Houdini 16.5 Nodes Dynamics nodes

Finite Element Solver 1.0 dynamics node

Sets and configures a Finite Element solver.

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The Finite Element Solver DOP sets objects to use the Finite Element solver.

If an object has this DOP as its Solver subdata, it will evolve itself as a cloth object.

This solver recognizes the following special subdata, if present:

It also recognizes most standard subdata:

Before each solve, the finite element solver reads the simulation state of each object from the attributes on the simulation geometry Geometry to get the previous state. After the solve, the new state is written to attributes the Geometry. In addition, the solver may maintain an EmbeddedGeometry. This would typically be a more higher-resolution geometry that moves and fractures along with Geometry. The embedded geometry can consists of polygons or tetrahedrons (or a mix of these two).

The finite element solver approximates the physics of continuous materials by splitting them up into a finite number of elements. In the case of the Solid object, the elements are determined by the tetrahedrons. In the case of the Cloth Object, the elements are determined by triangles and quadrangles. The resolution of the tetrahedrons and the orientations of the individual tetrahedrons have little influence over the overall movement; as long as the overall solid shape is the same the behavior is roughly the same (except for very coarse meshes). The finite element method (FEM) treats the elements an approximation of a continuous material. This property makes the results very predictable when you simulate the same shape using a lower-res and a higher-res mesh.

Parameters

Substepping

Substeps

This is the number of substeps per frame. The higher this value, the better the quality and the accuracy of your simulation will be. However, the solve time per frame may increase. When you're having trouble with a simulation’s quality or stability, the first thing to try is increasing the Substeps. One particular case where this may be needed is when you have fast moving objects that collide. When the relative velocity is large compared with the feature size, you may need to manually increase the Substeps to get better looking results.

Capabilities

Enable Collisions

When this is disabled, no collisions will happen in the simulation, regardless of any other collision settings on the solver or on any of the objects that are solved. This is a convenient toggle that allows you to quickly see how an object behaves without collisions.

Enable Fracturing

When this is disabled, no fracturing or tearing will occur on any of the Solid Objects or Cloth Objects attached to this solver.

Accuracy

Absolute Tolerance Implicit Solve

The absolute tolerance that is used to decide when the finite element solver has found a good enough approximate solution for the implicit integration step. This roughly indicates the amount of error allowed in the acceleration that is computed in each integration step. It is not recommended to increase this tolerance too much from its default.

Relative Tolerance Implicit Solve

The relative tolerance that is used to decide when the finite element solver has found a good enough approximate solution for implicit integration step. In contrast with the absolute tolerance, this parameter has no units. It is not recommended to increase this tolerance from its default, because this may produce results that are unstable or poor quality. In some types of simulations, it may be necessary to reduce the Relative Tolerance Implicit Solve to a lower value, say 0.0001 or even smaller. For example, the tolerance may need to be lowered in fracturing simulations with very stiff objects.

Max Collision Passes

This is the maximum number of times, within a single substep, that the solver is allowed to detect and resolve new collisions. The collision resolution step in each pass may introduce new, secondary collisions that must be resolved in later passes.

Attributes

Create Quality Attributes

This creates float-valued primitive attribute quality on the simulation geometry. This attribute allows you to find primitives that have a bad effect on the simulation. The closer the value is to 1, the better. Bad tetrahedrons have a value close to 0 and should be removed or improved.

Create Energy Attributes

This creates float-valued vertex attributes on the simulation geometry. These attributes potentialdensity, dissipationdensity, and kineticdensity show the potential energy density, the density of energy dissipation, and the kinetic energy, respectively.

Create Fracture Attributes

This allows the fracturecount point attribute to be created. Any point that was fractured at any time in the simulation will have a fracturecount of 1 or higher ever after.

Advanced

Float Precision

This determines the floating point precision that is used internally by the finite element solver. Float 32 bit uses less memory and is generally faster than Float 64 bit. However, the extra accuracy of 64-bit floating point numbers may be needed when you are simulating object with very high overall stiffness or when your geometry is positioned very far away from the origin.

Integrator Type

This determines the type of integration that is used. The default is Implicit, which is the most stable option for simulations that have fracturing stiff objects. The Implicit Order 2 allows more lively looking simulations of higher quality for about the same solve time. This is best used when doing finite element simulation of flesh on characters.

Max Fully Implicit Passes

For highly nonlinear finite element simulations, increasing this parameter may result in better quality without having to increase the sub-steps. This may be needed for simulations in which the deformations of the tetrahedrons are relatively large.

Attributes

The finite element solver will recognize and use attributes on the simulated geometry. In the DOP network, this simulation geometry is attached to the simulated object as a sim-data with name Geometry. When an object is created, then the geometry and all the corresponding attributes are read from the Initial Geometry. This includes the standard position and velocity point attributes P and v.

The finite element solve supports input attributes and output attributes. Some attributes, such as the simulation state, are both input and output attributes. The input attributes include multiplier attributes for material properties, fracture attributes, and attributes for controlling target positions and corresponding hard/soft constraints. The output attributes include optional attributes for tet quality, energy densities, FEM node forces, collision info attributes and fracture info attributes.

Material Property Multiplier Attributes

Each of the material properties of a simulated object can be locally modified using multiplier point attributes. As a rule, each of the material properties in the Model tab of an object can be affected by a multiplier attribute. As a rule, the name of the parameter is the name of the attribute. The name of the attribute is the name that is displayed after "Parameter:" when you hover over a parameter with your mouse cursor.

You can locally change the material properties of the object using point attributes. For example, you can make some polygons resists stretching and bending more than other polygons. These attributes work as multipliers for the parameters in the Model tab: The stiffness multiplier is a convenient way to modify the local stiffness for all object types that are recognized by the finite element solver:

Name Class Type Description
stiffness Point Float Multiplier for all types of stiffnesses.
dampingratio Point Float Multiplier for all damping ratios.
massdensity Point Float Multiplier for all mass densities.

For solid objects, the following multiplier point attributes can be used to modify the local behavior:

Name Class Type Description
solidstiffness Point Float Multiplier for both the shape stiffness and the volume stiffness of a Solid Object.
solidshapestiffness Point Float Multiplier for the shape stiffness of a Solid Object.
solidvolumestiffness Point Float Multiplier for the volume stiffness of a Solid Object.
solidmassdensity Point Float Multiplier for the mass density of a Solid Object.

Fracturing Control Attributes

When you create a simulation with fracturing, it is recommended to specify chunks of tetrahedrons that you want to stay together. Otherwise, the fracturing process may create a very large amount of separate pieces, many of which may consist of single tetrahedrons. For this purpose, you can assign a nonnegative integer to each chunk using the fracturepart attribute. In areas where you don’t want to specify parts, you can set fracturepart to -1, which means that each primitive in that region will become its own part. Real-life materials tend not to be equally strong everywhere. For realistic results, it is recommended to vary the Fracture Threshold locally using the vertex attribute fracturethreshold.

fracturepart Primitive Integer Partitions the object into unbreakable parts. Must be either -1 (no part) or a nonnegative number that indicates a part.
enablefracturing Point/Vertex Integer Locally enable/disable fracturing for points or vertices.
fracturethreshold Point/Vertex Float Multiplier for the object’s Fracture Threshold.

Drag Force Control Attributes

The behavior of the drag force can be modified locally using the following attributes:

normaldrag Primitive Float Multiplier for the object’s Normal Drag.
tangentdrag Primitive Float Multiplier for the object’s Tangent Drag.

Reference Attributes

The attribute restP can be used to specify the rest position for all the object points. The attribute materialP is similar to the rest position, but it must not be changed during a simulation. Among other things, materialP defines the mass and the strength of the internal forces that act within the simulation geometry. The attribute materialP can be thought of as an initial rest position. table>>

The attribute baseP can be used to specify a generic base position for all the object points. This attribute’s values must not be changed during a simulation. When the user does not specify baseP, the solver creates this point attribute based on the point positions on the creation frame. This attribute is used as a fallback; whenever the user does not specify materialP, the attribute baseP is read instead. In the same way, baseP is used as a fallback for when no restP or targetP attributes are provided. Finally, baseP is used to bind the simulated and the embedded geometry, in the embedded workflow (e.g., a T-pose). This embedded binding looks at the baseP position attribute on both the simulated geometry and the embedded geometry. If no baseP attribute is provided by the user on the embedded geometry, the solver creates the baseP attribute on the embedded geometry based on the position P at the creation frame.

The attribute initialpid stores the initial point index for each point. This is the point index at the creation time of the object. This attribute is created only when fracturing is enabled on both the object and the solver. The finite element solver uses this attribute for the options Import Rest Geometry and Import Target Geometry to transfer animated positions and velocities in SOPs to the current fractured topology in the simulated object.

Name Class Type Description
initialpid Point Integer Initial point index for each point.

Target Attributes

Target attributes can be used to make a simulated object partially follow a target animation. The attribute targetP can be used to specify a target position for each object point. When you use the Import Target Geometry option on the simulated object, the targetP will be set automatically every frame. Alternatively, you can create and modify these attributes yourself, using a Multi Solver and a SOP Solver. The target positions and velocities allow the user to mix animation and simulation in a very stable way (assuming the Target Strength and Target Damping parameters have been set on the object). You can set the Target Strength and Target Damping parameters on the object to express how strongly the object should match the target position and velocity, respectively. This is a way to create soft constraints. You can use the pintoanimation to create hard constraints that make the simulated points follow targetP exactly.

Name Class Type Description
targetP Point or Vertex Vector Target position of each point.
targetstrength Point Float Multiplier for the object’s Target Strength. If this attribute is missing, a multiplier of 1 is used at all points.
targetdamping Point Float Multiplier for the object’s Target Damping. If this attribute is missing, a multiplier of 1 is used at all points.
pintoanimation Point Int When 1, the point is hard constrained to the target animation (e.g., targetP). When zero, the point is unconstrained.

State Attributes

Below is a list of attributes that are maintained internally by the solver. Each of these attributes is written to at the end of each solve and read from at the start of the next solve. You should not modify any of these attributes yourself. When you do, the solver is likely to become unstable and you will get bad results. However, you can inspect the values in these attributes in your network for visualization or for the creation of secondary effects.

At each frame, the finite element solver computes a new physical state for each simulated object. The physical state of the object is represented by the point attributes P and v, representing the position and velocity, respectively. The solver’s integration scheme maintains additional attributes a for acceleration and j for jerk.

The point attributes P, v, a, and j store the current integration state of the object. These attributes should not be modified during the simulation because the finite element solver will become unstable and produce low-quality results.

Name Class Type Description
P Point Vector Do not modify! Current position of each object point.
v Point Vector Do not modify! Current velocity of each object point.
accel Point Vector Do not modify! Current acceleration of each object point.
jerk Point Vector Do not modify! Current jerk of each object point.

Embedded Geometry Attributes

These attributes are created on the Embedded Geometry of the Solid Object. The parent attribute is maintained by the embedding code itself, and should not be modified. The baseP point attribute can be provided on the Embedded Geometry by the user to control the binding between the simulated geometry and the embedded geometry. If no baseP is provided, it will be copied from the point positions stored in P at the creation frame. The alignment happens relative to the baseP point attribute on the simulated geometry. If the simulated geometry has a materialP vertex or point attribute, then this attribute takes precedence, allowing control per vertex, rather than per point, if necessary. When you want to ensure that embedded geometry ends up on the desired side of a fracture between simulated geometry, you can use the combination of vertex attributes baseP on the embedded geometry and restP on the simulated geometry. This allows you to line up the embedded geometry with the separate parts in the simulated geometry, for example using the Exploded View SOP. The fracturepart attribute allows you to make sure that the embedded geometry follows the right parts when it gets fractured. When both the simulated and the embedded geometry have the fracturepart attribute, the finite element solver will parent embedded geometry to simulated geometry that has the same fracture part.

Name Class Type Description
parent Primitive Float The index of a parent primitive in the simulated geometry.
baseP Point Float Base positions used for alignment with simulated mesh.
fracturepart Point or Vertex Float Optional user-specified fracture part ID.
P Point Float Positions that correspond to the deformed state.
v Point Float Velocities that correspond to the deformed state.
N Point or Vertex Float Normals that correspond to the deformed state.

Optional Output Attributes

These are attributes that are optionally generated by the solver, when the generation is enabled on the simulated object. These attributes can be useful for visualization, for example, using the Finite Element Visualization SOP. Additionally, these attributes may be used to create secondary effects, for example, particles flying off in regions where fracturing occurs. The optional output attributes are also expected by the Finite Element Visualization SOP.

The following attribute is generated when Create Quality Attributes is turned on:

Name Class Type Description
quality Primitive Float A quality metric between 0 (worst) and 1 (best)

Finite element simulation tends to be sensitive to the quality of the incoming primitives. Low quality primitives may slow down, destabilize or lock a finite element simulation. Low quality primitives are best avoided by using the Solid Embed as a tool to create your tet mesh. Although various quality metrics exist for tetrahedra, the one that’s generated by the solver in this attribute is the one that best matches Houdini’s finite element solution.

The solver generates energy-density attributes for each object that has Create Energy Attributes turned on. The material property settings in the Model tab and the corresponding multiplier attributes result in potential energy, energy dissipation and kinetic energy. For each of these three contributions, local densities are computed within the solver. These densities and quantities derived from them are used to determine the motion and behavior of the objects that are solved by the finite element solver.

Name Class Type Description
potentialdensity Point Float The local density of deformation energy
dissipationdensity Point Float The local density of the rate of energy loss
kineticdensity Point Float The local density of the kinetic energy

The potentialdensity attribute is directly affected by the stiffness parameters in the Model tab. The kineticdensity is proportional to the mass density that is specified for the object. The dissipationdensity is related to the damping settings.

If Create Fracture Attributes is enabled on the simulated object, then the fracturecount point attribute is created. The point attribute fracturecount maintains for each point, the number of times that the point has been involved in a fracture. So any point with a nonzero value of fracturecount has been involved fracturing.

Name Class Type Description
fracturecount Point Integer The number of times a point was fractured during the simulation

Legacy Attributes

In most situations where you want to influence a finite-element simulation, you will want to use soft constraints to achieve this, for example, target constraints, region constraints, or the target strength/damping settings on the object. These are first-class solver features that work in a stable way with the solver and should produce high quality results when used correctly. Purely for backwards compability, a force force attribute is still supported. Because the force force attribute lacks essential information that the solver needs, this attribute cannot be relied on when stability and quality are important. When setting up a new sim, alternatives such as soft targeting, region constraints and animated rest positions should be considered instead of the force force attribute.

Name Class Type Description
fexternal Force Vector External force density
force Force Vector Another name for external force density

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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.

See also

Dynamics nodes