Houdini 16.5 Nodes Dynamics nodes

Solid Configure Object dynamics node

Attaches the appropriate data for Solid Objects to an object.

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The Solid Configure Object DOP takes a simulation object and attaches the data which is needed for it to be used as a Solid Object.

This DOP is very similar to the Solid Object DOP, except it allows you to explicitly control the creation of the object using another DOP, such as the Empty Object DOP. This can be used for more advanced instancing or creating objects every 10 frames.

Parameters

Material

Shape Stiffness

This determines how strongly the Solid Object resists changes in shape. In the isotropic use case (Anisotropic Strength all 1), this physical constant is also known as shear modulus, modulus of rigidity, or Lame’s second parameter. If the units of length are set to meters, then the Shape Stiffness parameter has units of GPa.

Volume Stiffness

This determines how strongly the Solid Object resists changes in volume. In the isotropic use case (Anisotropic Strength all 1), this physical constant is also known as Lame’s first parameter. If the units of length are set to meters, then the Shape Stiffness parameter has units of GPa.

Damping Ratio

This value should lie between 0 and 1. It controls the rate of energy loss as a result of the rate of deformation. A value of 0 means that there is no loss of energy due to internal damping forces. A value of 1 means that the object is critically damped, in which case the object comes to rest in the quickest possible way without oscillating. The higher the damping ratio, the less the solid oscillates and the quicker the object’s motion will come to rest. The effect of damping is independent of your geometry’s resolution.

Mass Density

This is the mass per cubed length unit. The mass density can be made lower or higher in parts of the object using the primitive attribute volumemassdensity, which works as a multiplier for the parameter. The higher the mass density, the less the object tends to accelerate as a result of any internal or external forces (F = m a, Newton’s second law).

Enable Fracturing

This enables or disables all fracturing for this object.

Note

Before any fracturing can occur, Enable Fracturing must also be enabled on the Finite Element Solver.

Fracture Threshold

The amount of relative stretch that will cause the geometry to break up into separate parts during the simulation. For example, if the threshold is set to 0.1, the geometry will break in places where there is more than 10% stretch compared to the rest geometry.

Realistic solid objects are not equally strong everywhere, there are weak parts that tend to fracture before any other parts. To create these relatively weaker parts you can create a vertex attribute called fracturethreshold. This attribute is a multiplier for the Fracture Threshold parameter, so that you can still use the Fracture Threshold to control the overall strength of the object.

Friction Coefficient

The coefficient of friction of the object. A value of 0 means the object is frictionless. This governs how much the velocity that is tangential to the contact plane is affected by collisions. When two objects are in contact, the solver multiplies the friction coefficients of the involved object to get the effective friction coefficient for that contact.

Anisotropic Strength

These values allow you to make the internal forces of the Solid Object behave in an anisotropic way; in that case, the amount of stress will differ depending on the direction in which the object is deformed. An example of an anisotropic material is wood, which has a different strength along the grain than perpendicular to the grain. The materialuvw vertex or point attribute can be used to specify the internal directions of the Solid Object’s internal force model. For example, in the case of wood, the U direction could be aligned with the grain of the wood, while the VW coordinates are chosen perpendicular to the grain of the wood. In this case, the strength along the grain can be separately controlled using the U component of the Anisotropic Strength.

Geometry

Initial Geometry

This geometry determines the initial simulated state of the object. It determines the initial position and velocity for each of the points.

This is the geometry that is used for the computation of internal forces and for collision detection. Don’t use more tetrahedrons than you need to get a good looking motion; more tetrahedrons does not always translate to more quality. The fewer tetrahedrons you use, the better the simulation speed is. If extra detail needs to be added, it is recommended that you use the Embedded Geometry.

You can use the Tetrahedralize SOP to create a suitable input mesh. It is important that you enable the quality option on the Tetrahedralize SOP. Otherwise, the interior of your Solid Object won’t have enough degrees of freedom to be flexible.

Enable Embedding

Turns on/off the use of embedded geometry.

Embedded Geometry

This geometry is embedded into and deformed along with the simulated tetrahedral mesh.

Import Rest Geometry

This option allows you to specify and animate the rest positions that are used by the simulation inside the SOP network (without having to use a SOP solver). The option defines whether the rest positions should be imported from a SOP geometry node at each frame. When enabled, the solver will copy rest positions from the point attribute restP of the SOP geometry node onto the attribute restP on the simulation geometry at each frame. If no restP exists, then the 'P' attribute from the SOP geometry node is copied instead.

Rest Geometry Path

The path to the SOP node that will serve as the source of the rest positions.

Import Target Geometry

This option allows you to specify and animate the target positions that are used by the simulation inside the SOP network (without having to use a SOP solver). The option defines whether the target positions should be imported from a SOP geometry node at each frame. When enabled, the solver will copy target positions from the point attribute targetP of the SOP geometry node onto the attribute targetP on the simulation geometry at each frame. If no targetP exists, then the P attribute from the SOP geometry node is copied instead.

Target Geometry Path

The path to the SOP node that will serve as the source of the target positions. The positions should be stored in an attribute with the name targetP. If this attribute is not found, the P attribute is used as a fallback.

Stiffness

This coefficient determines how strongly the finite element solver tries to make the point positions match the target point positions. The solver creates an imaginary potential force for this purpose.

Damping

This coefficient determines how strongly the finite element solver tries to make the point velocities match the target point velocities. The solver creates an imaginary dissipation force for this purpose.

Collisions

Collide with objects

If enabled, the geometry in this object will collide with all other objects. These other objects may belong to the same solver or they may be be Static Objects, RBD Objects, or the Ground Plane. When the Collision Detection parameter on the Static Object is set to Use Volume Collisions, then the polygon vertices will be tested for collision against the signed distance field (SDF) of the Static Object. When Collision Detection is set to Use Surface Collisions, then geometry-based continuous collision detection is used. The geometry-based collisions collide points against polygons, and edges against edges.

When surface-based collisions are used, only polygons and tetrahedrons in the Static Object are considered. Other types of primitives, for example spheres, are be ignored. The geometry of the external objects (e.g. Static Object) is treated as being one-sided; only the outsides of the polygons, determined by the winding order, oppose collisions.

When volume-based collisions are enabled, only points will be colliding against the volumes, not the interiors of polygons and tetrahedrons. When colliding against small volumes, this may mean that you need to increase the number of points on your mesh to get accurate collision results.

Collide with other objects with same solver

When enabled, this object will collide with other objects that have the same solver. These collisions are handled using continuous collision detection, based on the geometry (polygons and/or tetrahedrons). For collisions between objects on the same solver, the polygons are treated as two-sided. Both sides of the polygons collide. The surface of a tetrahedral mesh only collides on one side: the outside.

Collide within this object

If disabled, no two tetrahedrons within this object can collide with each other.

Collide within each component

If disabled, no two tetrahedrons that belong on the same connected component may collide with each other.

Collide within each fracture part

This option only has an effect when fracturing is enabled on the solver. If disabled, no two tetrahedrons that belong on the same fracture part may collide with each other. Fracture parts are controlled by the integer-valued fracturepart primitive attribute.

Drag

Visualization

Collision Radius

Visualize the cloth’s collisionradius.

Collision Radius Color

Color of the cloth collisionradius guide geometry.

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

Inputs

First

The simulation objects to turn into solid objects by attaching the appropriate data.

Outputs

First

The simulation objects which were passed into this node are output with the data required for them to be considered cloth Objects attached.

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