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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
Click the RBD Cone Twist Constraint tool on the Rigid Bodies tab.
Select the object to constrain and press Enter to confirm your selection.
Select the position for the cone twist constraint and press Enter to confirm your selection.
You can hold Alt to detach the constraint from the construction plane.
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.
Identifies the RBD Object to be constrained.
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.
Specifies a location in world space used to initialize the local object space position of the constraint.
Specifies a location in world space used to initialize the local object space position of the constraint in the goal object.
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.
Disables collision detection between the constrained objects.
Max Up Rotation
The maximum rotation up or down in degrees.
Max Out Rotation
The maximum rotation from side to side in degrees.
The maximum twist in degrees.
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.
Constraint Force Mixing
Increase this to make the constraint spongier, and potentially increase the stability of the simulation. The constraint may be violated by an amount proportional to the force required to re-establish the constraint, times this parameter.
The rate at which the constraint corrects errors in position. A value of 1 will ensure that the constraint is always obeyed. It is recommended to keep bias between 0.2 and 0.5.
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.
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.
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.
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.
If enabled, the constraint will attempt to also guide the constrained object to a target orientation within the rotation limits.
Target Current Pose
The Motor Target will be set to the current orientation. This can be used to add stiffness to the constrained object and resist changes to its relative orientation.
Specifies the target orientation (relative to the goal anchor) that the motor should attempt to achieve.
Use Initial Motor 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 Motor Target
Specifies the motor target at the beginning of the timestep.
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.
Specifies the maximum impulse that the constraint solver can apply to achieve the Motor Target. Larger values will cause the motor to be stronger.
Motor Correction Time
Specifies how gradually the constraint attempts to correct deviations from the Motor Target.
Increasing this value makes the motor component of the constraint softer. A small positive value can increase the stability of the simulation.
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.
The color of the primary guide geometry.
The color of the secondary guide geometry. This includes the Softness Threshold and the indicator of the current twist.
Scales the guide geometry.
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.
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.
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.
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.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
This string contains a space separated list of the names of every object being processed by the current node.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
strcmp($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).
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.
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.
This sample creates a simple ragdoll using the cone twist constraint between pieces of the ragdoll.
The following examples include this node.