OGRPolygon::PointOnSurface craches on self-intersecting polygon

[ftrastour]

The function OGRPolygon::PointOnSurface crashes on "butterfly" polygons ( self intersecting ). It crashes because the call to GEOSPointOnSurface returns null when the polygon is self intersecting ans the following createFromGEOS failed.

Here is a fix :

int OGRPolygon::PointOnSurface( OGRPoint *poPoint ) const

{
    if( poPoint == NULL )
        return OGRERR_FAILURE;
 
#ifndef HAVE_GEOS
    return OGRERR_FAILURE;
#else
    GEOSGeom hThisGeosGeom = NULL;
    GEOSGeom hOtherGeosGeom = NULL;
     
    hThisGeosGeom = exportToGEOS();
 
    if( hThisGeosGeom != NULL )
    {
     	hOtherGeosGeom = GEOSPointOnSurface( hThisGeosGeom );

        //Check if the function has find a such point.
	if( ! hOtherGeosGeom ) {
          // GEOSPointOnSurface sometimes failed on degenerated polygons.
	  GEOSGeom_destroy( hThisGeosGeom ) ;
	  return OGRERR_FAILURE ;
	}

        ...

[warmerdam]

Chaitanya,

Could you review and apply the suggested changes in trunk, 1.7 and 1.6 branches?

Frédéric,

Would it be possible for you to paste in the WKT for a polygon that triggers this? I'd like Chaitanya to add a test of this to the test suite.

[chaitanya]

I fixed a similar situation in OGRPolygon::Centroid() too. Applied the patch in trunk (r19088), 1.7 branch (r19090) and 1.6 branch (r19091).

Frédéric, can you give me a sample polygon for testing?

[ftrastour]

Chaitanya,

here is a self intersecting WKT polygon :

POLYGON ((541777.73621003702 1789399.8326989261,541836.33734208031 1789280.1623735379,541867.88100016885 1789439.9155349836,541964.66262947978 1789325.2472433962,541777.73621003702 1789399.8326989261))

[chaitanya]

Added a test for centroid of butterfly polygon in trunk (r19981).

In any case, GEOS 3.1.1 has been returning non-null GEOSGeom objects in GEOSCentroid() and GEOSPointOnSurface() methods.

Currently PointOnSurface() method has neither C API interface nor python interface.

[Even Rouault]

Milestone 1.6.4 deleted