Bullet Solver dynamics node

Sets and configures a Bullet Dynamics solver.

On this page	Parameters Constraint Solver Sticky Collisions Inputs Outputs Locals
Since	12.0

The Bullet Solver DOP sets objects to use the Bullet Dynamics solver.

This solver can use simplified representation of the objects, such as boxes or spheres, or a composite of these simple shapes to make-up a more complex shape. This solver can use arbitrary convex shapes based on the geometry points of the object, and can also collide objects against affectors that are cloth, solid, or wire objects.

Note

For mutual interaction between wire and bullet objects, the Wire Solver’s Collision Handling parameter should be set to SDF.

Note

Collision of concave shapes is known to have some problems, and is not fully supported.

Warning

The Bullet Solver does not support scaling at the object level.

Parameters ¶

Time Scale

The scale value (multiplier) for the time step used by the solver. This can create bullet time-like effects where the physics of the Bullet solver runs at a different rate than the Houdini playbar. A value of 2 causes objects to fall twice as fast, and a value of 0.1 slows them down to a tenth of the speed. This parameter can be animated.

Number of Substeps

The number of substeps for each simulation step. This is used by Bullet internally. Increasing this number increases the resolution of the simulation.

Tip

Increasing the number of substeps is one way to fix the problem of collisions not being detected for quickly moving objects.

Sleeping Time

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.

Use Implicit Drag

Applies a more accurate damping for the drag described by the targetv and airresist point attributes, instead of applying it as an explicit force. This also affects how the targetw and spinresist attributes are applied for angular drag.

Enable Aging

For packed objects, the age point attribute will be updated for each time step and the objects flagged for deletion if they exceed their life attribute. For constraint networks, the age primitive attribute will be updated for each time step.

Constraint Solver ¶

Constraint Solver

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.

Constraint Iterations

The number of constraint iterations used by the solver. The more iterations, the more accurate the constraint and collision handling.

Solve Tolerance

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.

Contact CFM

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.

Contact ERP

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.

Split Impulse

Tries to make interpenetrating objects split without adding velocity (to keep objects from explosively flying apart).

Errors when solving positional and velocity constraints can introduce some extra energy to the system. Although this option removes most of the extra energy, it degenerates quality a little bit, in particular for stable stacking.

Penetration Threshold

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).

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.

Sticky Collisions ¶

If sticky collisions are turned on for an object, the solver creates constraints at the instant that a suitable collision occurs, attaching the objects together for the remainder of the timestep. At the end of the timestep, these new constraints are added into a constraint network’s geometry according to the External Constraint Network and Use Internal Constraint Network parameters.

Tip

The newstickycollisions primitive group identifies the constraints produced by sticky collisions during the last timestep. This group can be used to configure the properties of new constraints, for example, by using RBD Constraint Properties in a SOP solver. By default, the new constraints have a constraint_name attribute with a value of Glue.

The stickycollisions primitive group identifies all of the constraints produced by sticky collisions during the simulation.

Tip

The solver creates additional primitive attributes with information about the sticky collision that produced the constraint. The collision_depth float primitive attribute indicates the penetration depth of the contact point, and collision_impulse float primitive attribute indicates the impulse that would have been applied to resolve the collision.

The collision_pos, collision_normal and collision_relvel vector primitive attributes contain the contact point’s position, normal and relative velocity, respectively. The collision_vel vector point attribute contains the velocity at the contact point for each object.

External Constraint Network

Specifies the constraint network where constraints produced by sticky collisions should be emitted. If not specified, the first constraint network encountered by the solver will be used. If no constraint networks exist, sticky collisions will be turned off.

This constraint network must be a relationship (i.e. Attach Internal Constraints to Object is not turned on), and will receive any constraints not added to an internal constraint network by Use Internal Constraint Networks.

Use Internal Constraint Networks

If turned on, the solver will add constraints into the internal constraint network (ConstraintGeometry geometry subdata) for sticky collisions between two objects from the same DOP object. If the DOP object does not have ConstraintGeometry subdata, the constraint will be added to the External Constraint Network. An internal constraint network (ConstraintGeometry subdata) can be added to an object using a Constraint Network DOP with Attach Internal Constraints to Object turned on.

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 MMB 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