Outer ring detection fails when two rings share exactly one point.

[sdlime]

The code that detects inner and outer rings is used for converting shapeObj's to GML and GEOS geometries. There is a problem in cases where two rings touch at exactly 1 point (thing two squares touching at a corner). Polygons can touch at 1 point or less. The result is malformed GML output or degenerate GEOS geometries.

The cause is the point-in-polygon method in MapServer. It is degenerate when a point is on the edge of a polygon. It returns 0 or 1 consistently but you don't know if you're on the edge. So, in certain circumstances it's possible to have no outer rings. The outer ring needs to begin and end with a point that is present in another ring. A rare occurrence, but I've found a shapefile with just that case.

The solution is to test 2 points and perhaps 3 when checking for an outer ring. If the first two point-in-polygon tests yield the same result then you know that neither of them are on an edge. If, however, they yield opposite results then one must be on an edge (but you don't know which one). In this case you test a third point and since you know it can't be on an edge (as at most 1 can be).

Steve

[sdlime]

Fixed in the main trunk. I wonder if this should make it's way into the next 5.0 maintenance release?

Steve

[sdlime]

Changing milestone to 5.0 just in case there's another release for that version...

Steve

[sdlime]

Should be 5.0.4 release. Already fixed in 5.2 and higher...

Steve

[dmorissette]

No more 5.0 releases planned. Closing.