Label Lower Left Corner Of Square Polygon SLD and Geoserver

Question

I would like to label the lower left corner of rectangular shaped polygons in geoserver I have looked the the functions and I am wondering how I would set up variable to perform the calculation for anchor point

Here are the filter functions I was looking at https://docs.geoserver.org/latest/en/user/filter/function_reference.html

I am thinking I could use the Geometry:boundary function somehow but I cannot figure out how to get the corner geometry. Is this possible

Answer 1

If you can make sure during preprocessing/querying your data that the lower left corner is always the n-th (e.g. first) point in the geometry data, you could use the pointN function to get this point for labeling.

Not the cleanest solution, but should work...

Answer 2

I think you may need to write a custom function for this, something like this should work (seems mostly right with my test cases):

public class LowerLeftFunction implements Function {
  static FunctionName NAME = new FunctionNameImpl("lowerleft", Polygon.class,
  FunctionNameImpl.parameter("polygon", Polygon.class));

  ....

  @Override
  public <T> T evaluate(Object object, Class<T> context) {
    Expression polyExp = parameters.get(0);
    Polygon poly = polyExp.evaluate(object, Polygon.class);
    LineString ext = poly.getExteriorRing();
    Coordinate ll = new Coordinate(Double.POSITIVE_INFINITY, Double.POSITIVE_INFINITY);
    for (Coordinate c : ext.getCoordinates()) {
      if (c.x <= ll.x && c.y <= ll.y) {
        ll = c;
      }
    }
    //System.err.println(poly);
    Point pt = poly.getFactory().createPoint(ll);
    //System.err.println(pt);
    return Converters.convert(pt, context);
  }
 ....
}

I'll put the whole thing up on github when it has some tests and I get a chance later.