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Hybrid Object dynamics node

Creates a Hybrid Object from SOP Geometry.

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Overview

This object simulates both tetrahedrons and polygons in a single object. The polygons may share points with the tetrahedra. For example a tetrahedral mesh may have a skin envelope consisting of polygons. These polygons are simulated similar to a thin layer of solid material. The material properties of the polygons may be controlled separately from the material properties of the tetrahedra to create interesting effects (e.g., skin wrinkling) that are not easy to come by using tetrahedrons alone.

How to

To...Do this

Create a soft hybrid object

  1. Append a Solid Conform node to your geometry.

  2. In the Solid Conform node’s parameters, turn on Add surface triangles to make sure the polygonal skin is composed of triangles.

  3. On the Solid tab, click the Hybrid Object tool.

  4. Select the geometry to convert to an hybrid object and press Enter.

    This tool creates a Hybrid Object node in the current simulation and sets it up to import from the selected object.

Create a wrinkly hybrid object

  1. Append a Solid Conform node to your geometry.

  2. In the Solid Conform node’s parameters, turn on Add surface triangles to make sure the polygonal skin is composed of triangles.

  3. On the Solid tab, click the Organic Tissue tool.

  4. Select the geometry to convert to an hybrid object and press Enter.

    This tool creates a Hybrid Object node in the current simulation and sets it up to import from the selected object. It sets the physical properties of the new object to simulate the look of soft material contained inside a skin that can wrinkle and bunch.

Note

Once you convert geometry to a solid object, you can only transform, rotate, and scale it at the first frame.

Parameters

Model

Shell

Stiffness Multiplier

This is a convenient multiplier for both the Shape Stiffness and the Bend Stiffness of the shell polygons. This multiplier has no units.

Damping Ratio

This unitless parameter controls how quickly the shell polygons stop deforming.

Mass Density

This is the density of mass per volume for the polygons.

Thickness

This specifies the volume per surface area for the shell polygons.

Shape Stiffness

This determines how strongly the shell polygons resist local deformation in directions tangent to the shell.

Bend Stiffness

This determines how strongly the shell polygons resist local deformation in directions normal to the shell.

Solid

Stiffness Multiplier

This is a convenient multiplier for both the Shape Stiffness and the Volume Stiffness of the solid tets. This multiplier has no units.

Damping Ratio

This unitless parameter controls how quickly the solid tets stop deforming.

Mass Density

This is the density of mass per volume for the solid tets.

Shape Stiffness

This determines how strongly the solid tets resist local changes in shape.

Volume Stiffness

This determines how strongly the solid tets resist local changes in volume.

Contact

Friction

This controls the strength of friction forces at contacts

Deformation

Initial

Initial State

The path to the SOP node with the initial connectivity, position and velocity.

Rest

Rest Shape

The path to the SOP node that defines the rest shape.

Target

Target Deformation

The path to the SOP node with target deformation.

Target Strength

Strength density of the distributed soft-constraint force field that tries to match the target position.

Target Damping

Damping density of the distributed soft-constraint force field that tries to match the target velocity.

Attributes

Create Quality Attributes

This creates a primitive attribute 'quality' on the simulated geometry. The worst quality is 0, the best quality is 1. The better the quality of the primitives, the better the performance and stability of the solve will be.

Create Energy Attributes

This toggle allow the object to generate attributes that indicate the density of kinetic energy and potential energy. In addition, an attribute that indicates the density of the rate of energy loss is generated.

Create Force Attributes

This toggle allows force attributes to be generated.

Create Collision Attributes

Create Fracture Attributes

Creation

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

GuidedWrinkling Example for Hybrid Object dynamics node

This is a setup for guided wrinkling using the hybrid object. The first sim creates a detailed mesh consisting of both tets and triangles that doesn’t have any wrinkles yet. The second sim is targeted to the animation creates by the first sim and this adds in the wrinkles.

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The following examples include this node.

GuidedWrinkling Example for Hybrid Object dynamics node

See also

Dynamics nodes