slope_drop (FET003 Physics)#
Property |
Value |
|---|---|
Test name |
slope_drop |
Feature(s) |
FET003_BASE_PHYSX |
Engine |
Kit / Isaac Sim (>=2024.2.0) |
Test version |
3.0.0 |
Summary#
Places the asset on a 45-degree inclined plane, confirms the asset slides down the slope by detecting cumulative horizontal displacement, and verifies the asset does not tunnel through the surface.
What Pass Guarantees#
A reviewer, PM, or OEM can trust that the asset’s collision mesh registers contacts on a non-horizontal surface. The collider is geometrically correct enough to transmit lateral forces from an inclined plane, and the asset will not fall through slope or ramp surfaces in a physics scene.
What It Checks#
The test verifies two conditions during a single simulation run on a tilted surface.
First, sliding detection: the asset must accumulate a horizontal XY displacement from its starting position of at least 0.01 m. Horizontal movement is the direct signal that the collider is registering contact with the slope and that lateral forces are being transmitted correctly. An asset that does not move horizontally within the simulation time limit has not interacted with the slope surface.
Second, non-penetration: after sliding is detected, the test observes the asset’s bounding-box minimum Z coordinate for a 3.0-second window. If Z drops below the floor level minus the 0.1 m tolerance during that window, the asset has tunneled through the slope-to-floor transition area. Penetration is only checked after sliding is detected, so a brief below-floor position during the initial fall before the asset contacts the slope does not cause a false failure.
How It Works#
The test loads the asset in a blue room with a collision-enabled flat floor and a 45-degree slope. The asset is placed at the top of the slope. Physics is simulated at 240 fps with a camera following the asset from the side so the sliding motion is visible in the frame.
Each simulation frame, the test computes the cumulative XY displacement from the initial bounding-box center position. When that displacement reaches 0.01 m, sliding is confirmed. The test then continues simulating for a 3.0-second post-sliding window, watching the floor clearance. If no penetration occurs during that window, the test passes and the simulation exits. If the displacement threshold is never reached within the 10-second hard cap, the test fails.
A 120-second watchdog terminates any simulation that hangs.
Before physics starts, the pre-simulation safeguards check whether the asset has UsdPhysics.RigidBodyAPI and UsdPhysics.CollisionAPI. An asset with a world-anchor is reported as not applicable and skipped, with the reason recorded.
Key thresholds from config_defaults:
slope_angle_deg: 45.0 (slope angle in degrees)slope_friction: 0.5 (static friction coefficient applied to the slope and flat floor; dynamic friction is 0.4)floor_margin: 0.1 m (penetration tolerance)horizontal_movement_threshold: 0.01 m (minimum cumulative XY displacement to confirm sliding)post_horiz_seconds: 3.0 s (penetration observation window after sliding is detected)simulation_seconds: 10.0 s (hard upper cap)physics_fps: 240
Failure Cases#
Symptom |
Likely cause |
|---|---|
No horizontal movement detected within the time limit |
|
Asset penetrates the floor after sliding |
The collision mesh approximation is incorrect at the slope-to-floor transition area. Thin geometry or gaps in the collision mesh at the base can cause tunneling as the asset transitions from the slope to the flat floor. |
Physics simulation hangs (120 s watchdog) |
Self-penetrating geometry, missing or zero-volume colliders, or extreme values in mass or inertia properties stall the PhysX solver. |
How to Fix#
If the asset does not slide, apply UsdPhysics.RigidBodyAPI to the root prim and ensure at least one mesh has UsdPhysics.CollisionAPI. Verify that the collision mesh geometry makes contact with the slope surface and is not floating above it. Remove any FixedJoint anchoring the asset to the world. If friction values are explicitly set on the asset’s physics material, reduce them to allow sliding on a 45-degree surface.
If the asset penetrates the floor after sliding, change the collision mesh approximation to convex hull. Check the collision mesh for thin geometry or gaps at the bottom of the asset where it transitions from the slope to the flat floor. Review the test video to identify exactly where penetration occurs.
If the simulation hangs, open the asset in Kit standalone and step physics manually to surface the specific PhysX error. Check for self-penetrating geometry, missing or zero-volume colliders, and NaN or extreme values in physxRigidBody:mass and physxRigidBody:diagonalInertia.
Expected Result#
The asset is placed on a tilted ramp. It begins to slide down the slope under gravity, possibly tumbling as it goes. It stays on top of the slope surface for the entire clip. An asset that drops straight through the ramp or floats above it indicates a broken collision shape.
Notes and Caveats#
The 0.01 m horizontal displacement threshold filters out micro-jitter from the physics solver at high frame rates. Per-frame deltas at 240 fps are very small, so cumulative displacement from the initial position is tracked rather than per-frame velocity.
Penetration is only checked after horizontal movement is confirmed. A brief below-floor position during the initial drop before the asset contacts the slope does not trigger a penetration failure, which prevents false positives on assets that fall onto the slope from above.
The 0.1 m floor margin accounts for floating-point imprecision in collision resolution at the slope-to-floor boundary, which is a geometrically complex contact region.
The world-anchor pre-check reports the test as not applicable and skips it, with the reason recorded, for assets with a FixedJoint pinning them to the world. These assets cannot slide by design.
The slope surface is assigned a static friction coefficient of 0.5 and a dynamic friction coefficient of 0.4. These values are applied to the flat floor as well, ensuring deterministic behavior at the slope-to-floor transition. Very high friction values set directly on the asset’s physics material can prevent sliding and cause a false failure if they override the scene-level friction.