[postgis-devel] ST_AsMVT

[Björn Harrtell]

As a continuation of my work with ST_AsGeobuf (1) I've started work on
St_AsMVT (2), which is a function intended to output a Mapbox Vector Tile
(3).

A feature of Mapbox Vector Tiles are that they can contain several layers
of data within a tile, typically corresponding to a separate source schema.
So I'm thinking about how to specify the parameters to ST_AsMVT to support
fetching the data for a tile and this is my initial proposal:

CREATE FUNCTION ST_AsMVT(query text, extent box2d, resolution double,
buffer integer, pixelratio integer)

The query parameter is expected to produce rows with a specific schema like
follows:

CREATE TABLE output
(
  layer text,
  id bigint,
  geom geometry,
  attributes json
)

The above schema would allow unioning data from multiple tables with
differing attributes (represented using the json datatype, alternatively it
could be a hstore?). Note that it's up the the query to include geometries
to be considered (usually extent + buffer * resolution).

The extent parameter defines the geographic extent to be visible within the
tile (i.e does not include buffer).

The resolution parameter defines units per pixel used to translate
geometric coordinates into tile coordinates.

The buffer parameter defines how many pixels should be included outside
visible tile coordinates until clipping the geometry. This is used to avoid
border rendering artifacts.

The pixelratio parameter optionally multiplies tile coordinate space to
support hdpi displays. Default should probably be 1.

1) https://git.osgeo.org/gogs/postgis/postgis/pulls/4
2) https://git.osgeo.org/gogs/postgis/postgis/pulls/5
3) https://www.mapbox.com/vector-tiles/specification

/Björn
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