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Vector tile => raster tile backend for tilelive


Implements the tilelive API for rendering mapnik vector tiles to raster images.

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new Vector(options, callback)

  • xml: a Mapnik XML string that will be used to render vector tiles.
  • source: Optional, a uri string suitable for use with tilelive.load(). This is fallback source that will be used if no source is found as part of the Mapnik XML parameters.
  • base: Optional, basepath for Mapnik map. Defaults to __dirname.
  • format: Optional, target output format. Defaults to png8:m=h.
  • scale: Optional, Mapnik scale factor. Defaults to 1.

Code concepts

  • Backend z/x/y: a request for a raster tile at, say, 3/3/3 does not always mean 3/3/3 is requested from the backend source. The z/x/y requested from the backend source is referred in code by bz/bx/by and generally represent the same or lower zoom level. This allows for features like overzooming, maskLevel tiles, and scale factor adjustment.
  • Overzooming: if a tile beyond the maxzoom of the backend is requested, Vector will attempt to render the tile using the parent of the request at maxzoom.
  • maskLevel tiles: to avoid requiring many duplicate or empty vector tiles to be generated at high zoom levels, the backend source can specify a maskLevel. If a vector tile is not initially found at some z > maskLevel, Vector will issue an additional request to the backend using the parent tile of of the request at maskLevel. This allows a lower zoom level to "backfill" high zoom levels.
  • Scale factor adjustment: the scale argument decrements the backend zoom level such that the requested tile is the visual equivalent (when viewed on the proper dpi device) of its parent counterpart. For example, scale: 2 decrements bz by 1, scale: 4 decrements by 2, and so on.

Backend headers

The Backend and Vector sources set the x-vector-backend-object header to indicate the type of backend source tile loaded. Possible values:

value description
empty backend returned no tile
fillzoom tile from the fillzoom of the backend (backfilled from a lower zoom)
overzoom tile from the maxzoom of the backend (overzoomed)
default normal tile

Bonus constructors

There are two additional constructors provided for slightly different workflows for loading vector sources.

Vector.tm2z(uri, callback)

Load a vector source from a uri (filepath or HTTP url) to a tm2-generated tm2z style package.

Vector.xray(options, callback)

Load a vector source with autogenerated data inspection ("xray") styles. xml is not required -- only the uri to a vector source backend is necessary.