# Physics Constraints (bge.constraints)¶

Bullet Physics provides collision detection and rigid body dynamics for the Blender Game Engine.

Features:

• Vehicle simulation.

• Rigid body constraints: hinge and point to point (ball socket).

• Access to internal physics settings, like deactivation time, and debugging features

Note

Since this API is not well documented, it can be unclear what kind of values to use for setting parameters. In general, damping settings should be in the range of 0 to 1 and stiffness settings should not be much higher than about 10.

## Examples¶

For more examples of Bullet physics and how to use them see the pybullet forum.

## Basic Physics Constraint¶

Example of how to create a hinge Physics Constraint between two objects.

from bge import logic
from bge import constraints

# get object list
objects = logic.getCurrentScene().objects

# get object named Object1 and Object 2
object_1 = objects["Object1"]
object_2 = objects["Object2"]

# want to use Edge constraint type
constraint_type = 2

# get Object1 and Object2 physics IDs
physics_id_1 = object_1.getPhysicsId()
physics_id_2 = object_2.getPhysicsId()

# use bottom right edge of Object1 for hinge position
edge_position_x = 1.0
edge_position_y = 0.0
edge_position_z = -1.0

# rotate the pivot z axis about 90 degrees
edge_angle_x = 0.0
edge_angle_y = 0.0
edge_angle_z = 90.0

# create an edge constraint
constraints.createConstraint(physics_id_1, physics_id_2,
constraint_type,
edge_position_x, edge_position_y, edge_position_z,
edge_angle_x, edge_angle_y, edge_angle_z)


## Functions¶

bge.constraints.createConstraint(physicsid_1, physicsid_2, constraint_type, pivot_x=0.0, pivot_y=0.0, pivot_z=0.0, axis_x=0.0, axis_y=0.0, axis_z=0.0, flag=0)

Creates a constraint.

Parameters
• physicsid_1 (int) – The physics id of the first object in constraint.

• physicsid_2 (int) – The physics id of the second object in constraint.

• constraint_type (int) – The type of the constraint, see Create Constraint Constants.

• pivot_x (float) – Pivot X position. (optional)

• pivot_y (float) – Pivot Y position. (optional)

• pivot_z (float) – Pivot Z position. (optional)

• axis_x (float) – X axis angle in degrees. (optional)

• axis_y (float) – Y axis angle in degrees. (optional)

• axis_z (float) – Z axis angle in degrees. (optional)

• flag (int) – 128 to disable collision between linked bodies. (optional)

Returns

A constraint wrapper.

Return type

KX_ConstraintWrapper

bge.constraints.createVehicle(physicsid)

Creates a vehicle constraint.

Parameters

physicsid (int) – The physics id of the chassis object in constraint.

Returns

A vehicle constraint wrapper.

Return type

KX_VehicleWrapper

bge.constraints.exportBulletFile(filename)

Exports a file representing the dynamics world (usually using .bullet extension).

Parameters

filename (str) – File path.

bge.constraints.getAppliedImpulse(constraintId)
Parameters

constraintId (int) – The id of the constraint.

Returns

The most recent applied impulse.

Return type

float

bge.constraints.getVehicleConstraint(constraintId)
Parameters

constraintId (int) – The id of the vehicle constraint.

Returns

A vehicle constraint object.

Return type

KX_VehicleWrapper

bge.constraints.getCharacter(gameobj)
Parameters

gameobj (KX_GameObject) – The game object with the character physics.

Returns

Character wrapper.

Return type

KX_CharacterWrapper

bge.constraints.removeConstraint(constraintId)

Removes a constraint.

Parameters

constraintId (int) – The id of the constraint to be removed.

bge.constraints.setContactBreakingTreshold(breakingTreshold)

Note

Reasonable default is 0.02 (if units are meters)

Sets tresholds to do with contact point management.

Parameters

breakingTreshold (float) – The new contact breaking treshold.

bge.constraints.setDeactivationAngularTreshold(angularTreshold)

Sets the angular velocity treshold.

Parameters

angularTreshold (float) – New deactivation angular treshold.

bge.constraints.setDeactivationLinearTreshold(linearTreshold)

Sets the linear velocity treshold.

Parameters

linearTreshold (float) – New deactivation linear treshold.

bge.constraints.setDeactivationTime(time)

Sets the time after which a resting rigidbody gets deactived.

Parameters

time (float) – The deactivation time.

bge.constraints.setERPNonContact(erp)

Sets the Error Reduction Parameter (ERP) for non-contact constraints. The Error Reduction Parameter (ERP) specifies what proportion of the joint error will be fixed during the next simulation step. If ERP = 0.0 then no correcting force is applied and the bodies will eventually drift apart as the simulation proceeds. If ERP = 1.0 then the simulation will attempt to fix all joint error during the next time step. However, setting ERP = 1.0 is not recommended, as the joint error will not be completely fixed due to various internal approximations. A value of ERP = 0.1 to 0.8 is recommended.

Parameters

erp (float [0.0, 1.0]) – The ERP parameter for non-contact constraints.

bge.constraints.setERPContact(erp2)

Sets the Error Reduction Parameter (ERP) for contact constraints. The Error Reduction Parameter (ERP) specifies what proportion of the joint error will be fixed during the next simulation step. If ERP = 0.0 then no correcting force is applied and the bodies will eventually drift apart as the simulation proceeds. If ERP = 1.0 then the simulation will attempt to fix all joint error during the next time step. However, setting ERP = 1.0 is not recommended, as the joint error will not be completely fixed due to various internal approximations. A value of ERP = 0.1 to 0.8 is recommended.

Parameters

erp2 (float [0.0, 1.0]) – The ERP parameter for contact constraints.

bge.constraints.setCFM(cfm)

Sets the Constraint Force Mixing (CFM) for soft constraints. If the Constraint Force Mixing (CFM) is set to zero, the constraint will be hard. If CFM is set to a positive value, it will be possible to violate the constraint by pushing on it (for example, for contact constraints by forcing the two contacting objects together). In other words the constraint will be soft, and the softness will increase as CFM increases.

Parameters

cfm (float [0.0, 10000.0]) – The CFM parameter for soft constraints.

bge.constraints.setDebugMode(mode)

Sets the debug mode.

Parameters

mode (int) – The new debug mode, see Debug Mode Constants.

bge.constraints.setGravity(x, y, z)

Sets the gravity force.

Parameters
• x (float) – Gravity X force.

• y (float) – Gravity Y force.

• z (float) – Gravity Z force.

Sets the linear air damping for rigidbodies.

bge.constraints.setNumIterations(numiter)

Sets the number of iterations for an iterative constraint solver.

Parameters

numiter (int) – New number of iterations.

bge.constraints.setNumTimeSubSteps(numsubstep)

Sets the number of substeps for each physics proceed. Tradeoff quality for performance.

Parameters

numsubstep (int) – New number of substeps.

bge.constraints.setSolverDamping(damping)

Note

Very experimental, not recommended

Sets the damper constant of a penalty based solver.

Parameters

damping (float) – New damping for the solver.

bge.constraints.setSolverTau(tau)

Note

Very experimental, not recommended

Sets the spring constant of a penalty based solver.

Parameters

tau (float) – New tau for the solver.

bge.constraints.setSolverType(solverType)

Note

Very experimental, not recommended

Sets the solver type.

Parameters

solverType (int) – The new type of the solver.

bge.constraints.setSorConstant(sor)

Note

Very experimental, not recommended

Sets the successive overrelaxation constant.

Parameters

sor (float) – New sor value.

### Constants¶

bge.constraints.error

Symbolic constant string that indicates error.

Type

str

#### Debug Mode Constants¶

Debug mode to be used with setDebugMode().

bge.constraints.DBG_NODEBUG

No debug.

bge.constraints.DBG_DRAWWIREFRAME

Draw wireframe in debug.

bge.constraints.DBG_DRAWAABB

Draw Axis Aligned Bounding Box in debug.

bge.constraints.DBG_DRAWFREATURESTEXT

Draw features text in debug.

bge.constraints.DBG_DRAWCONTACTPOINTS

Draw contact points in debug.

bge.constraints.DBG_NOHELPTEXT

Debug without help text.

bge.constraints.DBG_DRAWTEXT

Draw text in debug.

bge.constraints.DBG_PROFILETIMINGS

Draw profile timings in debug.

bge.constraints.DBG_ENABLESATCOMPARISION

Enable sat comparision in debug.

bge.constraints.DBG_DISABLEBULLETLCP

Disable Bullet LCP.

bge.constraints.DBG_ENABLECCD

Enable Continous Collision Detection in debug.

bge.constraints.DBG_DRAWCONSTRAINTS

Draw constraints in debug.

bge.constraints.DBG_DRAWCONSTRAINTLIMITS

Draw constraint limits in debug.

bge.constraints.DBG_FASTWIREFRAME

Draw a fast wireframe in debug.

#### Create Constraint Constants¶

Constraint type to be used with createConstraint().

bge.constraints.POINTTOPOINT_CONSTRAINT
bge.constraints.LINEHINGE_CONSTRAINT
bge.constraints.ANGULAR_CONSTRAINT
bge.constraints.CONETWIST_CONSTRAINT
bge.constraints.VEHICLE_CONSTRAINT
bge.constraints.GENERIC_6DOF_CONSTRAINT