measure (library)

A small utility module — not a standalone binary — providing geometric measurement (length, area, volume) for MOAB entity types. Used internally by other tools in this directory (mbsize, etc.) and available for embedding in client code.

Source

• measure.hpp — public API
• measure.cpp — implementation

API

double edge_length(const double* start_vtx_coords,
                   const double* end_vtx_coords);
 
double measure(moab::EntityType type,
               int             num_vertices,
               const double*   vertex_coordinates);

<tt>edge_length(start, end)</tt>

Returns the Euclidean distance between two 3-D points. The two arguments must each point to three contiguous doubles holding {x, y, z}.

<tt>measure(type, num_vertices, coords)</tt>

Returns the geometric measure (length, area, or volume) of a single entity of the given moab::EntityType. coords is the flat XYZ coordinate array for the entity's vertices, in canonical MOAB ordering (3 × num_vertices doubles).

The function returns 0.0 for entity types it does not handle.

Supported entity types and formulas

Entity type	Measure	Formula
MBEDGE	length	\|p1 − p0\|
MBTRI	area	½ \|(p1−p0) × (p2−p0)\|
MBQUAD	area	Fan-triangulation around centroid (4 vertices)
MBPOLYGON	area	Fan-triangulation around centroid (num_vertices corners)
MBTET	volume	⅙ ((p1−p0) × (p2−p0)) · (p3−p0)
MBPYRAMID	volume	Two-tet decomposition through apex (vertex 4)
MBPRISM	volume	Three-tet decomposition
MBHEX	volume	Five-tet decomposition (canonical hex split)
anything else	—	returns 0.0

Conventions

• All formulas assume 3-D Cartesian coordinates. Spherical-polygon area on the unit sphere is not provided here — see moab::IntxMesh::IntxUtils for that.
• The polygon area formula uses fan triangulation around the centroid, which is signed-area-correct for convex polygons and a reasonable approximation for moderately non-convex ones. It does not handle self-intersecting polygons.
• Hex volume uses the canonical 5-tet decomposition; this matches Exodus / MOAB element ordering and is exact for trilinear hexes.

Usage example

#include "measure.hpp"
#include "moab/Core.hpp"
 
moab::Interface* mb = /* … */;
moab::Range cells;
mb->get_entities_by_dimension(0, 2, cells);
 
double total_area = 0.0;
for (moab::EntityHandle c : cells) {
    const moab::EntityHandle* conn = nullptr;
    int n = 0;
    mb->get_connectivity(c, conn, n);
 
    std::vector<double> coords(3 * n);
    mb->get_coords(conn, n, coords.data());
 
    total_area += measure(mb->type_from_handle(c), n, coords.data());
}

License

Original code copyright Lawrence Livermore National Laboratory (LLNS contract B545069 with UW–Madison). BSD-2-Clause; see source header for the full notice.