Houdini 17.0 Nodes Dynamics nodes

FEM Target Constraint dynamics node

Constrains an FEM object to a target trajectory using a hard constraint or soft constraint.

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This involves constraining a set of points on the soft body object to a target animation. The soft body object may be a Solid Object, Hybrid Object, or Wire Object.

The target animation may be specified by the point attribute targetP on the soft body object’s geometry. In the case of the Solid Object and the Hybrid object, the target animation may also be specified using the Target Deformation parameter instead of the targetP attribute.

This constraint provides a method for applying hard or soft constraints to individual points. If the Type is set to Hard, the constrained points will follow the target trajectory exactly under all circumstances, even if collisions happen. If the Type is set to Soft, the constrained points will approximately follow the target animation through the use of forces. The soft method is preferable for simulations that involve collisions. The higher the Target Strength, the larger the force that tries to match the simulated point positions to the target animation point positions. The Target Damping parameter can be fine tuned such that the soft method does not overshoot the target.

The Soft Controls on the Target Constraint work different from the Target Strength and Target Damping parameters on the Solid Object and the Hybrid Object. The Target Constraint looks at individual points and it applies lumped force contributions at these points; the coefficients are not densities, but they control the entire contribution at each constrained point. In contrast, the parameters on the Solid/Hybrid Object and the corresponding point multipliers attributes control the densities of fields that are distributed over the entire object.

Target Constraints should be used when precise control over individual points is desired and the Target Controls on the Solid/Hybrid Object should be used when a smooth falloff of targeting forces is desired over a contiguous object.

SBD Constraint is a digital asset.

Parameters

Constraint

Type

Whether to use a hard constraint or a soft constraint.

Constrained Object

The soft body object to be constrained.

Constrained Points

The group of points on the object’s geometry that are to be constrained. It is recommended to use the name of a group that is created in SOPs, instead of specifying a single point number or a hard-coded range (e.g., 1-99). This allows constraints to work in simulations where the topology is modified by a SOP solver.

Soft Controls

Strength

This determines how strongly the soft constraint matches the target positions.

Damping

This determines how strongly the soft constraint matches the target velocities.

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 if using pin constraints.

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.

Examples

The following examples include this node.

AnimatedClothPatch Example for Cloth Object dynamics node

This example shows how a piece of cloth that is pinned on four corners. These corners are constrained to the animated geometry.

BendCloth Example for Cloth Object dynamics node

This cloth example demonstrates how the stiffness of your cloth object can be defined by using the strong or weak bend parameters.

BendDamping Example for Cloth Object dynamics node

This cloth example demonstrates the use of the Damping parameter to control how quickly a cloth object will come to its rest position.

ClothUv Example for Cloth Object dynamics node

This is an example that shows how you can specify the warped and weft directions on a triangulated cloth planel using uv coordinates.

Because the uv directions are aligned with the xy directions of the grid, the result looks nearly identical to a quad grid, even though the mesh is triangulated.

The little blue and yellow lines visualize the directions of the cloth fabric. This is enabled in the Visualization tab of both cloth objects.

See also

Dynamics nodes