MS RFC 48: GEOTRANSFORM Geometry operations

Date:2008/11/01
Authors:Thomas Bonfort, Stephen Lime,
Contact:thomas.bonfort at camptocamp.com, Steve.Lime at DNR.State.MN.US
Status:Implemented (2009/02)
Last Edited:2009/02
Version:MapServer 5.4

Summary

The purpose of this RFC is to add the ability to MapServer to interpret the geometries based on a user-defined GEOMETRYTRANSFORM keyword, and to render the interpreted geometry with the selected styling

An example usage would be:

GEOMETRYTRANSFORM "start([geom])"

to treat the underlying geometry as a point rendered on the first vertex current feature.

The directive to activate this behavior is the GEOMETRYTRANSFORM keyword, that initially belongs to the STYLE object. Further developments could be to enable the keyword at the LAYER level.

Usages of this parameter include drawing bounding boxes of underlying geometries, or adding arrowheads/tails to lines.

Further developments would allow complex expressions to be passed to the keyword, allowing nested transformations to the underlying geometry, eg:

GEOMETRYTRANSFORM "difference([geom],buffer([geom],10))"

and binding to attributes, eg:

GEOMETRYTRANSFORM "buffer([geom],[distance])"

Detailed functionality

The GEOMETRYTRANSFORM keyword initially accepts a shortcut version of these parameters :

  • bbox : render a shapeObj representing the bounding box of the underlying geometry.
  • start/end : render a point symbol at the first/last vertex of the underlying geometry. By default, the symbol is oriented to match the orientation of the corresponding geometry segment. This would be used to add arrowheads/tails to linestrings.
  • vertices : render a point symbol at each vertex of the geometry. The default orientation matches the half angle of the corresponding segments. The start and end vertices are not included in the transformation.

These transforms are not initially implemented, but could be supported in the future:

  • buffer([geom],distance) : render a shapeObj representing the buffered geometry. “distance” specifies the buffer distance, in layer units.
  • centroid([geom]) : render a pointObj at the location of the centroid of the geometry
  • labelpoint([geom]) : render a pointObj at the location of the at the point where the geometry will/would be labelled.
  • convexhull([geom])
  • simplify([geom])

Implementation Details

Apart from the parsing and internal storage of which type should be used, the modifications of this RFC only affect the high-level rendering functions in mapdraw.c .

There are three main modifications:

  • AUTO is added to the ANGLE parser for the STYLE block. styleObj has an autoangle member added accordingly.
  • inside msDrawVectorLayer, a copy of the shape before being clipped by the current extent must be kept, as the whole shape must be used for all but the VERTICES type. Then, when looping through the different styles that can be applied, we switch off to the type-specific function if a GEOMETRYTRANSFORM keyword is set.
  • inside mapgeomtransform.c, the type-specific drawing function is added. Depending on the current style type, it either computes a new shape from the original shape (types bbox, convexhull or buffer) and passes it to the msDrawShadeSymbol, or calls msDrawMarkerSymbol for points of the original shape (types start, end, vertices, centroid) after having adjusted the orientation accordingly.

Affected Files

  • mapdraw.c
  • maplexer.c/.l
  • mapfile.c
  • mapserver.h
  • mapgeometrytransform.c is added, for the parsing of the transform expression, the transformation itself, and the calling of msDrawMarkerSymbol / msDrawShadeSymbol functions

Limitations

For line layers, the transformations do not fit in with the current cache mechanism that draws the first style of each class in a first pass for all shapes, and the remaining styles in a second pass.

MapScript implications

Getters and setters will have to be added to the different mapscripts.

A possible enhancement could also be to expose the transformation function to MapScript.

Dokumentation

completed

Backwards Incompatibility Issues

none expected.

Voting History

Vote completed on 2008/12/08:

  • +1: Lime, Szekeres, Woodbridge, Nacionales, Morissette
  • +0: Nicoletti