[pal] Offseting lines sometimes results in a multilinestring result

If the offset lines self-intersect or form isolated rings then we'll sometimes get multiline string results. In this case we just take the largest part as the offset curve instead of returning no offset line geometry
qgis · Apr 30, 2021 · 8d0d677 · 8d0d677
1 parent d25b409
commit 8d0d677
Showing 1 changed file with 34 additions and 0 deletions.
diff --git a/src/core/pal/pointset.cpp b/src/core/pal/pointset.cpp
@@ -558,6 +558,40 @@ void PointSet::offsetCurveByDistance( double distance )
   try
   {
     newGeos = GEOSOffsetCurve_r( geosctxt, mGeos, distance, 0, GEOSBUF_JOIN_MITRE, 2 );
+
+    // happens sometime, if the offset curve self-intersects
+    if ( GEOSGeomTypeId_r( geosctxt, newGeos ) == GEOS_MULTILINESTRING )
+    {
+      // we keep the longest part
+      const int nParts = GEOSGetNumGeometries_r( geosctxt, newGeos );
+      double maximumLength = -1;
+      const GEOSGeometry *longestPart = nullptr;
+      for ( int i = 0; i < nParts; ++i )
+      {
+        const GEOSGeometry *part = GEOSGetGeometryN_r( geosctxt, newGeos, i );
+        double partLength = -1;
+        if ( GEOSLength_r( geosctxt, part, &partLength ) == 1 )
+        {
+          if ( partLength > maximumLength )
+          {
+            maximumLength = partLength;
+            longestPart = part;
+          }
+        }
+      }
+
+      if ( !longestPart )
+      {
+        // something is really wrong!
+        GEOSGeom_destroy_r( geosctxt, newGeos );
+        return;
+      }
+
+      geos::unique_ptr longestPartClone( GEOSGeom_clone_r( geosctxt, longestPart ) );
+      GEOSGeom_destroy_r( geosctxt, newGeos );
+      newGeos = longestPartClone.release();
+    }
+
     const int newNbPoints = GEOSGeomGetNumPoints_r( geosctxt, newGeos );
     const GEOSCoordSequence *coordSeq = GEOSGeom_getCoordSeq_r( geosctxt, newGeos );
     std::vector< double > newX;