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ST_RayCast

Sample image of casting rays from points onto polylines

Overview

This is a PostGIS function that allows you to cast rays from point features to linestring or polygon features. Returned geometry can be of MULTIPOINT or MULTILINESTRING type. Parameters controlling the number of casted rays and the maximum ray distance are also be specified.

This function was developed on PostgreSQL version 9.6.10 and PostGIS version 2.4.

Installation

Execute the contents of ST_RayCast.sql as a query on your database. To confirm that it is working correctly, execute the contents of test/ST_RayCast_test.sql on your database. It will create the PostGIS extension if it does not already exist, as well as create some test data tables and run through a set of sample function calls, which should produce the following tables:

  • test_lines_pointoutput: Result of casting rays towards linestrings and returning the result as a MULTIPOINT table.
  • test_lines_lineoutput: Result of casting rays towards linestrings and returning the result as a MULTILINESTRING table.
  • test_polys_pointoutput: Result of casting rays towards polygons and returning the result as a MULTIPOINT table.
  • test_polys_lineoutput: Result of casting rays towards polygons and returning the result as a MULTILINESTRING table.

Usage / Parameters

Call the function as you would any other PostGIS function. There are also sample function calls in test/ST_RayCast_test.sql.

ST_RayCast(in_point GEOMETRY, in_boundaries GEOMETRY, out_geom_type TEXT, num_rays INTEGER, max_ray_dist FLOAT)

The parameters are as follows:

  • in_point GEOMETRY: Input points from which to cast the rays. Must be of POINT type.
  • in_boundaries GEOMETRY: Input linestrings against which the rays are casted. Must be of LINESTRING type. To use POLYGON geometries instead, wrap them in a call to ST_ExteriorRing (More specialized transformations may be necessary in the case of holes).
  • out_geom_type TEXT: Type of desired output geometry. Must be one of MULTIPOINT or MULTPOLYGON.
  • num_rays INTEGER: Number of rays to cast. Will be distributed equally 360° around the point.
  • max_ray_dist FLOAT: Maximum distance that a ray may be cast. boundary features further away than this will not be "seen" by a point.

Further Development

There are a couple of features that could augment this function. I hope to revisit this in the future to add them, but they may also serve as an opportunity to cut your teeth in PostGIS development for anyone who is willing, as I don't think they would be particularly hard:

  • Add an offset parameter for the ray angles, i.e. Make the first ray start at a value other than 0° and have each subsequent ray also be casted with this offset value.
  • Add a start and end angle for the ray angles, i.e. Allow for the raycasting to occur only within a specific angular window for "directional" sight.

Credits

Developed by Isaac Boates (iboates@gmail.com) with support from Stackoverflow user ThingumaBob.