Streams

Rather than materializing intermediate representations, streams transform geometry through function calls to minimize overhead. Streams must implement several methods to receive input geometry. Streams are inherently stateful; the meaning of a point depends on whether the point is inside of a line, and likewise a line is distinguished from a ring by a polygon. Despite the name “stream”, these method calls are currently synchronous.

geoStream(object, stream)

Source · Streams the specified GeoJSON object to the specified projection stream. While both features and geometry objects are supported as input, the stream interface only describes the geometry, and thus additional feature properties are not visible to streams.

stream.point(x, y, z)

Indicates a point with the specified coordinates x and y (and optionally z). The coordinate system is unspecified and implementation-dependent; for example, projection streams require spherical coordinates in degrees as input. Outside the context of a polygon or line, a point indicates a point geometry object (Point or MultiPoint). Within a line or polygon ring, the point indicates a control point.

stream.lineStart()

Indicates the start of a line or ring. Within a polygon, indicates the start of a ring. The first ring of a polygon is the exterior ring, and is typically clockwise. Any subsequent rings indicate holes in the polygon, and are typically counterclockwise.

stream.lineEnd()

Indicates the end of a line or ring. Within a polygon, indicates the end of a ring. Unlike GeoJSON, the redundant closing coordinate of a ring is not indicated via point, and instead is implied via lineEnd within a polygon. Thus, the given polygon input:

{
  "type": "Polygon",
  "coordinates": [[[0, 0], [0, 1], [1, 1], [1, 0], [0, 0]]]
}

Will produce the following series of method calls on the stream:

stream.polygonStart();
stream.lineStart();
stream.point(0, 0);
stream.point(0, 1);
stream.point(1, 1);
stream.point(1, 0);
stream.lineEnd();
stream.polygonEnd();

stream.polygonStart()

Indicates the start of a polygon. The first line of a polygon indicates the exterior ring, and any subsequent lines indicate interior holes.

stream.polygonEnd()

Indicates the end of a polygon.

stream.sphere()

Indicates the sphere (the globe; the unit sphere centered at ⟨0,0,0⟩).