Crop (bounding box / half-space)

meshioplusplus.crop(mesh, ...) extracts the part of a mesh inside a region — an axis-aligned bounding box or a half-space (a plane, keeping one side) — pruning the now-unused points and remapping connectivity and all data. It is a mesh operation (like split and merge), not a file format, and uses only standard C++/numpy, so it runs under every mesh backend.

import meshioplusplus

mesh = meshioplusplus.read("domain.vtu")

# axis-aligned bounding box
inside = meshioplusplus.crop(mesh, bbox=[0, 0, 0, 1, 1, 1])

# half-space: keep the side where (p - point) . normal >= 0
half = meshioplusplus.crop(mesh, plane=([0.5, 0, 0], [1, 0, 0]))

# keep a cell if ANY node is inside (default is ALL)
loose = meshioplusplus.crop(mesh, bbox=[0, 0, 0, 1, 1, 1], mode="any")

meshioplusplus.write("cropped.vtu", inside)

Region and mode

argument	meaning
bbox	(xmin, ymin, zmin, xmax, ymax, zmax) or (lo3, hi3)
plane	(point3, normal3) — keeps the side where (p − point) · normal ≥ 0
mode	"all" (default) keeps a cell only if every node is inside; "any" keeps it if any node is inside
record_ids	attach crop:original_point_id point_data and crop:original_cell_id cell_data of the source indices

A point is inside the bbox when lo ≤ p ≤ hi component-wise. The point-inside test is parallelized; exactly one of bbox/plane must be given.

What changes

• Only the kept cells remain; unused points are pruned and connectivity is remapped so it stays valid.
• point_data / cell_data are subset to the kept points/cells; field_data is preserved.
• point_sets / cell_sets are remapped, dropping entries outside the kept region (done in the Python layer).

The numpy fallback handles rectangular cell blocks; ragged/polyhedron blocks are handled by the C++ core only.

CLI

meshioplusplus crop in.vtu out.vtu --bbox 0,0,0,1,1,1
meshioplusplus crop in.vtu out.vtu --plane 0.5,0,0,1,0,0 --mode any
meshioplusplus crop in.vtu out.vtu --bbox 0,0,0,1,1,1 --record-ids

Values that begin with - (e.g. a negative bbox corner) need the --bbox=-1,-1,-1,1,1,1 form so argparse doesn't read them as a flag. See the CLI reference.

Other languages

• C API — mio_crop_bbox(mesh, lo, hi, mode, record_ids) and mio_crop_plane(mesh, point, normal, mode, record_ids) (mode: 0 = all, 1 = any). See the C API reference.
• Fortran — mesh%crop_bbox(lo, hi, mode=...) and mesh%crop_plane(point, normal, mode=...). See the Fortran reference.
• WebAssembly / JavaScript — cropBbox(mesh, lo, hi, mode, recordIds) and cropPlane(mesh, point, normal, mode, recordIds). See the WebAssembly reference.