[BUG] Incorrect center of mass calculation #251
Description
Bug Description
While debugging an issue related towp.sim.parse_urdf and its loading of warp.sim.Mesh instances , I noticed that the function in the constructor which gets called to compute the inertia if the compute_inertia flag is True (default behaviour), returns unexpected values.
When looking at the code, I found that the compute_mesh_inertia function in warp.sim.inertia.py does not return the center of mass, but the arithmetic mean of all vertices. These are not equal for most meshes.
Lines 256 to 260 in 7de9179
A short example showing the problem:
import warp as wp
import warp.render
from warp.sim.inertia import compute_mesh_inertia
# Unit Cube
vertices = [
[1.0, 0.0, 0.0],
[1.0, 0.0, 1.0],
[0.0, 0.0, 1.0],
[0.0, 0.0, 0.0],
[1.0, 1.0, 0.0],
[1.0, 1.0, 1.0],
[0.0, 1.0, 1.0],
[0.0, 1.0, 0.0],
]
indices = [
1,
2,
3,
7,
6,
5,
4,
5,
1,
5,
6,
2,
2,
6,
7,
0,
3,
7,
0,
1,
3,
4,
7,
5,
0,
4,
1,
1,
5,
2,
3,
2,
7,
4,
0,
7,
]
mass, com, I, volume = compute_mesh_inertia(
density=1000, vertices=vertices, indices=indices, is_solid=True
)
print("Initial values", mass, com, I, volume, sep="\n")
# Add vertex between [0.0, 0.0, 0.0] and [1.0, 0.0, 0.0]
vertices.append([0.5, 0.0, 0.0])
last = 8
indices[15:18] = [0, 8, 7]
indices.extend([8, 3, 7])
indices[18:21] = [0, 1, 8]
indices.extend([8, 1, 3])
mass, com, I, volume = compute_mesh_inertia(
density=1000, vertices=vertices, indices=indices, is_solid=True
)
# mass and volume stay the same (approx.)
# center of mass gets shifted by (0.0, -1/15, -1/15)
# Inertia is also impacted
print("Updated Values", mass, com, I, volume, sep="\n")
# Triangle split can be seen in wireframe mode (Press letter x)
renderer = wp.render.OpenGLRenderer()
for i in range(1000):
print("FRAME", i)
renderer.begin_frame(i * 0.01)
renderer.render_mesh("Mesh", points=vertices, indices=indices)
renderer.end_frame()
renderer.paused = True
renderer.update()By splitting one triangle into two, the center of mass and inertia are unexpectedly changed.
System Information
Warp 1.2.2 initialized:
CUDA Toolkit 11.8, Driver 12.2
Devices:
"cpu" : "x86_64"
"cuda:0" : "NVIDIA GeForce RTX 4090" (24 GiB, sm_89, mempool enabled)