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[VERY BREAKING CHANGE] fix a comically old friction bug in convex manifolds. #410

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RossNordby merged 1 commit into from
Aug 8, 2026
Merged

[VERY BREAKING CHANGE] fix a comically old friction bug in convex manifolds. #410

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30 changes: 12 additions & 18 deletions BepuPhysics/Constraints/Contact/ContactConvexTypes.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -472,11 +472,10 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact0.OffsetA, prestep.Contact0.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration0, ref wsvA);
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact1.OffsetA, prestep.Contact1.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration1, ref wsvA);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 2f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, out var offsetToManifoldCenterA);
TangentFrictionOneBody.Solve(x, z, offsetToManifoldCenterA, inertiaA, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA));
TwistFrictionOneBody.Solve(prestep.Normal, inertiaA, maximumTwistImpulse, ref accumulatedImpulses.Twist, ref wsvA);
Expand Down Expand Up @@ -636,11 +635,10 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact1.OffsetA, prestep.Contact1.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration1, ref wsvA);
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact2.OffsetA, prestep.Contact2.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration2, ref wsvA);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 3f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact2.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, prestep.Contact2.Depth, out var offsetToManifoldCenterA);
TangentFrictionOneBody.Solve(x, z, offsetToManifoldCenterA, inertiaA, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA) +
accumulatedImpulses.Penetration2 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact2.OffsetA));
Expand Down Expand Up @@ -810,11 +808,10 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact2.OffsetA, prestep.Contact2.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration2, ref wsvA);
PenetrationLimitOneBody.Solve(inertiaA, prestep.Normal, prestep.Contact3.OffsetA, prestep.Contact3.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration3, ref wsvA);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 4f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2 + accumulatedImpulses.Penetration3);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2 + accumulatedImpulses.Penetration3);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact2.OffsetA, prestep.Contact3.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, prestep.Contact2.Depth, prestep.Contact3.Depth, out var offsetToManifoldCenterA);
TangentFrictionOneBody.Solve(x, z, offsetToManifoldCenterA, inertiaA, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA) +
accumulatedImpulses.Penetration2 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact2.OffsetA) +
Expand Down Expand Up @@ -1135,12 +1132,11 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact0.OffsetA, prestep.Contact0.OffsetA - prestep.OffsetB, prestep.Contact0.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration0, ref wsvA, ref wsvB);
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact1.OffsetA, prestep.Contact1.OffsetA - prestep.OffsetB, prestep.Contact1.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration1, ref wsvA, ref wsvB);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 2f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, out var offsetToManifoldCenterA);
Vector3Wide.Subtract(offsetToManifoldCenterA, prestep.OffsetB, out var offsetToManifoldCenterB);
TangentFriction.Solve(x, z, offsetToManifoldCenterA, offsetToManifoldCenterB, inertiaA, inertiaB, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA, ref wsvB);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA));
TwistFriction.Solve(prestep.Normal, inertiaA, inertiaB, maximumTwistImpulse, ref accumulatedImpulses.Twist, ref wsvA, ref wsvB);
Expand Down Expand Up @@ -1312,12 +1308,11 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact1.OffsetA, prestep.Contact1.OffsetA - prestep.OffsetB, prestep.Contact1.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration1, ref wsvA, ref wsvB);
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact2.OffsetA, prestep.Contact2.OffsetA - prestep.OffsetB, prestep.Contact2.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration2, ref wsvA, ref wsvB);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 3f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact2.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, prestep.Contact2.Depth, out var offsetToManifoldCenterA);
Vector3Wide.Subtract(offsetToManifoldCenterA, prestep.OffsetB, out var offsetToManifoldCenterB);
TangentFriction.Solve(x, z, offsetToManifoldCenterA, offsetToManifoldCenterB, inertiaA, inertiaB, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA, ref wsvB);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA) +
accumulatedImpulses.Penetration2 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact2.OffsetA));
Expand Down Expand Up @@ -1499,12 +1494,11 @@ public static void Solve(in Vector3Wide positionA, in QuaternionWide orientation
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact2.OffsetA, prestep.Contact2.OffsetA - prestep.OffsetB, prestep.Contact2.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration2, ref wsvA, ref wsvB);
PenetrationLimit.Solve(inertiaA, inertiaB, prestep.Normal, prestep.Contact3.OffsetA, prestep.Contact3.OffsetA - prestep.OffsetB, prestep.Contact3.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration3, ref wsvA, ref wsvB);
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
var premultipliedFrictionCoefficient = new Vector<float>(1f / 4f) * prestep.MaterialProperties.FrictionCoefficient;
var maximumTangentImpulse = premultipliedFrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2 + accumulatedImpulses.Penetration3);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (accumulatedImpulses.Penetration0 + accumulatedImpulses.Penetration1 + accumulatedImpulses.Penetration2 + accumulatedImpulses.Penetration3);
FrictionHelpers.ComputeFrictionCenter(prestep.Contact0.OffsetA, prestep.Contact1.OffsetA, prestep.Contact2.OffsetA, prestep.Contact3.OffsetA, prestep.Contact0.Depth, prestep.Contact1.Depth, prestep.Contact2.Depth, prestep.Contact3.Depth, out var offsetToManifoldCenterA);
Vector3Wide.Subtract(offsetToManifoldCenterA, prestep.OffsetB, out var offsetToManifoldCenterB);
TangentFriction.Solve(x, z, offsetToManifoldCenterA, offsetToManifoldCenterB, inertiaA, inertiaB, maximumTangentImpulse, ref accumulatedImpulses.Tangent, ref wsvA, ref wsvB);
var maximumTwistImpulse = premultipliedFrictionCoefficient * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
accumulatedImpulses.Penetration0 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact0.OffsetA) +
accumulatedImpulses.Penetration1 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact1.OffsetA) +
accumulatedImpulses.Penetration2 * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact2.OffsetA) +
Expand Down
9 changes: 3 additions & 6 deletions BepuPhysics/Constraints/Contact/ContactConvexTypes.tt
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Expand Up @@ -261,10 +261,7 @@ for (int i = 0; i < contactCount; ++i)
PenetrationLimit<#=suffix#>.Solve(inertiaA<#=bodyCount == 2 ? ", inertiaB" : ""#>, prestep.Normal, prestep.Contact<#=i#>.OffsetA, <#if(bodyCount == 2) {#>prestep.Contact<#=i#>.OffsetA - prestep.OffsetB, <#}#>prestep.Contact<#=i#>.Depth, positionErrorToVelocity, effectiveMassCFMScale, prestep.MaterialProperties.MaximumRecoveryVelocity, inverseDtWide, softnessImpulseScale, ref accumulatedImpulses.Penetration<#=i#>, ref wsvA<#if (bodyCount == 2) {#>, ref wsvB<#}#>);
<#}#>
Helpers.BuildOrthonormalBasis(prestep.Normal, out var x, out var z);
<#if (contactCount > 1) {#>
var premultipliedFrictionCoefficient = new Vector<float>(1f / <#=contactCount#>f) * prestep.MaterialProperties.FrictionCoefficient;
<#}#>
var maximumTangentImpulse = <#=contactCount > 1 ? "premultipliedFrictionCoefficient" : "prestep.MaterialProperties.FrictionCoefficient"#> * (<#for (int i = 0; i < contactCount; ++i) {#>accumulatedImpulses.Penetration<#=i#><#if(i < contactCount - 1){#> + <#}}#>);
var maximumTangentImpulse = prestep.MaterialProperties.FrictionCoefficient * (<#for (int i = 0; i < contactCount; ++i) {#>accumulatedImpulses.Penetration<#=i#><#if(i < contactCount - 1){#> + <#}}#>);
<#if (contactCount > 1) {#>
FrictionHelpers.ComputeFrictionCenter(<#for (int i = 0; i < contactCount; ++i) {#>prestep.Contact<#=i#>.OffsetA, <#}#><#for (int i = 0; i < contactCount; ++i) {#>prestep.Contact<#=i#>.Depth, <#}#>out var offsetToManifoldCenterA);
<#if (bodyCount == 2) {#>
Expand All @@ -277,9 +274,9 @@ for (int i = 0; i < contactCount; ++i)
<#if (contactCount == 1) {#>
//If there's only one contact, then the contact patch as determined by contact distance would be zero.
//That can cause some subtle behavioral issues sometimes, so we approximate lever arm with the contact depth, assuming that the contact surface area will increase as the depth increases.
var maximumTwistImpulse = <#=contactCount > 1 ? "premultipliedFrictionCoefficient" : "prestep.MaterialProperties.FrictionCoefficient"#> * accumulatedImpulses.Penetration0 * Vector.Max(Vector<float>.Zero, prestep.Contact0.Depth);
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * accumulatedImpulses.Penetration0 * Vector.Max(Vector<float>.Zero, prestep.Contact0.Depth);
<#} else {#>
var maximumTwistImpulse = <#=contactCount > 1 ? "premultipliedFrictionCoefficient" : "prestep.MaterialProperties.FrictionCoefficient"#> * (
var maximumTwistImpulse = prestep.MaterialProperties.FrictionCoefficient * (
<#for (int i = 0; i < contactCount; ++i) {#>
accumulatedImpulses.Penetration<#=i#> * Vector3Wide.Distance(offsetToManifoldCenterA, prestep.Contact<#=i#>.OffsetA)<#=i == contactCount - 1 ? ");" : " +"#>
<#}#>
Expand Down
2 changes: 1 addition & 1 deletion Demos/Demos/Cars/CarDemo.cs
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Expand Up @@ -62,7 +62,7 @@ public override void Initialize(ContentArchive content, Camera camera)
const float wheelBaseWidth = x * 2;
const float wheelBaseLength = frontZ - backZ;

playerController = new SimpleCarController(SimpleCar.Create(Simulation, properties, new Vector3(0, 10, 0), bodyShapeIndex, bodyInertia, 0.5f, wheelShapeIndex, wheelInertia, 2f,
playerController = new SimpleCarController(SimpleCar.Create(Simulation, properties, new Vector3(0, 10, 0), bodyShapeIndex, bodyInertia, 0.35f, wheelShapeIndex, wheelInertia, 1f,
new Vector3(-x, y, frontZ), new Vector3(x, y, frontZ), new Vector3(-x, y, backZ), new Vector3(x, y, backZ), new Vector3(0, -1, 0), 0.25f,
new SpringSettings(5f, 0.7f), QuaternionEx.CreateFromAxisAngle(Vector3.UnitZ, MathF.PI * 0.5f)),
forwardSpeed: 75, forwardForce: 6, zoomMultiplier: 2, backwardSpeed: 30, backwardForce: 4, idleForce: 0.25f, brakeForce: 7, steeringSpeed: 1.5f, maximumSteeringAngle: MathF.PI * 0.23f,
Expand Down
2 changes: 1 addition & 1 deletion Demos/Demos/FrictionDemo.cs
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Expand Up @@ -95,7 +95,7 @@ public override void Initialize(ContentArchive content, Camera camera)
var boxDescription = BodyDescription.CreateDynamic(RigidPose.Identity, new Vector3(20, 0, 0), shape.ComputeInertia(1), Simulation.Shapes.Add(shape), 1e-2f);

int boxCount = 100;
float maximumFriction = 3;
float maximumFriction = 0.75f;
for (int i = 0; i < 100; ++i)
{
//Drop the boxes in a line with varying friction.
Expand Down
2 changes: 1 addition & 1 deletion Demos/Demos/Tanks/TankDemo.cs
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Expand Up @@ -93,7 +93,7 @@ public override void Initialize(ContentArchive content, Camera camera)
SuspensionSettings = new SpringSettings(2.5f, 1.5f),
WheelShape = wheelShapeIndex,
WheelInertia = wheelInertia,
WheelFriction = 2f,
WheelFriction = 1f,
TreadSpacing = 1f,
WheelCountPerTread = 5,
WheelOrientation = QuaternionEx.CreateFromAxisAngle(Vector3.UnitZ, MathF.PI * -0.5f),
Expand Down
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