Constraints

Physics Constraint Component

The child is Component/Bone 1 and the parent is Component/Bone 2 !!

Stability

To have a stable chain of physics constraints (eg: multiple capsules forming a cable) you should have everything parent correctly.

For example, if you have a cable, a player capsule and a static cube, the chain should be Cube -> Cable Capsule -> ... -> Cable Capsule -> Player Capsule as Parent -> Child.

Adriel

“The solver will iterate trying to solve the chain, then after that it’ll do the projection step to pull every child towards its parent. You want everything pulled towards the solved state which is a chain of things from the cube to the player”

You can (and should) try to bump the number in the physics solver settings (in Project Settings).

Breaking

Before calling OnConstraintBroken inside UPhysicsConstraintComponent::OnConstraintBrokenWrapper (there is also UPhysicsConstraintComponent::OnConstraintBrokenHandler) the breaking state is pulled in FPhysicsSolverBase::PullPhysicsStateForEachDirtyProxy_External in a loop iterating LatestData->DirtyJointConstraints and calling FJointConstraintPhysicsProxy::PullFromPhysicsState on each entry.

Inside FPhysScene_Chaos::OnSyncBodies, if the bIsBreaking in FJointConstraint, FOutputData is true the FConstraintBrokenDelegateWrapper wrapper will be called (see OnConstraintBrokenWrapper above).

About bIsBroken in OutputData

It reflects if IsConstraintEnabled return false (bIsBroken = !IsConstraintEnabled)

This output data is pulled and set in FJointConstraintPhysicsProxy::PullFromPhysicsState (see above) by reading the buffer FDirtyJointConstraintData.

The actual set of bIsBreaking (and the rest of the FDirtyJointConstraintData Buffer) is inside FJointConstraintPhysicsProxy::BufferPhysicsResults where it calls IsConstraintBreaking on a FPBDJointConstraintHandle (FPhysScene_Chaos::OnSyncBodies).

This calls FPBDJointConstraints::IsConstraintBreaking on a FConstraintContainer (templated to FPBDJointConstraints for FPBDJointConstraintHandle), which just reads the value of bBreaking on the element stored in ConstraintStates.

This bBreaking state is set by FPBDJointConstraints::SetConstraintBroken (only called by FPBDJointConstraints::BreakConstraint (by the way this also disables the constraint) and FixConstraint). The only usage of this break function is FPBDJointConstraints::SetSolverResults which is called in Chaos::Private::ScatterOutputImpl (called by FPBDJointContainerSolver::ScatterOutput).

This function will set the value of the argument bIsBroken by calling IsBroken() on the solver (templated to FPBDJointCachedSolver here). This function just returns the value of the variable bIsBroken of the cached solver. This variable is only set by SetIsBroken() which is only called in Chaos::Private::ApplyPositionConstraintsImpl (called by FPBDJointContainerSolver::ApplyPositionConstraints, search for Solver.SetIsBroken).

This Solver.SetIsBroken call occurs because GetJointShouldBreak returned true. Since GetJointShouldBreak is called on a FPBDJointCachedSolver its doing some math to get the linear and angular impulse so you can’t simply read some variables to get the values.

Debug values of LinearImpulse and AngularImpulse in GetJointShouldBreak

(Example for LinearImpulse)

• Add a conditional breakpoint by using LinearForceSq > LinearThresholdSq at the last line of the first if block
• To get the value of LinearForceSq run the following (LinearImpulse.X*LinearImpulse.X + LinearImpulse.Y*LinearImpulse.Y + LinearImpulse.Z*LinearImpulse.Z) / (Dt * Dt * Dt * Dt). This mimics the math used to get LinearForceSq.

Motor

Orientation, Position and Velocity Target of the motors are relative to the constraint origin.

Constraint on body instance

See Constraint

Commands & debug

Some interesting commands, a lot more available

• Stat ChaosConstraintSolver
• Stat ChaosConstraintDetails

See Console commands & debugging in /Physics for more Chaos commands