UPrimitiveComponent::ComputePenetration returns wrong MTD when the shape is a Box

Development World Creation
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1

Turns out UPrimitiveComponent::ComputePenetration returns wrong values when the UPrimitiveComponent is a box. It’s weird because, for example, checking a Capsule or a Sphere against a Box works fine, but checking the same Box against the Capsule/Sphere returns worng values. Boxes against other boxes also returns wrong values.

Doesn’t happen always (depends on the position and the size of the shapes), but in a lot of cases.
I can share a small project that reproduces the issue.

Here’s a screenshot that shows the issue:

The highlighted log shows the MTD that the Box is returning, the next MTD in the log is what the capsule returns against the same collision, wich makes more sense.

2

Hi TS100101,

Please post the small project here. If you’re not comfortable posting it on a public site, feel free to private message me a link to download it on the Forums.

https://forums.unrealengine.com/

3

Hi TJ, thanks for your answer.

Here’s the project :

4

I’ve been looking into the source code and seems like the problem is that the function assumes that the CollisionShape is always a capsule.

I’ve written my own modified ComputePenetration function and seems to work fine in all cases. Here’s the code in case someone wants to take a look :

bool UPrimitiveComponent::ComputePenetrationCustom(FMTDResult & OutMTD, UPrimitiveComponent& CollisionPrimitive)
{
#if WITH_PHYSX
	const PxRigidActor* PRigidBody0 = CollisionPrimitive.BodyInstance.GetPxRigidActor();
	if (PRigidBody0 == NULL || PRigidBody0->getNbShapes() == 0)
	{
		return false;
	}
	const PxTransform PGlobalPose0 = PRigidBody0->getGlobalPose();

	const PxRigidActor* PRigidBody1 = BodyInstance.GetPxRigidActor();
	if (PRigidBody1 == NULL || PRigidBody1->getNbShapes() == 0)
	{
		return false;
	}
	const PxTransform PGlobalPose1 = PRigidBody1->getGlobalPose();

	// Get all the shapes from the actors
	// TODO: we should really pass the shape from the overlap info since doing it this way we do an overlap test twice
	TArray<PxShape*> PShapes0;
	PShapes0.AddZeroed(PRigidBody0->getNbShapes());
	int32 NumTargetShapes0 = PRigidBody0->getShapes(PShapes0.GetData(), PShapes0.Num());

	TArray<PxShape*> PShapes1;
	PShapes1.AddZeroed(PRigidBody1->getNbShapes());
	int32 NumTargetShapes1 = PRigidBody1->getShapes(PShapes1.GetData(), PShapes1.Num());

	for (int32 PShapeIdx0 = 0; PShapeIdx0 < PShapes0.Num(); ++PShapeIdx0)
	{
		const PxShape * PShape0 = PShapes0[PShapeIdx0];
		check(PShape0);

		// Calc shape global pose
		PxTransform PGeomPose0 = PGlobalPose0.transform(PShape0->getLocalPose());
		GeometryFromShapeStorage GeomStorage0;
		PxGeometry * PGeom0 = GetGeometryFromShape(GeomStorage0, PShape0, true);

		if (PGeom0)
		{
			for (int32 PShapeIdx1 = 0; PShapeIdx1 < PShapes1.Num(); ++PShapeIdx1)
			{
				const PxShape * PShape1 = PShapes1[PShapeIdx1];
				check(PShape1);

				// Calc shape global pose
				PxTransform PGeomPose1 = PGlobalPose1.transform(PShape1->getLocalPose());
				GeometryFromShapeStorage GeomStorage1;
				PxGeometry * PGeom1 = GetGeometryFromShape(GeomStorage1, PShape1, true);

				if (PGeom1)
				{
					PxVec3 POutDirection;
					bool bSuccess = PxGeometryQuery::computePenetration(POutDirection, OutMTD.Distance, *PGeom0, PGeomPose0, *PGeom1, PGeomPose1);
					if (bSuccess)
					{
						if (POutDirection.isFinite())
						{
							OutMTD.Direction = P2UVector(POutDirection);
							return true;
						}
						else
						{
							UE_LOG(LogPhysics, Warning, TEXT("UPrimitiveComponent::ComputePenetration: MTD returned NaN"));
							return false;
						}
					}
				}
			}
		}
	}
#endif

	return false;
}