# triangulate

triangulate, a C code which triangulates a polygonal region, by Joseph O'Rourke.

The polygon is defined by an input file which gives the coordinates of the nodes of the polygon.

Because the calculation is to be exact, the coordinates are expected to be integers. This requirement may cause problems!

No consecutive pair of vertices should be equal; when describing a polygon, sometimes the first and last vertices are equal. For this program, that is not the case. To describe a square, your input file should contain four pairs of coordinates, for instance.

The vertices should be listed in counterclockwise order. If you list them in clockwise order, then the area will probably come out negative, and the program will refuse to proceed.

It is possible to create a "polygon" that has zero area. The program will refuse to process such an object.

The polygon does not need to be convex. However, you should be careful not to specify a polygon which crosses itself, since this means the interior of the polygon is not well defined, and hence a triangulation is not well defined. As a simple example of such a problem, consider the four vertices of a square in counterclockwise order: V1, V2, V3, V4, and list them instead as V1, V3, V2, V4. This shape cannot be triangulated.

The output of the program is a PostScript file which shows the polygon and its triangulation. The text of the PostScript file includes comments that describe the triangulation in terms of the pairs of vertices that were connected to make each diagonal.

### Usage:

triangulate < input.txt > output.ps
where
• input.txt lists the (integer) vertex coordinates;
• output.ps is a PostScript image of the triangulation.

### Languages:

triangulate is available in a C version and a MATLAB version.

### Related Data and Programs:

POLYGON, a dataset directory which contains examples of polygons.

TRIANGLE, a C code which triangulates a set of points.

Joseph ORourke

### Reference:

• Joseph ORourke,
Computational Geometry in C,
Second Edition,
Cambridge, 1998,
ISBN: 0521649765,
LC: QA448.D38.

### Source Code:

Last revised on 23 August 2019.