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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 is a union of two different rigid body engines, the RBD engine and the Bullet engine. The RBD engine uses volumes and is useful for complicated, deforming, stacked, geometry. The Bullet engine offers simpler collision shapes and is suitable for fast, large-scale simulations.
Details on the first engine are found in the RBD Solver.
The RBD and Bullet engines also have support for voronoi fracturing. For more information see the Voronoi Fracture Solver help. You must have a Voronoi Fracture Configure Object for voronoi fracturing to work.
Details on the second engine are found in the Bullet Solver.
Choose between a Bullet or RBD Solver.
Scales the timestep used by the solver. This parameter can be animated. This can be used to create bullet-time like effects where the physics of the Bullet solver run at a different rate than the Houdini playbar. A value of 2 will cause objects to fall twice as fast, and 0.1 will slow them to a tenth the speed.
Number of Substeps
The number of substeps for each simulation step, used by Bullet internally. Increasing this number will increase the resolution of the simulation.
In Bullet’s documentation:
maxSubSteps = substeps + 1 fixedTimeStep = timeStep / substeps
Houdini adds 1 to
maxSubSteps to account for roundoff errors during division when substeps > 1.
See Stepping the World.
When an object’s speed has been below its linear and angular speed thresholds for this amount of time, the object is eligible to be deactivated and put to sleep. This can improve performance for simulations where there are some stationary objects. NOTE
An object can only be put to sleep once any nearby objects and objects it is constrained to are also eligible to be put to sleep.
Contact Breaking Threshold
Distance threshold used by the Bullet engine when determining whether a cached contact point should be discarded. Adjusting this value according to the scene scale may also improve performance, as it influences the margin added to objects' bounding boxes.
Specifies which constraint solver Bullet will use to resolve collisions and constraints. Both solvers parallelize the workload, but differ in the strategy they use to do so. Parallel Gauss-Seidel (Islands) will be faster in cases that involve many small "islands" of interacting objects (for example, a large number of small separate book stacks), whereas Parallel Gauss-Seidel (Graph Coloring) should perform better when such "islands" are few and large (such as a huge collapsing building).
Although results obtained with these solvers will generally not be identical, qualitative differences should be minor.
The more iterations you use, the more accurate the constraint and collision handling will be.
Allows the constraint solver to terminate before performing the full number of Constraint Iterations if it is close enough to the solution. Larger values can increase performance at the cost of accuracy.
Randomize Constraint Order
Specifies that the constraints should be randomly reordered before each of the Constraint Iterations. This may improve stability, but incurs a minor performance hit.
Ensure Islands are Independent
Specifies that the solver should ensure that changes to an island of interacting objects (including adding, removing, or repositioning objects) do not cause other islands to produce different simulation results, unless those changes cause the objects to interact. Otherwise, the solver only guarantees that resimulating with the exact same input to the solver will produce the same results. Enabling this option may incur a minor performance hit, and may change the simulation results slightly.
Increasing the CFM (constraint force mixing) parameter will make contact constraints softer, and may increase the stability of the simulation. Contact constraints may be violated by an amount proportional to this parameter times the force that is needed to enforce the constraint.
Specifies what proportion of the constraint error for contact constraints will be fixed during the next simulation step. If ERP (error reduction parameter) is set to 0, constrained objects will drift apart as the simulation proceeds. If ERP is set to 1, the solver will attempt to fix all constraint error during the next simulation step (however, this may result in instability in some situations). A value between 0.1 and 0.8 is recommended for most simulations.
Tries to make interpenetrating objects split without adding velocity (to keep objects from explosively flying apart).
See Split Impulse.
Split Impulse only applies when objects interpenetrate by more than this distance. This number should be negative (representing less than 0 distance between the objects).
See Split Impulse.
Split Impulse ERP
Overrides the Error Reduction Parameter for contact constraints where the penetration distance is within the Penetration Threshold and Split Impulse is enabled.
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.
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.
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.
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.
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.
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.
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.
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 $FF 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.
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.
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.
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 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 Piece Volume
The minimum volume for any piece geometry created by this solver. This can avoid creating geometry that is too small for the RBD Solver to handle in a stable manner.
The tolerance to use when fusing clustered pieces together.
Stamp Interior Primitives With Creation Time
Creates an attribute called
creationtime on all newly-created interior primitives that stores their creation time. This can be useful for effects such as emitting particles for debris for a short amount of time after the initial fracture of a primitive.
Allow Fracturing From Feedback
Usually fracturing is driven only by objects that solve before this object in the frame. If two-way coupling between different solvers is done using feedbacks, this option allows those feedback effects to also trigger fracturing.
Fracture Ignores Resting Objects
Impacts generated by objects merely resting on each other will not be considered for fracturing.
This value is used to initialize the pseudo-random sequence used to generate fracture points during the simulation. It is useful for generating different simulations from the same network.
Convert to Poly
The fracturing only works with polygonal geometry. This option auto-converts the geometry into polygons at the given LOD. This conversion only happens when the fracturing occurs.
Each data option parameter has an associated menu which specifies how that parameter operates.
Use the value from the Default Operation menu.
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.
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
$tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
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.
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
* before connecting it to this node. This option makes it
convenient to have all objects feeding into a solver node affect
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.
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.
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
to vary the solver parameters across the objects. Setting this toggle will
create a separate solver per object, allowing
$OBJID to vary as expected.
The objects to solve.
Microsolvers attached to this input will run after the main solve step.
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($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.
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.
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.
In this case, this value is set to the name of the relationship the data to which the data is being attached.
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.
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.
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.
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.
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.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
This string contains a space separated list of the names of every object being processed by the current node.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
strcmp($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).
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.
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.
The following examples include this node.
This example demonstrates how different anchor positions can affect pin constraints.
This example demonstrates how to use pin constraints to create hinges between objects.
This example shows how to create a simple network of soft constraints, which are used to allow an object to bend before breaking.
This example demonstrates two fluids with different densities and viscosities interacting with a solid object.
This sample creates a simple ragdoll using the cone twist constraint between pieces of the ragdoll.
This example demonstrates the how the shatter, RBD Fractured Object, and Debris shelf tools can be used to create debris emanating from fractured pieces of geometry.
First, the Shatter tool (from the Model tool shelf) is used on the glass to define the fractures. Then the RBD Fracture tool is used on the glass to create RBD objects out of the fractured pieces. Then the Debris tool is used on the RBD fractured objects to create debris.
This is an example of how to use the RBD Glue Object node to create an RBD object that automatically breaks apart on collision. It also demonstrates one technique for breaking a model into pieces appropriate for this sort of simulation.
This example demonstrates the friction parameter on an RBD Object.
This example demonstrates the use of the Initial State parameter of an RBD object.
This example shows how to modify the "active" point attribute of an RBD Packed Object to change objects from static to active.
This example shows how to use animated packed primitives in an RBD Packed Object and set up a transition to active objects later in the simulation.
This example shows how to remove objects from the simulation that are inside a bounding box.
This example shows how to limit the speed of specific objects in the simulation.
In this chain simulation, the individual chain links react to one another in an RBD sim.
This sample creates a box which can only slide and rotate on one axis, using the Slider Constraint.
This example demonstrates the use of the RBD State node to inherit velocity from movement and collision with other objects in a glued RBD fracture simulation.
This example demonstrates how to use the Ripple Solver and Ripple Object nodes. Bulge SOPs are used to deform a grid to create initial geometry and rest geometry for the Ripple Object which is then piped into the Ripple Solver.
This example demonstrates how to use the Script Solver node to scale fractured pieces of an RBD sim over time.
This example uses static object nodes in an RBD simulation of a grid falling and bouncing off three spheres before it hits the ground.
This example actually includes eight examples of ways that you can use voronoi fracturing in Houdini. In particular, it shows how you can use the Voronoi Fracture Solver and the Voronoi Fracture Configure Object nodes in your fracture simulations. Turn on the display flags for these examples one at a time to play the animation and dive down into each example to examine the setup.
This example shows how you can break a sphere into packed objects for use in a rigid body simulation using the Assemble SOP.
Here is an example of accumulating and fading an attribute
This example shows how to create a low res - high res set up to support RBD objects. The two main methods are to reference copy the DOP Import SOP and feed in the high res geometry or to use point instancing with an Instance Object.
This example demonstrates a creating points for each matching record in the DOP simulation. This lets us create a point for each object or a point for each impact.
This example shows how to create packed primitives with animated transforms from deforming geometry that represents rigid motion. The result is ideal for colliders in a rigid body simulation.
This example shows how to use the gluecluster SOP and glue constraint networks to cluster together the pieces of a voronoi fracture. This allows clustering to be used with Bullet without introducing concave objects.
This example demonstrates how to use the TimeShift SOP to achieve a slow-motion effect during a fracture simulation.