This low-level C++ library provides primitives for representing points and regions on the unit sphere, as well as support for partitioning the sphere. It can be used to answer the following sorts of questions:
- Is point X inside region Y?
- Do two regions A and B intersect?
- Which pieces of the sphere does region C overlap?
Regions can be serialized to binary strings, so that they may be stored efficiently in files or VARBINARY database columns. They can also be approximated with simpler regions - for example, one can ask for the bounding circle of a convex polygon.
Python bindings that expose most of the C++ API are also provided via pybind11.
There are 3 different classes for points - [LonLat](\ref lsst::sphgeom::LonLat) for spherical coordinates, [Vector3d](\ref lsst::sphgeom::Vector3d) for Cartesian vectors in ℝ³ (not constrained to lie on the unit sphere), and [UnitVector3d](\ref lsst::sphgeom::UnitVector3d) for vectors in ℝ³ with unit ℓ² norm.
Four basic spherical [Region](\ref lsst::sphgeom::Region) types are provided:
- [Box](\ref lsst::sphgeom::Box), a longitude/latitude angle box
- [Circle](\ref lsst::sphgeom::Circle), a small circle defined by a center and opening angle/chord length
- [Ellipse](\ref lsst::sphgeom::Ellipse), the intersection of an elliptical cone with the unit sphere
- [ConvexPolygon](\ref lsst::sphgeom::ConvexPolygon), a convex spherical polygon with unit vector vertices and great circle edges
In addition to the spherical regions, there is a type for 3-D axis aligned boxes, [Box3d](\ref lsst::sphgeom::Box3d). All spherical regions know how to compute their 3-D bounding boxes, which makes it possible to insert them into a 3-D R-tree. This is used by the exposure indexing task in the daf_ingest package to spatially index exposure bounding polygons using the SQLite 3 R*tree module.
A region can also determine its spatial [relationship](\ref lsst::sphgeom::Relationship) to another region, and test whether or not it contains a given unit vector.
This library also provides support for assigning points to pixels (a.k.a. cells or partitions) in a [Pixelization](\ref lsst::sphgeom::Pixelization) (a.k.a. partitioning) of the sphere, and for determining which pixels intersect a region.
Currently, the [Chunker](\ref lsst::sphgeom::Chunker) class implements the partitioning scheme employed by Qserv. The [HtmPixelization](\ref lsst::sphgeom::HtmPixelization) class implements the HTM (Hierarchical Triangular Mesh) pixelization. The [Q3cPixelization](\ref lsst::sphgeom::Q3cPixelization) and [Mq3cPixelization](\ref lsst::sphgeom::Mq3cPixelization) classes implement the original Quad Tree Cube indexing scheme and a modified version with reduced pixel area variation.
For instructions on how to contribute, see http://dm.lsst.org/#contributing (or just send us a pull request).
For help, see http://dm.lsst.org/#support.