Deformable volumes don't have the same drop speed as rigids

[kv-bh]

At small timestep values, the speeds at which a rigid body and deformable volume fall do not match up. This happens even when linear damping is set to 0. It's unclear why this happens, perhaps some sort of precision loss due to floating point error. Is this behavior expected? Would this be a bug of some sort?

Here is a video demonstrating the issue:

demo.mp4

The blue sphere is the deformable, the orange is a rigid body. When the time step is 0.001, the two bodies fall at approximately the same speed, but when the step is decreased to 0.00001, they are noticeably out of sync.

Working based off of 5ca9f47, here is a patch for the SnippetDeformableVolume snippet that I used to create these two tests:

Patch

diff --git a/physx/snippets/snippetdeformablevolume/SnippetDeformableVolume.cpp b/physx/snippets/snippetdeformablevolume/SnippetDeformableVolume.cpp
index 77f0569b..c6a9db7e 100644
--- a/physx/snippets/snippetdeformablevolume/SnippetDeformableVolume.cpp
+++ b/physx/snippets/snippetdeformablevolume/SnippetDeformableVolume.cpp
@@ -54,6 +54,8 @@ static PxMaterial*				gMaterial			= NULL;
 static PxPvd*					gPvd				= NULL;
 PxArray<DeformableVolume>		gDeformableVolumes;
 
+constexpr int steps = 1; // Set this to 100 to test with a smaller dt
+
 void addDeformableVolume(PxDeformableVolume* deformableVolume, const PxTransform& transform, const PxReal density, const PxReal scale)
 {
 	PxVec4* simPositionInvMassPinned;
@@ -126,6 +128,7 @@ static PxDeformableVolume* createDeformableVolume(const PxCookingParams& params,
 		addDeformableVolume(deformableVolume, PxTransform(PxVec3(0.f, 0.f, 0.f), PxQuat(PxIdentity)), 100.f, 1.0f);
 		deformableVolume->setDeformableBodyFlag(PxDeformableBodyFlag::eDISABLE_SELF_COLLISION, true);
 		deformableVolume->setSolverIterationCounts(30);
+		deformableVolume->setLinearDamping(0.0f);
 	}
 	return deformableVolume;
 }
@@ -143,16 +146,27 @@ static void createDeformableVolumes(const PxCookingParams& params)
 	
 	PxReal maxEdgeLength = 1;
 
+	/*
 	createCube(triVerts, triIndices, PxVec3(0.0, 9, 0), 2.5);
 	PxRemeshingExt::limitMaxEdgeLength(triIndices, triVerts, maxEdgeLength);
 	createDeformableVolume(params, triVerts, triIndices);
+	*/
 
-	createSphere(triVerts, triIndices, PxVec3(0, 4.5, 0), 2.5, maxEdgeLength);
+	createSphere(triVerts, triIndices, PxVec3(-3.0, 10.0, 0.0), 2.5, maxEdgeLength);
 	createDeformableVolume(params, triVerts, triIndices);
 
+	/*
 	createConeY(triVerts, triIndices, PxVec3(0.1, 11.5, 0), 2.0f, 3.5);
 	PxRemeshingExt::limitMaxEdgeLength(triIndices, triVerts, maxEdgeLength);
 	createDeformableVolume(params, triVerts, triIndices);
+	*/
+
+	PxShape* shape = gPhysics->createShape(PxSphereGeometry(2.5f), *gMaterial);
+	PxRigidDynamic* obj = gPhysics->createRigidDynamic(PxTransform(PxVec3(3.0f, 10.0f, 0.0f)));
+	obj->attachShape(*shape);
+	obj->setRigidBodyFlag(PxRigidBodyFlag::eKINEMATIC, false);
+	gScene->addActor(*obj);
+	shape->release();
 }
 
 void initPhysics(bool /*interactive*/)
@@ -219,10 +233,13 @@ void initPhysics(bool /*interactive*/)
 
 void stepPhysics(bool /*interactive*/)
 {
-	const PxReal dt = 1.0f / 60.f;
+	const PxReal dt = 0.001 / steps;
 
-	gScene->simulate(dt);
-	gScene->fetchResults(true);
+	for (int i = 0; i < steps; ++i)
+	{
+		gScene->simulate(dt);
+		gScene->fetchResults(true);
+	}
 
 	for (PxU32 i = 0; i < gDeformableVolumes.size(); i++)
 	{
diff --git a/physx/snippets/snippetdeformablevolume/SnippetDeformableVolumeRender.cpp b/physx/snippets/snippetdeformablevolume/SnippetDeformableVolumeRender.cpp
index bd526a0c..cfc7626e 100644
--- a/physx/snippets/snippetdeformablevolume/SnippetDeformableVolumeRender.cpp
+++ b/physx/snippets/snippetdeformablevolume/SnippetDeformableVolumeRender.cpp
@@ -69,6 +69,7 @@ void renderCallback()
 		Snippets::renderDeformableVolume(dv->mDeformableVolume, dv->mPositionsInvMass, true, rcaColor);
 	}
 
+	Snippets::showFPS();
 	Snippets::finishRender();
 }
 
@@ -85,11 +86,12 @@ void exitCallback()
 
 void renderLoop()
 {
-	sCamera = new Snippets::Camera(PxVec3(10.0f, 10.0f, 10.0f), PxVec3(-0.6f, -0.2f, -0.7f));
+	sCamera = new Snippets::Camera(PxVec3(0.0f, 10.0f, 20.0f), PxVec3(0.0f, 0.0f, -1.0f));
 
 	Snippets::setupDefault("PhysX Snippet Deformable Volume", sCamera, NULL, renderCallback, exitCallback);
 
 	initPhysics(true);
+	Snippets::initFPS();
 	glutMainLoop();
 
 	cleanup();