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The RBD Solver DOP sets objects to use the Rigid Body Dynamics solver.
If an object has this DOP as its “Solver” subdata, it will evolve
itself as an 
RBD Object.
This solver makes extensive use of volumetric representations of
object. It is recommended that the object’s geometry have a 
Volumetric
Representation DOP attached to it to allow this
representation to be tailored to the needs of the geometry.
This solver uses the “Position” subdata, which it expects to be the
type generated by a 
Motion DOP or 
Position
DOP.
The Stack solver requires a total number of passes equal to Collision Passes + (Contact Passes + Shock Propagation) * SubContact Passes.
Notes ¶
-
RBD simulation processes all collisions at the start of the time step rather than at the exact time that the collision occurs. This simplifies complicated interactions, but can result in visual artifacts. A fast moving object may appear to bounce back from a surface before it reaches it, for example, as the impulse from hitting the surface is applied to the objects position at the start of the time step.
Increasing substepping will reduce these artifacts by ensuring the object is closer to the surface when the collision is detected.
-
Increase substepping if objects move large distances within a single frame. An object should move less than half of its smallest important feature in a single step to avoid temporal aliasing problems.
Increase the Maximum Substeps to allow the automatic substep calculation to determine the right substepping to avoid temporal aliasing. This is a function of the velocity of your objects and the resolution of the volume representation of those objects. It is then scaled by the CFL condition - a smaller CFL condition will result in more substeps.
Increasing the Minimum Substeps can ensure that the substepping never takes fewer than the given number of substeps. This is required if deforming collision geometry is used because the collision geometry is not taken into account in the automatic sub step computation.
Parameters ¶
Collision Passes
The stack solver iterates over all objects looking for ballistic collisions. Because resolving one collision may create a collision elsewhere, this cannot be resolved in a single pass with a local solution.
The stack solver will thus repeat the collision resolution until either no collisions are found, or this pass count is reached.
Even if a collision is not fully resolved with these passes, it will still be cleaned up in the Contact Pass. The main difference is that it will become inelastic.
Contact Passes
The stack solver iterates over all objects, looking for cases where resting contact requires an acceleration to be adjusted.
Multiply stacked objects are common, so this often has complicated interrelationships, so requires multiple passes to converge.
SubContact Passes
Resting objects have a higher stability requirement than bouncing objects. Thus, the object is not immediately brought to a standstill, but slowed over multiple iterations to allow the system to stabilize.
This is the number of steps to do this for every contact pass.
Shock Propagation
These passes are very similar to Contact Passes.
The main difference is that if a book were resting on a table, the table would be assigned infinite mass in this pass. This prevents the table from shifting into the ground, allowing the system to converge faster.
As a rule of thumb, set this to the expected maximum number of stacked objects. If you plane to have ten tables stacked on top of one another in a stable configuration, a value of 10 can help ensure that the stacking is fully resolved.
If your objects come to rest appropriately, but then seem to slowly start to sink through each other, increasing Shock Propagation can be the right answer.
Resolve Penetrations
These passes are a final attempt to prevent any interpenetration. Like Shock Propagation, it is attempted to process objects from bottom up.
If a book is resting on a table and is penetrating the table, the book will be moved to lie outside of the table. This will be performed even if the book is at rest on the table.
The penetration recover repeated until there are no more penetrations up to the maximum number provided by this parameter.
The SubContact Passes is used to slowly feather the objects apart. Rather than immediately moving the book outside of the table, it is done in over the given number of subcontact passes. This is done to attempt to stabilize the process when complicated overlaps occur.
Use Point Velocity for Collisions
Determines if changes in the point positions will be used in collision resolution. Note that this is different from the Inherit Velocity option of RBD State. This flag only governs if velocity attributes are used for collisions, not for setting up the initial velocities.
When this is set, the object is inspected for any per point velocity attribute. If present, it is assumed to be a local deformation vector and is used to improve collision response.
If no point velocities are present, the geometry is compared between the two frames to manually calculate the per-point velocity. Note that if your deformation is a function of $F you may not get expected results as that is a step function, use $T instead.
Use Volume Velocity for Collisions
Determines if changes to the volumetric representation will be used in collision resolution.
When this is set the volumetric representation is compared between this frame and the previous frame. The difference is used to compute a velocity of the surface’s deformation. This allows deforming objects to interact plausibly.
Note
This method can handle changing topologies, but cannot
discover tangential deformational velocities.
Glue Ignores Resting Objects
When objects are resting on top of one another, they still receive impacts due to the force of gravity. This option prevents these from being added to the glue impulse, making it easier to prevent things from falling apart under their own gravity.
Add Impact Data
During the RBD solving process, numerous impacts are calculated between the RBD objects. These are normally not recorded in order to save time.
If this is set, however, all such impacts will be recorded by attaching an Impacts/RBDImpacts data to the objects that collide.
Culling Method
In simulations with a large number of objects, it is helpful to use various space partitioning schemes to reduce the work in finding collisions. This option selects one of these schemes.
None means that no attempt at spatial subdivision will occur.
Sphere means the objects will be treated as spheres and trivial intersection detection will be done with these spheres. This is fast, but with long skinny objects could cause false positives.
OBB means Oriented Bounding Boxes. While this provides a tight bound on long skinny objects, building the spatial partitioning tree is slow and will often exceed the benefits.
Contact Grouping Method
Controls whether and how Houdini groups similar points together when it calculates point collisions.
If you set this parameter to a value other than “none”, Houdini will treat similar points (that is, points within the distance specified in the Contact grouping tolerance below) as a single point for the purpose of calculating collisions.
This is useful when you have an object such as a cube, where the geometry points (the corners of the cube) are spaced far apart. One corner might impact a ground plane first, then the cube bounces and rotates so the opposite corner hits, which bounces and rotates, causing jitter when the cube hits.
If you set the Contact Grouping Method to “Average”, Houdini will calculate the hit based on an average point between the corners, giving a more stable result with less jitter.
This is similar to the effect of turning on Edge
representation in the Surface tab of an RBD
Object node. If you have sparse geometry
with sharp edges, such as a cube, you may want to turn on both
these options.
To see the effect of contact groupings, create a simulation
where you drop a cube onto a ground plane. Attach an 
RBD
Visualization DOP to see the
resulting impacts.
None
Calculate collisions for each point independently. Do not attempt to merge similar collision points.
Most central point
Group similar points together as the one point that is most in-line with the center of mass of the object. This uses only points from the original geometry and biases collision points to stable points.
Average point
Average similar points together to calculate the collision point.
This reflects the geometry of the actual collision better than “Most central point”, but may result in a point that does not lie on the original geometry.
Contact Grouping Tolerance
The distance within which points are grouped together when Contact grouping method is not “none”.
Minimum Substeps
The RBD Solver will break a full timestep into at least this number of substeps.
By increasing this, you can guarantee a minimum fineness to the substepping. This can be used if for some reason the automatic computations are too coarse.
Maximum Substeps
The RBD Solver will not break the simulation down into more substeps than this.
It is a very good idea to always have a maximum to ensure frames will be finished regardless of their complexity. Lowering this ceiling can ensure a minimum computation time at the expense of accuracy.
CFL Condition
The CFL Condition is a factor used for automatically determining what size substep a scene requires. The idea is that any substep should not allow any objects to interpenetrate by more than one voxel cell.
This condition is met when this parameter is at 1. A value of 10 would allow a substep to interpenetrate by as many as 10 voxel cells. This could allow objects to tunnel through each other rather than properly bounce.
Parameter Operations
Each data option parameter has an associated menu which specifies how that parameter operates.
Use Default
Use the value from the Default Operation menu.
Set Initial
Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity.
Set Always
Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming.
You can also use this setting in
conjunction with the local variables for a parameter value to
modify a value over time. For example, in the X Position, an
expression like $tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
Set Never
Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input.
For example, an 
RBD State
DOP may want to animate just the mass of an
object, and nothing else. The Set Never option could be used
on all parameters except for Mass, which would use Set
Always.
Default Operation
For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.
This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.
Make Objects Mutual Affectors
All objects connected to the first input of this node become mutual affectors.
This is equivalent to using an 
Affector
DOP to create an affector relationship between
* and * before connecting it to this node. This option makes it
convenient to have all objects feeding into a solver node affect
each other.
Group
When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.
Data Name
Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a “/” (or several), that indicates traversing inside subdata.
For example, if the 
Fan Force DOP has the default Data Name
“Forces/Fan”. This attaches the data with the name “Fan” to an
existing piece of data named “Forces”. If no data named “Forces”
exists, a simple piece of container data is created to hold the
“Fan” subdata.
Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.
Unique Data Name
Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.
With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.
If an object
needs to have several Fan Forces blowing on it, it is
much easier to use the Unique Data Name feature to ensure that
each fan does not overwrite a previous fan rather than trying to
change the Data Name of each fan individually to avoid
conflicts.
On the other hand, if an object is known to have RBD
State data already attached to it, leaving this
option turned off will allow some new RBD State
data to overwrite the existing data.
Solver Per Object
The default behavior for solvers is to attach the exact same solver to all of the objects specified in the group. This allows the objects to be processed in a single pass by the solver, since the parameters are identical for each object.
However, some objects operate more logically on a single object at
a time. In these cases, one may want to use $OBJID expressions
to vary the solver parameters across the objects. Setting this
toggle will create a separate solver per object, allowing $OBJID
to vary as expected.
Setting this is also required if stamping the parameters with a Copy Data DOP.
Inputs ¶
First Input
This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.
If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.
All Other Inputs
If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.
The specific types of subdata that are meaningful vary from node to
node. Click 
an input connector to see a list of available
data nodes that can be meaningfully attached.
Outputs ¶
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Locals ¶
channelname
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname)
If the data does not already exist, then a value of zero or an empty string will be returned.
DATACT
This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.
DATACF
This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.
RELNAME
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to the name of the relationship to which the data is being attached.
RELOBJIDS
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.
RELOBJNAMES
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.
RELAFFOBJIDS
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.
RELAFFOBJNAMES
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.
ST
The simulation time for which the node is being evaluated.
Depending on the settings of the 
DOP Network
Offset Time and Scale Time parameters,
this value may not be equal to the current Houdini time
represented by the variable T.
ST 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 $FF == 1.
SF
The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.
Depending on the settings of the DOP Network parameters,
this value may not be equal to the current Houdini frame number
represented by the variable F. Instead, it is equal to
the simulation time (ST) divided by the simulation timestep size
(TIMESTEP).
TIMESTEP
The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.
SFPS
The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
SNOBJ
The number of objects in the simulation. For nodes that
create objects such as the 
Empty Object DOP,
SNOBJ increases for each object that is evaluated.
A good way to guarantee unique object names is to use an expression
like object_$SNOBJ.
NOBJ
The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.
NOBJ may return 0 if the node does not
process each object sequentially (such as the 
Group
DOP).
OBJ
The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies 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
is -1 if the node does not process objects sequentially (such
as the Group DOP).
OBJID
The unique 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. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.
This value is also the best way to look up information on an object using the dopfield expression function.
OBJID is -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
The simulation time (see variable ST) at which the current object was created.
To check if an object was created
on the current timestep, the expression $ST == $OBJCT should
always be used.
This value is zero if the node does not process
objects sequentially (such as the Group DOP).
OBJCF
The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.
This value is zero if the node does not process objects
sequentially (such as the Group DOP).
OBJNAME
A string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named “myobject”,
specifying strcmp($OBJNAME, "myobject") == 0 in the activation field
of a DOP will cause that DOP to operate on only those 20 objects.
This value is the empty string if the node does not process objects
sequentially (such as the Group DOP).
DOPNET
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 DOP,
you could write the expression:
$tx + 0.1
…to make the object move 0.1 units along the X axis at each timestep.
Examples ¶
DegreesOfFreedom Example for RBD Solver dynamics node
This example demonstrates the use of the Constraint Type parameter on the RBD Constraint node. This parameter controls the number of degrees of freedom the constrained object has.
PaddleWheel Example for RBD Solver dynamics node
This example combines a number of elements and features of RBD to create a simulation of a paddle wheel being hit by a large number of falling objects.
This example demonstrates features such as resolving penetrations, gluing simple objects together to create more complex objects, grouping of objects, and constraints.
