Fix heightfield scene BVH bounds missing upper z-range

[kevinzakka]

The scene-level BVH bounds for heightfield geoms were computed with _compute_box_bounds(pos, rot, size), which creates an AABB symmetric around the geom position. However, heightfield mesh vertices have z-values in [0, max_z], extending upward from the geom origin rather than being centered on it. This caused the upper half of the surface to fall outside the bounding volume, so rays aimed at the top of the heightfield skipped intersection entirely.

Fix by offsetting the AABB center in-place using the existing half-extent. Since hfield z-values start at 0, size[2] (the z half-extent) equals the center offset:

hfield_center = pos + rot @ wp.vec3(0.0, 0.0, size[2])
_compute_box_bounds(hfield_center, rot, size)

This is a minimal 2-line change in _compute_bvh_bounds with no new arrays, fields, or plumbing required.

In local coordinates:

• Mesh z range: [0, max_z]
• Old AABB z range: [-max_z/2, +max_z/2] (centered at geom origin, misses the top half)
• New AABB z range: [0, max_z] (centered at max_z/2, tight fit)

Note: This works because MuJoCo's compiler always normalizes hfield_data to [0, 1] by subtracting the minimum value (ref), so pmin_z = 0 and center_z = half_z = size[2].

Thanks to @slambert-bd for flagging this.

[StafaH]

LGTM!

[thowell]

maybe something like rot[:, 2] * size[2]?

[kevinzakka]

Done, thanks Taylor.