Cell-level isosurface extraction
isoap
Cell-level isosurface extraction tools for arbitrary polyhedral cells.
Overview
isoap extracts isosurfaces on arbitrary polyhedral cells from scalar values assigned to the cell vertices. These values may be discrete volume fractions produced by a volume-of-fluid method, samples of an implicit mathematical function, or signed distances to a scanned surface.
The method uses a general polygon-tracing procedure that is valid for convex and non-convex polyhedra, including cells with non-planar faces. It may be regarded as an extension of the marching cubes technique to polyhedra of arbitrary shape and produces consistent reconstructions in ambiguous configurations. Within AIR Toolkits, isoap provides cell-level polygonal representations of interfaces for reconstruction and geometrical operations, while its potential applications also extend to CFD visualization, medical imaging and other fields based on scalar data on irregular grids.
Main capabilities
- Isosurface extraction from scalar values assigned to the vertices of arbitrary polyhedral cells.
- Input data from volume fractions, sampled implicit functions and signed distances to scanned surfaces.
- General polygon tracing on convex and non-convex polyhedra, including cells with non-planar faces.
- Consistent treatment of ambiguous configurations in polyhedra of arbitrary shape.
- Handling of multiple disconnected contours and generation of oriented polygonal output for subsequent geometrical operations.
Adoption by VTK
The polygon-tracing isosurface extraction algorithm implemented in isoap has been adopted by VTK for polyhedral contouring and clipping, where it is identified as the López Polygon-Tracing Algorithm. VTK documentation cites the original publication and isoap as the reference open-source implementation.
Kitware described the change and the wider polyhedron-processing improvements in its June 2026 technical article, “Polyhedron Processing Improvements in VTK”.
Software characteristics and availability
- Language
- Fortran
- Compiler
- GNU Fortran; historical Intel and NAG tests
- Platforms
- Primarily Linux and macOS
- Dependencies
- None
Version 1.1 was published on 24 June 2026 and is distributed exclusively through Zenodo. Version 2 is in preparation.
Citation and license
When using isoap v1.1, cite the version-specific software release:
The polygon-tracing method implemented in isoap is described in:
isoap is distributed under the GNU General Public License v3 or later (GPL-3.0-or-later).