Wrong Collision Result between complicated mesh and sphere

[chenyanzz]

Version

• OS: macos 13.4.1
• conda_env: python 3.10
• pip_package: hppfcl==2.2.0

Bug Description

Collision Object 1 is the mesh "model.stl" inside the zip

Collision Object 2 is a Sphere center at (-2, 3, 0), with a radius of 50

Actually the two models DOES have a small slice of space between them,

However, the collide(obj1, obj2) returns a wrong answer:result.isCollision() == True.

And it shows the collision point is (-30.44375973, -43.73033919, 1.37608287)

(It is hard to show that there isn't a collision in a non-convex model in one image, so you can test it in your methods)

I draw the situation on blender:

The yellow dot is the collision point hpp-fcl decleared to be. But it is outside the Sphere.

And, However, There is NOT a collison at all.

The blender project is "test.blend" in zip

Testing Env

"test.py" also in the zip file

import hppfcl
import numpy

mesh_stl_path = '/in_zip_file/model.stl'
ball_pos = (-2, 3, 0)
ball_d = 100

# create objects
mesh_model = hppfcl.MeshLoader(hppfcl.BV_OBB).load(mesh_stl_path)
ball_model = hppfcl.Sphere(ball_d / 2)
mesh_obj = hppfcl.CollisionObject(mesh_model)
ball_obj = hppfcl.CollisionObject(ball_model)
place = hppfcl.Transform3f(numpy.array(ball_pos))
ball_obj.setTransform(place)

# try collide
req = hppfcl.CollisionRequest()
result = hppfcl.CollisionResult()
req.enable_contact = True
req.num_max_contacts = 1
hppfcl.collide(mesh_obj, ball_obj, req, result)

print('isCollision:', result.isCollision())
if result.isCollision():
    contacts = result.getContacts()
    contacts = list(map(lambda c: c.pos, contacts))
    print('contacts:', contacts)

Qs

• Is there some mistake for me ?
• Is there any approach to quick-fix such situation ?
• Is it the problem within 'BVHModel' or 'gjk collide test' ?

Attachment ( zip file of everything )

hppfcl-bugreport.zip

[jcarpent]

At first glance, it might be a problem of normal orientation in the case of BVHs.
This issue would need time to be deeply investigated. Unfortunately, we are lacking of man power to handle this issue rapidly.

[lmontaut]

Hi @chenyanzz,
There doesn't seem to be a mistake from your side. I may have identified the issue but I need more time to debug it.
It has to do with a specialized function which BVH-sphere collisions call.
In the mean time, a quick fix is to replace Sphere by Ellipsoid in your code, like so:

# ball_model = hppfcl.Sphere(ball_d / 2)
ball_model = hppfcl.Ellipsoid((ball_d / 2) * numpy.ones(3))

[chenyanzz]

@lmontaut
Well, By Using Ellipsoid, the contact point comes to the ball at [ 33.32329434, -26.13812054, -19.20840025], but still results in 'collided' (/cry)

[lmontaut]

Hi @chenyanzz, apologies for the late reply.
I can't seem to reproduce your bug using an ellipsoid instead of a ball.
If I run your code on my side but replace the ball by an ellipsoid:

# ball_model = hppfcl.Sphere(ball_d / 2)
ball_model = hppfcl.Ellipsoid((ball_d / 2) * numpy.ones(3))

I get the following output:

isCollision: False

I am also on macos 13.4.1. I ran your code using the latest hppfcl v. 2.3.5 installed using conda.

I will have a look to fix the bug for the ball case.

[jcarpent]

@lmontaut Any news on this issue?