polygons - example of using the zappem.net/pub/math/polygon package

Overview

This repository provides a small example of using the zappem.net/pub/math/polygon package.

The example program creates a space containing 5 overlapping triangles (one of which is considered a “hole”), and then computes the outline of these overlapping polygons. You can run the included example as follows:

$ go run polygons.go
2024/05/05 18:35:27 wrote result to "dump.png"

The result shows the pre-outlined polygons on the left, and the post-outlined polygons on the right:

Note, the hole (blue triangle) on the left, does not survive the Union merging, because it is overwritten by a triangle that envelops it. On the other hand, the gap between the three partially overlapping triangles on the left emerges as a hole within the red polygon outline after the Union operation.

The polygons.go example can preserve fully internal holes, using the --swallow=false flag. This operation extracts holes before merging the shapes and then adds them back after generating the union of what remains:

$ go run polygons.go --swallow=false
2026/08/30 07:55:27 wrote result to "dump.png"

To yield this:

The program can also save the result in .polys format, which is just a JSON represention of the polygons.Shapes structure. That can be used with the svgpoly/examples.go tool to generate SVG output. If ../svgpoly is where your clone of that repo is, you can do this to generate an SVG:

$ go run polygons.go --swallow=false --dest=dump.polys
2026/08/30 08:31:19 wrote result to "dump.polys"
$ cd ../svgpoly
$ go run examples/outline.go --after=false --before --hatch .1 --hatch-angle 20 --poly ../polygons/dump.polys --osvg dump.svg

Which generates this SVG image:

License info

The polygons example program is distributed with the same BSD 3-clause license as that used by golang itself.

Reporting bugs

Use the github polygons bug tracker.