Wire Object dynamics node

Creates a Wire Object from SOP Geometry.

All Parameters Outputs Local variables Example files

See also: Wire Configure Object, Wire Physical Parameters, Wire Force Model, Wire/Volume Collider 3 more , Wire/Wire Collider, Wire Visualization, Wire Solver

The Wire Object DOP creates a Wire Object inside the DOP simulation. It creates a new object and attaches the subdata required for it to be a properly conforming Wire Object.

The SOP geometry used to define wire objects are expected to contain a set of curves. These curves may be closed curves (eg. polygons) and will be connected if multiple curves share a common point. This lets wire objects describe structures such as ropes, trees, bridges, and spider webs.

Using Wire Object

  1. Select the objects to convert to wire objects and press Enter to confirm your selection.

  2. Click the Wire Object tool from the Wires tab.

Attributes

You can create attributes on the wire object’s InputGeometry to influence its behavior. Most of these attributes allow fine-tuning of the wire by scaling values set in this node. Point, primitive, or detail attributes of the same name will be used if the vertex attributes are not present.

Name Class Type Description Scaling Factor
angvel Point Vector

Initial angular velocity of each point measured in degrees per second.

 No
mass Point Float Mass of each point. No
density Point Float Density of each point. Yes
orient Point Float[4]

Initial orientation of each point. This value is stored as a quaternion.

 No
v Point Vector Initial velocity of each point. No
width Edge (vertex) Float Width of each edge. Yes
klinear Edge (vertex) Float Defines how strongly the wire resists stretching. Yes
damplinear Edge (vertex) Float

Defines how strongly the wire resists oscillation due to stretching forces.

 Yes
kangular Edge (vertex) Float Defines how strongly the wire resists bending. Yes
dampangular Edge (vertex) Float

Defines how strongly the wire resists oscillation due to bending forces.

 Yes
fexternal Point Vector Defines an external force applied to each point. No
texternal Point Vector Defines an external torque applied to each point. No
nocollide Edge (vertex) Integer Non-zero values disable collision detection for the edge. No

Parameters

Creation Frame Specifies Simulation Frame

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

Creation Frame

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Number of Objects

Instead of making a single object, one can create a number of identical objects. You can set each object’s parameters individually by using the $OBJID expression.

Object Name

The name for the created object. This is the name that shows up in the details view and is used to reference this particular object externally.

Note

While it is possible to have many objects with the same name, this complicates writing references, so it is recommended to use something like $OBJID in the name.

Solve On Creation Frame

For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created.

Usually this parameter will be turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

SOP Path

The path to a SOP (or an Object, in which case the display SOP is used) which will be the rest geometry for this object.

Initial Pose

The path to a SOP (or an Object, in which case the display SOP is used) which will be the initial pose for this simulation object.

Use Deforming Geometry

Causes the geometry for the object to be pulled from the chosen SOP at each timestep.

The positions of the points will be used to set the initial wire state, but will be simulated after that. However, attributes from the animated geometry will be used.

Use Object Transform

The transform of the object containing the chosen SOP is applied to the geometry.

Initial State

Position

Initial position in world space of the object.

Rotation

Initial orientation of the object. This is in RX/RY/RZ format.

Pivot

Local space position around which rotation is applied.

Velocity

Initial velocity of the object.

Angular Velocity

Initial angular velocity of the object.

Physical

Compute Mass

Determines if the mass will be calculated automatically from the object’s density and volume.

Density

The mass of a wire object is its volume times its density. The volume is affected by the width parameter.

Mass

The absolute mass of the object.

Width

The width of the wire object defines the diameter of each cylindrical section.

Bounce

The elasticity of the object. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill.

Friction

The coefficient of friction of the object. A value of 0 means the object is frictionless. This governs how much the tangential velocity is affected by collisions.

Dynamic Friction Scale

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction.

Force Model

Linear Spring Constant

This parameter defines how strongly the wire resists stretching.

Linear Damping Constant

This parameter defines how strongly the wire resists oscillation due to stretch forces.

Angular Spring Constant

This parameter defines how strongly the wire resists bending.

Angular Damping Constant

This parameter defines how strongly the wire resists oscillation due to bending forces.

Adjust For Length

Enabling this parameter will adjust spring and damper strengths according to segment lengths. This allows wire flexibility behavior to be independent of segment resolution.

Collisions

Wire/Volume Collisions

If enabled, the wire object will be prevented from touching or passing through any affectors that have a Volume collider label (e.g., RBD Objects or the ground plane). This can make the simulation slower.

Volume Offset

The wire/volume collisions uses a Volumetric representation of the affector.

This is only an approximation of the affector’s geometry, however. To reduce artifacts where parts of the affector are visible through the wire, it can be useful to offset the volume and make it larger.

A positive value here makes the affector object act larger, while a negative value makes it act smaller. This is equivalent to changing the Offset parameter of a Volume DOP on the affector object.

Wire/Wire Collisions

If enabled, the wire object will be prevented from touching or passing through all of its wire affectors. This can make the simulation much slower.

Self Collisions

If enabled, the wire object will be prevented from touching or passing through itself. This can make the simulation much slower.

Visualization

Force Scale

This is used to define the scale of the force lines drawn in the viewport. Use a small value if the lines are too long and distracting, and a large value if you can’t see any lines.

External Force

Turn this on to see external forces, applied by DOPs Force nodes (such as the Fan DOP).

External Force Color

Use this parameter to choose the color for external forces in the viewport.

External Torque

Turn this on to see external torques, applied by DOPs Force nodes (such as the Drag DOP).

External Torque Color

Use this parameter to choose the color for external torques in the viewport.

Internal Force

Turn this on to see internal forces generated by a Wire Solver to resist stretching.

Internal Force Color

Use this parameter to choose the color for internal forces in the viewport.

Internal Torque

Turn this on to see internal torques generated by a Wire Solver to resist bending.

Internal Torque Color

Use this parameter to choose the color for internal torques in the viewport.

Collision Force

Turn this on to see the force preventing collisions in the viewport. This includes wire/volume collisions, wire/wire collisions and self-collisions.

Collision Force Color

Use this parameter to choose the color for collision forces in the viewport.

Constraint Force

Turn this on to see forces generated by a Wire Solver to satisfy constraints.

Constraint Force Color

Use this parameter to choose the color for constraint forces in the viewport.

Constraint Torque

Turn this on to see torques generated by a Wire Solver to satisfy constraints.

Constraint Torque Color

Use this parameter to choose the color for constraint torques in the viewport.

Impacts

Turn this on to see impacts in the viewport. The impacts may appear in strange locations: they are shown at the position where a collision would have happened.

Impacts Scale

This is used to define the scale of the lines drawn in the viewport to show impacts.

Use a small value if the lines are too long and distracting, and a large value if you can’t see the lines.

Impacts Color

Use this parameter to choose the color for impacts in the viewport.

Show Substep Impacts

Use this to show all impacts during a DOPs step. The wire solver takes many substeps per DOPs step. If this is cleared, only the impacts for the current substep are shown.

Axis

Turn this on to see each point’s orientation.

Axis Scale

This is used to define the scale of the axis lines drawn in the viewport. Use a small value if the lines are too long and distracting, and a large value if you can’t see any lines.

X Axis Color

Use this parameter to choose the color for local x-axis.

Y Axis Color

Use this parameter to choose the color for local y-axis.

Z Axis Color

Use this parameter to choose the color for local z-axis.

Outputs

First

The wire object created by this node is sent through the single output.

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.

Example files

CompressedSpring

$HFS/houdini/help/examples/nodes/dop/wireobject/CompressedSpring.cmd

Load | Launch

This example demonstrates how an initial pose may be specified for a wire object.

Usages in other examples

Example name Example for
FurBallWorkflow

Fur surface node

Load | Launch

FluidWireInteraction

Fluid Force dynamics node

Load | Launch

AnimatedSkin

Wire Glue Constraint dynamics node

Load | Launch

BeadCurtain

Wire Solver dynamics node

Load | Launch

BendingTree

Wire Solver dynamics node

Load | Launch