[Bug]: MeshSet particle sampling ignores member rotations

[prexhu]

Bug Description

Particles sampled for gs.morphs.MeshSet do not receive the rotation specified by each member's eulers. Only the translation in poss is applied.

The visual mesh receives the complete rotation and translation, causing the rendered mesh and its physical particles to have different orientations. This also affects Hybrid entities generated from URDF links: particles belonging to rotated links remain aligned with the original local axes.

Steps to Reproduce

import numpy as np
import trimesh

import genesis as gs


gs.init(backend=gs.cpu)

pos = np.array((0.2, 0.1, 0.2))
mesh = trimesh.creation.box(extents=(0.04, 0.12, 0.04))

scene = gs.Scene(
    mpm_options=gs.options.MPMOptions(
        lower_bound=(-0.5, -0.5, -0.5),
        upper_bound=(0.5, 0.5, 0.5),
        particle_size=0.01,
    ),
    show_viewer=False,
)

entity = scene.add_entity(
    morph=gs.morphs.MeshSet(
        files=(mesh,),
        poss=(pos,),
        eulers=((0.0, 0.0, 90.0),),
    ),
    material=gs.materials.MPM.Elastic(sampler="regular"),
)

particle_extents = np.ptp(entity.init_particles, axis=0)
print("Particle extents:", particle_extents)

np.testing.assert_allclose(
    particle_extents,
    (0.11, 0.03, 0.03),
    atol=1e-6,
)

Expected Behavior

The 90-degree rotation around the Z axis should rotate the box's long axis from Y to X. The particle extents should therefore be approximately: [0.11, 0.03, 0.03]

The physical particles and visual mesh should have the same orientation.

Screenshots/Videos

I have also written a URDF file to test the particle rotation.

Error: Particle rotation.

Which should be like:

Environment

• OS: Ubuntu 22.04.5 LTS, x86_64
• CPU: AMD Ryzen 9 9950X 16-Core Processor
• GPU-driver version: N/A
• CUDA / CUDA-toolkit version: N/A

Release version or Commit ID

genesis-world 1.3.1
e4e8013