103 lines
4.7 KiB
Markdown
103 lines
4.7 KiB
Markdown
# polygon-clipping
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Apply boolean Polygon clipping operations (`intersection`, `union`, `difference`, `xor`) to your Polygons & MultiPolygons.
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[](https://github.com/mfogel/polygon-clipping/actions) [](https://codecov.io/gh/mfogel/polygon-clipping) [](https://opensource.org/licenses/MIT) [](https://www.npmjs.com/package/polygon-clipping)
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## Quickstart
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<!-- prettier-ignore-start -->
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```javascript
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const polygonClipping = require('polygon-clipping')
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const poly1 = [[[0,0],[2,0],[0,2],[0,0]]]
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const poly2 = [[[-1,0],[1,0],[0,1],[-1,0]]]
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polygonClipping.union (poly1, poly2 /* , poly3, ... */)
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polygonClipping.intersection(poly1, poly2 /* , poly3, ... */)
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polygonClipping.xor (poly1, poly2 /* , poly3, ... */)
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polygonClipping.difference (poly1, poly2 /* , poly3, ... */)
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```
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<!-- prettier-ignore-end -->
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## API
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```javascript
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/* All functions take one or more [multi]polygon(s) as input */
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polygonClipping.union (<geom>, ...<geoms>)
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polygonClipping.intersection(<geom>, ...<geoms>)
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polygonClipping.xor (<geom>, ...<geoms>)
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/* The clipGeoms will be subtracted from the subjectGeom */
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polygonClipping.difference(<subjectGeom>, ...<clipGeoms>)
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```
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### Input
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Each positional argument (`<geom>`) may be either a Polygon or a MultiPolygon. The [GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-3.1) is followed, with the following notes/modifications:
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- MultiPolygons may contain touching or overlapping Polygons.
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- rings are not required to be self-closing.
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- rings may contain repeated points, which are ignored.
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- rings may be self-touching and/or self-crossing. Self-crossing rings will be interpreted using the [non-zero rule](https://en.wikipedia.org/wiki/Nonzero-rule).
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- winding order of rings does not matter.
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- inner rings may extend outside their outer ring. The portion of inner rings outside their outer ring is dropped.
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- inner rings may touch or overlap each other.
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### Output
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For non-empty results, output will always be a MultiPolygon containing one or more non-overlapping, non-edge-sharing Polygons. The [GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-3.1) is followed, with the following notes/modifications:
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- outer rings will be wound counter-clockwise, and inner rings clockwise.
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- inner rings will not extend outside their outer ring.
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- rings will not overlap, nor share an edge with each other.
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- rings will be self-closing.
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- rings will not contain repeated points.
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- rings will not contain superfluous points (intermediate points along a straight line).
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- rings will not be self-touching nor self-crossing.
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- rings _may_ touch each other, but _may not_ cross each other.
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In the event that the result of the operation is the empty set, output will be a MultiPolygon with no Polygons: `[]`.
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## Correctness
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Run: `npm test`
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The tests are broken up into unit tests and end-to-end tests. The end-to-end tests are organized as GeoJSON files, to make them easy to visualize thanks to [GitHub's helpful rendering of GeoJSON files](https://help.github.com/articles/mapping-geojson-files-on-github/). Browse those tests [here](test/end-to-end).
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## Performance
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The Martinez-Rueda-Feito polygon clipping algorithm is used to compute the result in `O((n+k)*log(n))` time, where `n` is the total number of edges in all polygons involved and `k` is the number of intersections between edges.
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## Settings
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Global settings are set via environment variables.
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- **POLYGON_CLIPPING_MAX_QUEUE_SIZE** and **POLYGON_CLIPPING_MAX_SWEEPLINE_SEGMENTS**: Aims to prevent infinite loops - usually caused by floating-point math round-off errors. Defaults are 1,000,000.
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## Changelog
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This project adheres to [Semantic Versioning](https://semver.org/).
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<!-- prettier-ignore-end -->
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The full changelog is available at [CHANGELOG.md](https://github.com/mfogel/polygon-clipping/blob/master/CHANGELOG.md).
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## Authors
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- [Mike Fogel](https://github.com/mfogel)
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- [Alexander Milevski](https://github.com/w8r)
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- [Vladimir Ovsyannikov](https://github.com/sh1ng)
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## Sponsors
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- [Alantgeo](https://www.alantgeo.com.au/)
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- [EasyTerritory](https://www.easyterritory.com/)
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Please contact [Mike Fogel](https://github.com/mfogel) if you or your company is interested in sponsoring work on specific bug fixes or feature requests.
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## Based on
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- [A new algorithm for computing Boolean operations on polygons](paper.pdf) by Francisco Martinez, Antonio Jesus Rueda, Francisco Ramon Feito (2009)
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