Labels: Data defined position from geometry column

[mwtoews]

I would like to use a geometry (POINT) attribute to position label, using the example postgres schema:

CREATE TABLE region
(
  gid serial NOT NULL,
  geometry geometry, -- SELECT AddGeometryColumn('public','region','geometry',26910,'POLYGON',2);
  label geometry, -- SELECT AddGeometryColumn('public','region','label',26910,'POINT',2);
  ...
);

Here, there are two geometry columns: a polygon for the shape in the map, and a point to specify and store the label location.

The present versions (1.0.1 to 1.2.0 unstable) support X/Y coordinates from two numeric columns. (This is found in Layer Properties > Labels > Data defined position).

The enchantment requested in this ticket is to add another pull-down for a data defined position, which would be a geometry column. This seems more natural than defining two numeric columns for X and Y coordinates.

I suggest first keeping it simple, supporting only POINT geometries. However, I can also envision enabling MULTIPOINT (for multiple places of the same label), and perhapps [MULTI]LINESTRING for text along a path (creeks, rivers) if the text fits the length. Furthermore, the centroid can easily be extracted from all other geometry types ([MULTI]POLYGONs, COLLECTIONs, etc.) to represent the coordinate for the label (i.e., this is the "else"/"default" case for other geometry types).