VTK UnstructuredGrid#

.vtu read + write eager

Summary of the specification#

.vtu is the XML serial form of a VTK UnstructuredGrid: an arbitrary mix of cell types in one dataset. A <VTKFile type="UnstructuredGrid"> root holds an <UnstructuredGrid> with one or more <Piece> elements, each declaring NumberOfPoints and NumberOfCells. A piece carries <Points> with a three-component coordinate DataArray, and <Cells> with three named arrays — connectivity, offsets and types — where types holds one VTK cell type code per cell. <PointData> and <CellData> hold named attribute arrays. Every DataArray is inline ASCII, inline base64, or a reference into a single appended binary blob, optionally zlib-compressed.

Specification at a glance#

root	<VTKFile type=”UnstructuredGrid”>
pieces	one or more <Piece NumberOfPoints= NumberOfCells=>
cells	connectivity, offsets and types DataArrays
cell types	one VTK type code per cell
encodings	ascii, base64, appended (raw or base64), optionally zlib-compressed
indices	0-based

Read the full VTK XML specification ↗

Reading#

import polyxios as px

mesh = px.read("grid.vtu")
mesh.element_types     # mixed cell types, mapped from the VTK codes

Writing#

px.write(mesh, "out.vtu")                 # base64 payloads (default)
px.write(mesh, "out.vtu", binary=False)   # inline ASCII

option	meaning
`binary`	`True` (the default) writes base64-encoded payloads; `False` writes inline ASCII, which is larger but diffable.

Quirks worth knowing#

Multiple <Piece> elements are concatenated into one PolyData, with each piece’s connectivity shifted by the running vertex count.
VTK cell type codes with no polyxios equivalent are dropped rather than guessed at.
lazy=True raises LazyReadError; the payload may be compressed or base64-encoded, neither of which can be memory-mapped.
Header counts are validated against the file size before any array is allocated.