Interface Projection

s2/projections.go:26–72

Projection defines an interface for different ways of mapping between s2 and r2 Points. It can also define the coordinate wrapping behavior along each axis.

// Projection defines an interface for different ways of mapping between s2 and r2 Points.
// It can also define the coordinate wrapping behavior along each axis.
type Projection interface {
	// Project converts a point on the sphere to a projected 2D point.
	Project(p Point) r2.Point

	// Unproject converts a projected 2D point to a point on the sphere.
	//
	// If wrapping is defined for a given axis (see below), then this method
	// should accept any real number for the corresponding coordinate.
	Unproject(p r2.Point) Point

	// FromLatLng is a convenience function equivalent to Project(LatLngToPoint(ll)),
	// but the implementation is more efficient.
	FromLatLng(ll LatLng) r2.Point

	// ToLatLng is a convenience function equivalent to LatLngFromPoint(Unproject(p)),
	// but the implementation is more efficient.
	ToLatLng(p r2.Point) LatLng

	// Interpolate returns the point obtained by interpolating the given
	// fraction of the distance along the line from A to B.
	// Fractions < 0 or > 1 result in extrapolation instead.
	Interpolate(f float64, a, b r2.Point) r2.Point

	// WrapDistance reports the coordinate wrapping distance along each axis.
	// If this value is non-zero for a given axis, the coordinates are assumed
	// to "wrap" with the given period. For example, if WrapDistance.Y == 360
	// then (x, y) and (x, y + 360) should map to the same Point.
	//
	// This information is used to ensure that edges takes the shortest path
	// between two given points. For example, if coordinates represent
	// (latitude, longitude) pairs in degrees and WrapDistance().Y == 360,
	// then the edge (5:179, 5:-179) would be interpreted as spanning 2 degrees
	// of longitude rather than 358 degrees.
	//
	// If a given axis does not wrap, its WrapDistance should be set to zero.
	WrapDistance() r2.Point

	// WrapDestination that wraps the coordinates of B if necessary in order to
	// obtain the shortest edge AB. For example, suppose that A = [170, 20],
	// B = [-170, 20], and the projection wraps so that [x, y] == [x + 360, y].
	// Then this function would return [190, 20] for point B (reducing the edge
	// length in the "x" direction from 340 to 20).
	WrapDestination(a, b r2.Point) r2.Point

	// We do not support implementations of this interface outside this package.
	privateInterface()
}

// PlateCarreeProjection defines the "plate carree" (square plate) projection,
// which converts points on the sphere to (longitude, latitude) pairs.