Houdini 19.0 Nodes Dynamics nodes

FEM Attach Constraint dynamics node

Constrains a set of points on the surface of one FEM object to a set of points on the surface on another FEM object or a static object.

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Since 18.5

The FEM Attach Constraint SOP node glues the points of a source object to relative locations on the surface of a target object. This constraint can glue together two FEM objects, an FEM object to an animated static object, or all the components of a single FEM object. For example, you can use this constraint to attach muscles to bones or create skin wrinkling when paired with the FEM Slide Constraint DOP.

In addition to a direct attachment, you can can also set the Distance Threshold parameter to 0 to allow a certain amount of relative movement to the source object before the constraint starts to restrict its motion.

This constraint is more flexible than the FEM Fuse Constraint DOP, which requires a point-to-point correspondence.

Example: Constrained box sticking to the surface of the torus

Parameters

Coefficients

Strength

Controls how quickly the constraint point adapts its position to the relative position that it is sliding against.

Damping

Controls how quickly the constraint point adapts its velocity to the goal.

Source

Object

Specifies the FEM Object to constrain.

Registration Attribute

Specifies the position attribute that is used to compute all link lengths and directions.

Target

Object

Specifies the target FEM Object or Solid Object. Choose the same object as ***A*** to allow sliding between connected components of this object.

Registration Attribute

Specifies the position attribute that is used to compute all constraint distances and directions.

Filter

Point Group

Specifies the name of the group outside of which no points are constrained.

Distance Threshold

When on, the distance threshold is used when determining which points to constrain. Only points that have a goal surface within the specified distance threshold are constrained.

Distance Threshold Attribute

When on, specifies the name of a multiplier point attribute for the Distance Threshold.

Filter by Source Border Side

When on, source points are only constrained when the link to the closest target point touches ***that*** source point on the Source Border Side.

Source Border Side

Specifies the side of the source mesh border on which links are allowed: Interior or Exterior.

Filter by Source Self Occlusion

When on, source points are only constrained when the link to the closest target point does not intersect the source border.

Filter by Target Border Side

When on, source points are only constrained when the link to the closest target point touches ***that*** target point on Target Border Side.

Target Border Side

Specifies the side of the target mesh border on which links are allowed: Interior or Exterior.

Behavior

Attach to Local Space

When on, constrains a rigidly animating Static Object to a spatial location outside the mesh instead of a location on the surface.

Enable Strength Multiplier Attribute

When on, Strength is locally scaled by a point attribute.

Strength Multiplier Attribute

Specifies the name of the multiplier point attribute for the Strength.

Enable Damping Multiplier Attribute

When on, Damping is locally scaled by a point attribute

Damping Multiplier Attribute

Specifies the name of the multiplier point attribute for the Damping.

Rest Initialization

Determines how rest is initialized: based on being zero, based on a reference position, or based on an absolute rest length.

Rest Distance

Specifies the absolute rest length for each of the constraint links.

Enable Rest Multiplier Attribute

When on, locally multiply the rest length using a point attribute.

Output Attributes

Enable Target Position Attribute

When on, allows the target position of each constraint link to be written to a point attribute on the source.

Target Position Attribute

Specifies the name of the attribute to which the target position is written at each frame.

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

Controls the color of the guide geometry.

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 MMB 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.

See also

Dynamics nodes