This is a library for manipulating geometric shapes. Unlike many geometry libraries, S2 is primarily designed to work with spherical geometry, i.e., shapes drawn on a sphere rather than on a planar 2D map. (In fact, the name S2 is derived from the mathematical notation for the unit sphere.) This makes it especially suitable for working with geographic data.
The library consists of:
Basic representations of angles, intervals, latitude-longitude points, unit 3D vectors, and conversions among them.
Various shapes over the unit sphere, such as spherical caps ("discs"), latitude-longitude rectangles, polylines, and polygons. These are collectively known as "regions".
Support for spatial indexing of collections of geometry, and algorithms for testing containment, finding nearby objects, finding intersections, etc.
A hierarchical decomposition of the sphere into regions called "cells". The hierarchy starts with the six faces of a projected cube and recursively subdivides them in a quadtree-like fashion.
The ability to approximate arbitrary regions as a collection of cells. This is useful for building inverted indexes that allow queries over arbitrarily shaped regions.
The implementations attempt to be precise both in terms of mathematical definitions (e.g. whether regions include their boundaries, representations of empty and full regions) and numerical accuracy (e.g. avoiding cancellation error).
Note that the intent of this library is to represent geometry as a mathematical abstraction. For example, although the unit sphere is obviously a useful approximation for the Earth's surface, functions that are specifically related to geography are not part of the core library (e.g. easting/northing conversions, ellipsoid approximations, geodetic vs. geocentric coordinates, etc).
See http://godoc.org/github.com/golang/geo for specific package documentation.
For an analogous library in C++, see https://code.google.com/archive/p/s2-geometry-library/, and in Java, see https://github.com/google/s2-geometry-library-java
Status of the Go Library
This library is principally a port of the C++ S2 library, adapting to Go idioms where it makes sense. We detail the progress of this port below relative to that C++ library.
ℝ¹ - One-dimensional Cartesian coordinates
Full parity with C++.
ℝ² - Two-dimensional Cartesian coordinates
Full parity with C++.
ℝ³ - Three-dimensional Cartesian coordinates
Full parity with C++.
S¹ - Circular Geometry
- Angle - Missing Arithmetic methods, Trigonometric methods, Conversion to/from s2.Point, s2.LatLng, convenience methods from E5/E6/E7
- Interval - Missing ClampPoint, Complement, ComplementCenter, HaussdorfDistance
S² - Spherical Geometry
Approximately ~40% complete.
Complete These files have full parity with the C++ implementation.
- s2stuv.go (AKA half of s2.h in C++) - This file is a collection of helper and conversion methods to and from ST-space, UV-space, and XYZ-space.
Mostly Complete Files that have almost all of the features of the original C++ code, and are reasonably complete enough to use in live code. Up to date listing of the incomplete methods are documented at the end of each file.
- Cell - Missing Subdivide, BoundUV, DistanceToEdge, ChordDistance.
- CellUnion - Missing Union, Intersection, etc.
- Edgeutil - Missing Distance methods, LongitudePruner, FaceSegments, ClosestPair.
- Point - Missing TurningAngle, Rotate, some Area methods.
- Polyline - Missing Projection, Intersects, Interpolate, etc.
- Rect (AKA s2latlngrect in C++) - Missing Centroid, Distance, InteriorContains.
- RegionCoverer - Missing FloodFill and SimpleCovering.
- s2_test.go (AKA s2testing and s2textformat in C++) - Missing ConcentricLoopsPolygon and Fractal test shape generation. This file is a collection of testing helper methods.
In Progress Files that have some work done, but are probably not complete enough for general use in production code.
- Loop - Loop has basic skelton complete and some tendons, but missing most things. Normalize, Invert, Area, Centroid, Projection, Distance, Contains, Intersects, Union, etc. A significant portion of these are under review now.
- Polygon - Polygon is at the partial skeleton phase, the fields all exist, and some basic methods are implemented, but it's missing almost everything. Init with multiple loops, Area, Centroid, Distance, Projection, Intersection, Union, Contains, Normalized, etc.
- PolylineSimplifier - Initial work has begun on this.
- s2predicates.go - Initial work has begun on this. This file is a collection of helper methods used by other parts of the library.
- ShapeIndex - Currently has only the minimal skeleton pieces submitted, but changes are out for review for the remainder of it.
Not Started Yet. These files (and their associated unit tests) have dependencies on most of the In Progress files before they can begin to be started.
- Builder - Next on the list. This is a robust tool for creating the various Shape types from collection of simpler S2 types.
- BuilderGraph - Next on the list.
- BuilderLayers - Next on the list.
- BuilderSnapFunctions - Next on the list.
- Error - May not be needed as it's just declaring a number of predefined Errors.
- ShapeUtil - Most of this will end up in s2_test.