objects when setting the Max Impulse for the constraint. This makes it simpler to set up motors with a similar strength for different pairs of objects.

Max Impulse

Specifies the maximum impulse that the constraint solver can apply to achieve the Motor Target. Larger values will cause the motor to be stronger.

Correction Time

Specifies how gradually the constraint attempts to correct deviations from the Motor Target.

Constraint Force Mixing

Increasing this value makes the motor component of the constraint softer. A small positive value can increase the stability of the simulation.

Guide Options

Show Softness Threshold

Show where the constraint begins to take effect. This is only used if Softness is greater than 0 and less than 1.

Color

The color of the primary guide geometry.

Secondary Color

The color of the secondary guide geometry. This includes the Softness Threshold and the indicator of the current twist.

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

RagdollExample Example for Cone Twist Constraint dynamics node

This sample creates a simple ragdoll using the cone twist constraint between pieces of the ragdoll.

Dynamics nodes