Why zero hit distance in PxVehiclePhysXRoadGeometryQueryUpdate is interpreted as non hit?

[Tooflevor]

PxVehiclePhysXRoadGeometryQueryUpdate in a file physx\source\physxvehicle2\src\physxRoadGeometry\VhPhysXRoadGeometryFunctions.cpp has specific check for zero hit distance in the raycast query

PhysX/physx/source/physxvehicle2/src/physxRoadGeometry/VhPhysXRoadGeometryFunctions.cpp

Lines 112 to 131 in 4d33492

	if(buff.hasBlock && buff.block.distance != 0.0f)
	{
	const PxPlane hitPlane(v + w * buff.block.distance, buff.block.normal);
	roadGeomState.plane = hitPlane;
	roadGeomState.hitState = true;
	PxMaterial* hitMaterial;
	roadGeomState.friction = computeMaterialFriction(buff.block.shape, buff.block.faceIndex, materialFrictionParams,
	hitMaterial);
	roadGeomState.velocity = computeVelocity(*buff.block.actor, buff.block.position);
	if (physxRoadGeometryState)
	{
	copyHitInfo(buff.block, hitMaterial, *physxRoadGeometryState);
	}
	}
	else
	{
	if (physxRoadGeometryState)
	physxRoadGeometryState->setToDefault();
	}

Something similar is in the sweep query type:

PhysX/physx/source/physxvehicle2/src/physxRoadGeometry/VhPhysXRoadGeometryFunctions.cpp

Lines 156 to 223 in 4d33492

	if (buff.hasBlock && buff.block.distance > 0.0f)
	{
	//Sweep started outside scene geometry.
	const PxPlane hitPlane(buff.block.position, buff.block.normal);
	roadGeomState.plane = hitPlane;
	roadGeomState.hitState = true;
	PxMaterial* hitMaterial;
	roadGeomState.friction = computeMaterialFriction(buff.block.shape, buff.block.faceIndex, materialFrictionParams,
	hitMaterial);
	roadGeomState.velocity = computeVelocity(*buff.block.actor, buff.block.position);
	if (physxRoadGeometryState)
	{
	copyHitInfo(buff.block, hitMaterial, *physxRoadGeometryState);
	}
	}
	else if (buff.hasBlock && buff.block.distance < 0.0f)
	{
	//The sweep started inside scene geometry.
	//We want to have another go but this time starting outside the hit geometry because this is the most reliable
	//way to get a hit plane.
	//-buff.block.distance is the distance we need to move along buff.block.normal to be outside the hit geometry.
	//Note that buff.block.distance can be a vanishingly small number. Moving along the normal by a vanishingly
	//small number might not push us out of overlap due to numeric precision of the overlap test.
	//We want to move a numerically significant distance to guarantee that we change the overlap status
	//at the start pose of the sweep.
	//We achieve this by choosing a minimum translation that is numerically significant.
	//Any number will do but we choose the wheel radius because this ought to be a numerically significant value.
	//We're only sweeping against the hit shape and not against the scene
	//so we don't risk hitting other stuff by moving a numerically significant distance.
	const PxVec3 unitDir = -buff.block.normal;
	const PxF32 maxDist = PxMax(wheelParams.radius, -buff.block.distance);
	const PxGeometry& geom0 = convMeshGeom;
	const PxTransform pose0(T.p + buff.block.normal*(maxDist*1.01f), T.q);
	const PxGeometry& geom1 = buff.block.shape->getGeometry();
	const PxTransform pose1 = buff.block.actor->getGlobalPose()*buff.block.shape->getLocalPose();
	PxGeomSweepHit buff2;
	const bool b2 = PxGeometryQuery::sweep(
	unitDir, maxDist*1.02f,
	geom0, pose0, geom1, pose1, buff2, PxHitFlag::eDEFAULT | PxHitFlag::eMTD);
	if (b2 && buff2.distance > 0.0f)
	{
	//Sweep started outside scene geometry.
	const PxPlane hitPlane(buff2.position, buff2.normal);
	roadGeomState.plane = hitPlane;
	roadGeomState.hitState = true;
	PxMaterial* hitMaterial;
	roadGeomState.friction = computeMaterialFriction(buff.block.shape, buff.block.faceIndex, materialFrictionParams,
	hitMaterial);
	roadGeomState.velocity = computeVelocity(*buff.block.actor, buff.block.position);
	if (physxRoadGeometryState)
	{
	copyHitInfo(buff.block, hitMaterial, *physxRoadGeometryState);
	}
	}
	else
	{
	if (physxRoadGeometryState)
	physxRoadGeometryState->setToDefault();
	}
	}
	else
	{
	if (physxRoadGeometryState)
	physxRoadGeometryState->setToDefault();
	}

So, if hit distance is exactly zero, result is like there are no hit at all.

But according to PxLocationHit and documentation, zero hit distance is a valid query result. So, why is it ignored?

[msauter-nvidia]

In the sweep case, this looks like a bug and we will have a look at it. For raycasts, it's a different story. The ray does not start at the wheel center but at the top of the wheel (see PxVehicleComputeSuspensionRaycast). The motivation for this is to handle scenarios where the wheel at max compression penetrates into the ground. Penetrations are only handled up to a certain distance (the wheel diameter, approximately). Beyond that, it's treated as no collision which was just a design decision. Since distance=0 means initial overlap, we ignore it at that point (see PxLocationHit::hadInitialOverlap()) (but just to be clear, for raycasts it would mean the wheel is sort of penetrating the ground by a distance of a wheel diameter at this point).

[Tooflevor]

Yeah, for raycasts it seems okay.
Initially, I stumbled upon zero hit distance in sweep case, and I am more interested in this one. Raycast just gave me the thought that I don't understand something.
Do I need to make an issue or something?

[msauter-nvidia]

It's tracked as a bug on our side and will probably get tackled in 5.2. Thanks.