VTP — VTK XML PolyData (.vtp)

The VTK XML PolyData format: the same XML container as VTU with a <PolyData> grid holding <Verts>/<Lines>/<Polys>/<Strips> connectivity+offsets sections instead of <Cells>. Surface meshes only.

Format name	vtp
Extensions	.vtp
Read / Write	✓ / ✓
Extra dependencies	— (zlib compression via the optional zlib build in C++, stdlib zlib in Python)

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("surface.vtp")
meshioplusplus.vtp.write("out.vtp", mesh, binary=True, compression="zlib")

• binary — base64 "binary" DataArrays (default True) vs ASCII.
• compression — "zlib" (default), None, or "lzma" (Python-only; the C++ path handles None/"zlib").
• header_type — VTU-style header integer type; leave None (default UInt32) for the C++ fast path.

Cell types

PolyData section	meshio++ types
Verts	vertex (single-node rows only; poly-vertices are rejected)
Lines	line (two-node rows only; polylines are rejected)
Polys	triangle (3 nodes), quad (4), polygon (≥5)
Strips	not supported (rejected)

Anything else — volume cells, quadratic cells, polyhedra — cannot be held by PolyData and raises WriteError in both the C++ and Python writers.

Data mapping

• point_data / cell_data round-trip as in VTU. Cell data follows VTK's canonical PolyData cell order — Verts, then Lines, then Polys — in both directions; the writer reorders the mesh's blocks (and their cell_data) into that partition, preserving relative order within each section.
• Ragged (jagged) polygon blocks are written as-is; on read, polygons are grouped by node count into rectangular blocks.

Quirks & limitations

• PolyData has no cell-type array, so the triangle/quad vs 3-/4-noded-polygon distinction cannot survive a round-trip: a polygon cell with 3 or 4 nodes reads back as triangle/quad. (Genuine ≥5-noded polygons are unaffected.)
• Triangle strips, poly-vertex/poly-line rows, multiple <Piece>s, <AppendedData>, and lzma compression are not handled by the C++ reader (the Python fallback additionally covers lzma).
• The C++ writer's output layout mirrors the VTU writer (same <DataArray> encoding, %.11e ASCII floats, 32 KiB zlib blocks).