Flipping normals

flip_normals(mesh, region=None) reverses the winding of every cell, or of the cells of a cell region, with no point moved (roadmap 7.8.2). repair orients a surface consistently and outward; this is the operation for the opposite orientation, or for one part of a surface flipped on purpose.

import meshioplusplus

inward = meshioplusplus.flip_normals(scan)
patched, report = meshioplusplus.flip_normals(scan, "lid", return_report=True)
report["num_flipped"]

Each cell's nodes are permuted by the same table mirror uses (detail/cell_flip.hpp's block_flip): a triangle or a polygon reverses its ring after node 0, a higher-order cell keeps its mid-edge and mid-face nodes in their slots, a volume cell turns inside out. A Side region entry (cell, facet) moves with its facet, as in mirror. Points, the block order, the cell data and the other regions are untouched. Every permutation is an involution, so flipping twice gives the input back, bit for bit (tests/cpp/test_flip_normals.cpp). An unknown region is refused by name.

Surfaces

C++ flip_normals (operations/flip_normals.hpp), Python meshioplusplus.flip_normals (C++ core only), both CLIs (flip-normals IN OUT [--region NAME] [--json]), the FlipNormals pipeline step (Region), C mio_flip_normals(mesh, region, &num_flipped), Fortran, Julia, R, Octave/MATLAB, WASM flipNormals and the MCP tool flip_normals.