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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
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:
||Point||Float||Multiplier for all types of stiffnesses.|
||Point||Float||Multiplier for all damping ratios.|
||Point||Float||Multiplier for all mass densities.|
For solid objects, the following multiplier point attributes can be used to modify the local behavior:
||Point||Float||Multiplier for both the shape stiffness and the volume stiffness of a Solid Object.|
||Point||Float||Multiplier for the shape stiffness of a Solid Object.|
||Point||Float||Multiplier for the volume stiffness of a Solid Object.|
||Point||Float||Multiplier for the mass density of a Solid Object.|
Material Space Attributes
materialP can be thought of as the positions of the simulated object in the material space.
materialP is the undeformed configuration relative to which the current position
P determines the deformation of a simulated object.
materialP must stay the same throughout the entire simulation.
The finite element solver relies on assumption that the materialP` attribute remains unchanged from frame to frame; it should never be modified externally (e.g., through a SOP solver) otherwise bad simulation results will be produced.
In the case where no Rest Shape is specified and nor
restP attribute is provided,
materialP can be thought of as a permanent rest position.
If no animation of the rest position is required in a sim, only
materialP should be specified (no
At any stage in the simulation, it is the mapping from
materialP to the current
P that determines the deformation of tets in simulated objects.
The deformation in turn defines the energy stored inside the object.
materialuvw can be used to specify a UVW parametrization of the material space.
The U, V and W directions that are implied by
materialuvw matter if the anisotropic controls are used or when the fiber controls are used on a simulated object.
For the FEM muscle simulation use case, the fiber controls are an important tool for controlling muscle contraction.
materialuv attribute can be used to specify UV directions for cloth. This attribute is essential for triangle meshes, in particular, to define the warped and weft directions for cloth.
||Point||Vector||Material position of each point.|
||Point or Vertex||Vector||Local material uvw coordinates for each point or vertex of a tet.|
||Point or Vertex||Vector||Local material uvw coordinates for each point or vertex of a polygon or polysoup.|
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
||Primitive||Integer||Partitions the object into unbreakable parts. Must be either -1 (no part) or a nonnegative number that indicates a part.|
||Point/Vertex||Integer||Locally enable/disable fracturing for points or vertices.|
||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:
||Primitive||Float||Multiplier for the object’s Normal Drag.|
||Primitive||Float||Multiplier for the object’s Tangent Drag.|
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
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.
restP can be used to specify an animated rest position for all the object points. For example, at each frame
restP may be modified in a SOP Solver before the finite element solver. Among other things this makes it possible to create plastic deformation kinds of effects. When the rest should stay the same during an entire simulation the attribute
restP should not be used. In that case, it is sufficient to specify only an attribute
materialP, which would act as a permanent, unchanged rest position. If no attribute
materialP is specified, the solver falls back to the
baseP attribute that gets automatically created at the creation frame.
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.
||Point||Integer||Initial point index for each point.|
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
||Point or Vertex||Vector||Target position of each point.|
||Point||Float||Multiplier for the object’s Target Strength. If this attribute is missing, a multiplier of 1 is used at all points.|
||Point||Float||Multiplier for the object’s Target Damping. If this attribute is missing, a multiplier of 1 is used at all points.|
When 1, the point is hard constrained to the target animation (e.g.,
fiberscale point attribute acts as a multiplier for the rest strain in the fiber direction.
Among other things, this is useful for FEM muscle simulations.
fiberscale is changed from 1 to 0.5, then the muscle wants to be half as long as before in the direction of the fiber.
If you animate the
fiberscale in a SOP Solver such that it decreases from 1 to a smaller value, you will cause a muscle contraction in the sim.
fiberstiffness point attribute acts as a multiplier for the stiffness along the fiber direction.
The fiber direction of the material is determined by the W axis of the materialuvw coordinates.
fiberstiffness works as a multiplier on top of all the other material property multipliers, including the anisotropic multipliers.
fiberstiffness changed from 1 to 10, then the stiffness along the fiber direction becomes 10 stronger than before.
This can be used to control how strong and how quick the effect of muscle flexing using the fiberscale attribute takes effect.
For fiberscale/fiberstiffness to have the desired effect, it is important that UVW directions are specified.
A material-space UVWs for FEM muscles can be specified using the
materialuvw point/vertex attribute.
materialuvw as a vertex attribute, you are able to provide a local UVW space for each individual tet, which gives you to option of providing a separate UVW frame to each tet.
Multiplier for stiffness along the fiber direction, the W direction implied by
Multiplier for the rest strain along the fiber direction, the W direction implied by
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
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
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.
||Point||Vector||Do not modify! Current position of each object point.|
||Point||Vector||Do not modify! Current velocity of each object point.|
||Point||Vector||Do not modify! Current acceleration of each object point.|
||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.
parent attribute is maintained by the embedding code itself, and
should not be modified.
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.
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.
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.
||Primitive||Float||The index of a parent primitive in the simulated geometry.|
||Point||Float||Base positions used for alignment with simulated mesh.|
||Point or Vertex||Float||Optional user-specified fracture part ID.|
||Point||Float||Positions that correspond to the deformed state.|
||Point||Float||Velocities that correspond to the deformed state.|
||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:
||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.
||Point||Float||The local density of deformation energy|
||Point||Float||The local density of the rate of energy loss|
||Point||Float||The local density of the kinetic energy|
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.
||Point||Integer||The number of times a point was fractured during the simulation|
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.
||Force||Vector||External force density|
||Force||Vector||Another name for external force density|