Cloth Stitch Constraint dynamics node

Creates a number of constraints on a cloth object with a single node.

All

Parameters

Inputs

Outputs

Local variables

This is most often done to stitch two panels of cloth together, or to constrain a whole edge of a piece of cloth rather than one or two points. It can also be useful for setting up tear-able cloth.

If you only need to create one or two constraints, the Cloth Constraint DOP may be more useful. It’s a good place to start for understanding cloth constraints generally.

Using Cloth Stitch Constraint

Click the Cloth Stitch Constraint tool from the Cloth tab.
Select the dynamic points defining one side of the seam, and press Enter to confirm your selection.
Select the optional dynamic points defining the second side of the seam, and press Enter to confirm your selection.

Parameters

Anchors

Stitch Using Seam Attributes

Turning on this parameter builds constraints within a single cloth object using seam point attributes.

Seam attributes can be built using the Cloth Create Seam and related SOPs. Most other parameters are disabled when this is turned on.

Cloth Point Group

This parameter specifies which points in the cloth object are to be constrained.

This parameter can be a point group, a numeric range (such as 5-15), or a combination of the two specified in a space separated list.

Goal Anchor

Controls how the goal positions of the cloth constraints will be determined.

Points on Goal Object Geometry

The cloth points will be constrained to match the positions of points on another simulation object. You must enter the Goal Object Name and the Goal Point Group.

If the other object is also a cloth object and you want mirrored constraints (i.e., both objects to be constrained to each other), you should also specify the Cloth Object Name, and apply this same stitch constraint to both objects.

Points on Input Geometry

The cloth points will be constrained to match the positions of the corresponding points on the input geometry of the cloth object.

Points on Same Cloth Object Geometry

The cloth points will be constrained to match the positions of other points on the same cloth object. You must enter the Goal Point Group.

Goal Point Group

This parameter specifies which points on the goal object will be the goal positions for the cloth points.

Cloth Object Name

This parameter specifies the name of the cloth object to be constrained.

Both the goal and cloth objects need to be explicitly named when creating a two-way constraint as data needs to be attached to each one referring to the other.

Goal Object Name

This parameter specifies the name of the goal object.

Build Mirror Constraints

When Goal Anchor is set to Points on Same Cloth Object Geometry, this parameter allows you to control how the Cloth Point Group and Goal Point Group interact.

If this is turned on, both groups will be constrained to each other. If it is turned off, the cloth points will be constrained to the goal points, but not vice versa.

This parameter is disabled for other settings of Goal Anchor. You can still get mirror constraints if you're using Points on Goal Object Geometry, but you do it by attaching the same constraint to both objects. The solver will automatically reverse the roles of “goal” and “cloth” in that case.

Constraint Type

Controls if/how the constrained object can move in reaction to the pull of the “goal” (the point to which the object is constrained).

Free to Move in Any Direction	The constraint has no effect.
Constrained to Plane (Specify Normal)	The object can move along a plane defined by the goal location and the normal you specify in the Constraint Direction parameter.
Constrained to Line (Specify Direction)	The object can move along an axis defined by the goal location and the direction you specify in the Constraint Direction parameter.
Constrained to Point	The object exactly follows the goal.

Constraint Direction

When Constraint Type is Constrained to Plane, this value defines the normal of a plane centered at the goal location.

When Constraint Type is Constrained to Line, this value defines the direction of an axis running through the goal location.

Relationship

Relationship Type

How Houdini constrains the object to the goal point.

Hard Constraint

Keeps a constant distance between the object and the goal, as if they were attached by a rigid wire.

Spring Constraint

Constrains the object as if it was attached to the goal by a spring.

The Spring Strength parameter controls how much the spring constrains the object. If this value is too low, the spring will not be able to pull the object, and it will act as if it wasn’t constrained at all.

For a 1 unit object at the default density, the spring strength should be around 3000-8000 to hold up an object against default gravity.

Increase Spring Damping to decrease unwanted oscillation and yo-yoing of the object.

Two State Constraint

Acts as a hard constraint until certain forces are met, then acts like a spring constraint.

The constraint switches between a hard and a spring constraint based on the motion of the constrained object:

When the constraint is hard, if the “pull” of the object goes beyond the Two State Max Force value, the hard constraint will “let go” and turn into a spring constraint.
When the constraint is a spring, if the object gets within the Two State Min Distance of the goal, the spring will turn into a hard constraint.

Two State Initial State

When using a two state constraint relationship, this parameter controls whether the constraint starts as a hard constraint or as a spring constraint.

Hard Constraint	The two state constraint is initially treated like a hard constraint.
Spring Constraint	The two state constraint is initially treated like a spring constraint.

Two State Max Force

Sets the force threshold for changing a two state constraint from the hard constraint state to the spring constraint state.

Two State Min Distance

Sets the distance threshold for changing a two state constraint from the spring constraint state to the hard constraint state.

Spring Strength

Controls how strongly the spring constraint acts on the constrained object to return the anchors to the rest length separation.

Spring Damping

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

Spring Rest Length

Sets the rest length of the spring relationship. The force applied on a constrained object by a solver will tend to keep the object this distance away from the goal.

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	Sets the color of the constraint in the viewport. Using different colors for hard and spring constraint relationships makes it easy to see when a two state constraint switches from one state to the other.
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.

Local variables

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:

$positionx + 0.1

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

Usages in other examples

Example name	Example for
ClothSeam	Cloth Create Seam surface node	Load \| Launch
SimpleSwitch	Switch Solver dynamics node	Load \| Launch
StitchedClothPatches	Cloth Solver dynamics node	Load \| Launch
SimpleFan	Fan Force dynamics node	Load \| Launch