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Detailed Description
The collision type of both the robot gripper and the apple has been set to SDF Mesh, with all collision-related parameters left at their default values. In most scenarios, the gripper achieves tight and well-fitted contact with the apple. However, occasionally a visible gap appears between the gripper and the apple while the apple remains firmly grasped. What causes this phenomenon, and what approaches can be taken to prevent it?
Hi, that gap is expected PhysX behaviour with contact offsets, not a bug.
Every collider has a contact offset (the distance at which PhysX starts generating contacts) and a rest offset (the distance at which two shapes settle). For rigid bodies rest offset defaults to 0, so at rest the surfaces should touch. But contact offset defaults to auto — PhysX computes it from the shape’s extent, which can be several millimetres. Under an active grasp, the finger and apple are held apart within that contact band, and whether the joint drive fully closes it each frame varies with solver convergence — hence the intermittent gap while the object stays firmly held.
To tighten it, on both the gripper fingers and the apple:
python
from pxr import PhysxSchema
api = PhysxSchema.PhysxCollisionAPI.Apply(prim)
api.CreateContactOffsetAttr(0.002) # ~2 mm; keep > rest offset
api.CreateRestOffsetAttr(0.0) # or a small negative value for flush contact
Set the smallest contact offset that stays stable — too small invites tunnelling. If it still drifts, raise the articulation’s Solver Position Iteration Count (default 32 → 64/128). Since you’re on SDF Mesh, also try increasing SDF Resolution on both shapes for a tighter surface fit.
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