Soft joint limits

[AccessViolationEnjoyer]

I'm seeing odd results in the behavior of joints with soft/spring limits.

I have a body with 1kg mass, limits set to (1, 1), stiffness set to 10 and damping set to 0. There is a D6 joint with the linear x-axis set to eLIMITED. In this case, everything is fine. The body oscillates with a period of 2 seconds.

However, if I make the spring critically damped by setting the damping to 2*sqrt(k*m) = 2*sqrt(10*1) ~= 0.6325, the behavior is not what I expect. The body overshoots. In fact, I can set the damping to a much higher value and the body still overshoots.

It looks like the damping isn't being applied correctly. Is this to be expected, or should I log an issue?

Snippet to reproduce is below:

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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// Copyright (c) 2008-2023 NVIDIA Corporation. All rights reserved.
// Copyright (c) 2004-2008 AGEIA Technologies, Inc. All rights reserved.
// Copyright (c) 2001-2004 NovodeX AG. All rights reserved.  

// ****************************************************************************
// This snippet illustrates simple use of physx
//
// It creates a number of box stacks on a plane, and if rendering, allows the
// user to create new stacks and fire a ball from the camera position
// ****************************************************************************

#include <ctype.h>
#include <iostream>
#include "PxPhysicsAPI.h"
#include "../../lowlevel/api/include/PxsMaterialManager.h"
#include "../snippetcommon/SnippetPrint.h"
#include "../snippetcommon/SnippetPVD.h"
#include "../snippetutils/SnippetUtils.h"
#include "../snippetrender/SnippetCamera.h"

using namespace physx;

static PxDefaultAllocator		gAllocator;
static PxDefaultErrorCallback	gErrorCallback;
static PxFoundation* gFoundation = NULL;
static PxPhysics* gPhysics = NULL;
static PxDefaultCpuDispatcher* gDispatcher = NULL;
static PxScene* gScene = NULL;
static PxMaterial* gMaterial = NULL;
static PxPvd* gPvd = NULL;
static PxD6Joint* joint = NULL;
static float currentTime = 0;


void initPhysics(bool interactive)
{
	PX_UNUSED(interactive);
	gFoundation = PxCreateFoundation(PX_PHYSICS_VERSION, gAllocator, gErrorCallback);

	gPvd = PxCreatePvd(*gFoundation);
	PxPvdTransport* transport = PxDefaultPvdSocketTransportCreate(PVD_HOST, 5425, 10);
	gPvd->connect(*transport, PxPvdInstrumentationFlag::eALL);

	gPhysics = PxCreatePhysics(PX_PHYSICS_VERSION, *gFoundation, PxTolerancesScale(), true, gPvd);

	PxSceneDesc sceneDesc(gPhysics->getTolerancesScale());
	sceneDesc.gravity = PxVec3(0.0f, 0.0f, 0.0f);
	gDispatcher = PxDefaultCpuDispatcherCreate(0);
	sceneDesc.cpuDispatcher = gDispatcher;
	sceneDesc.filterShader = PxDefaultSimulationFilterShader;
	sceneDesc.solverType = PxSolverType::ePGS;

	gScene = gPhysics->createScene(sceneDesc);

	gMaterial = gPhysics->createMaterial(0.5f, 0.5f, 0.6f);

	PxRigidDynamic* horizontal = gPhysics->createRigidDynamic(PxTransform(PxVec3(0.0, 2.0, 0.0)));
	horizontal->attachShape(*gPhysics->createShape(PxBoxGeometry(PxVec3(1, 1, 1)), *gMaterial, true));
	horizontal->setSolverIterationCounts(128, 8);
	horizontal->setLinearDamping(0.0);
	horizontal->setAngularDamping(0.0);
	horizontal->setSleepThreshold(0);
	physx::PxRigidBodyExt::setMassAndUpdateInertia(*horizontal, 1.0);

	joint = physx::PxD6JointCreate(*gPhysics, horizontal, PxTransform(PxIdentity), nullptr, horizontal->getGlobalPose());
	joint->setMotion(PxD6Axis::eX, PxD6Motion::eLIMITED);
	joint->setLinearLimit(physx::PxD6Axis::eX, physx::PxJointLinearLimitPair(1.0, 1.0, physx::PxSpring(10.0, 6.325)));

	gScene->addActor(*horizontal);
}

void stepPhysics(bool /*interactive*/)
{
	float dt = 1.0f / 100.0f;

	std::cout << currentTime << ": " << joint->getRelativeTransform().p.x << ", " << joint->getRelativeLinearVelocity().x << std::endl;
	
	gScene->simulate(dt);
	gScene->fetchResults(true);
	currentTime += dt;
}

void cleanupPhysics(bool /*interactive*/)
{
	PX_RELEASE(gScene);
	PX_RELEASE(gDispatcher);
	PX_RELEASE(gPhysics);
	if (gPvd)
	{
		PxPvdTransport* transport = gPvd->getTransport();
		gPvd->release();	gPvd = NULL;
		PX_RELEASE(transport);
	}
	PX_RELEASE(gFoundation);

	printf("SnippetHelloWorld done.\n");
}

void keyPress(unsigned char key, const PxTransform& camera)
{
	PX_UNUSED(key);
	PX_UNUSED(camera);
}

int snippetMain(int, const char* const*)
{
#ifdef RENDER_SNIPPET
	extern void renderLoop();
	renderLoop();
#else
	static const PxU32 frameCount = 100;
	initPhysics(false);
	for (PxU32 i = 0; i < frameCount; i++)
		stepPhysics(false);
	cleanupPhysics(false);
#endif

	return 0;
}

[vreutskyy]

Hi @kermado
Thanks for the detailed report. Will check it.

[gyeomannvidia]

A fix is coming for this. In the meantime, it is simple to locally replicate the fix.

The bug was that we limited soft limit impulses to be positive. We do need hard limits impulses to be positive but soft limits should be allowed to be positive and negative.

CPU

Open ExtConstraintHelper.h.
In the function
void addLimit(Px1DConstraint* c, const PxJointLimitParameters& limit)
move the following line of code
c->minImpulse = 0.0f;
so that it only executes when the limit is not a soft limit:
c->solveHint = PxConstraintSolveHint::eINEQUALITY;
c->mods.bounce.restitution = limit.restitution;
c->mods.bounce.velocityThreshold = limit.bounceThreshold;
c->minImpulse = 0.0f;

GPU

For gpu, the fix is similar. Open PxgConstraintHelper.h and repeat the pattern above in the function:
PX_CUDA_CALLABLE void addLimit(Px1DConstraint* c, const PxgJointLimitParameters& limit)const