Fix #10605 (round corners when using simple line with offset)

python/core/qgsgeometry.sip

@@ -314,7 +314,21 @@ class QgsGeometry
         of segments used to approximate curves */
     QgsGeometry* buffer( double distance, int segments ) /Factory/;
 
-    /** Returns an offset line at a given distance and side from an input line. */
+    /** Returns a buffer region around the geometry, with additional style options.
+     * @param segments    For round joins, number of segments to approximate quarter-circle
+     * @param endCapStyle Round (1) / Flat (2) / Square (3) end cap style
+     * @param joinStyle   Round (1) / Mitre (2) / Bevel (3) join style
+     * @param mitreLimit  Limit on the mitre ratio used for very sharp corners
+     * @note added in 2.4
+     * @note needs GEOS >= 3.3 - otherwise always returns 0
+     */
+    QgsGeometry* buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit ) /Factory/;
+
+    /** Returns an offset line at a given distance and side from an input line.
+     * See buffer() method for details on parameters.
+     * @note added in 2.4
+     * @note needs GEOS >= 3.3 - otherwise always returns 0
+     */
     QgsGeometry* offsetCurve( double distance, int segments, int joinStyle, double mitreLimit ) /Factory/;
 
     /** Returns a simplified version of this geometry using a specified tolerance value */

src/core/qgsgeometry.cpp

@@ -5594,10 +5594,30 @@ QgsGeometry* QgsGeometry::buffer( double distance, int segments )
   CATCH_GEOS( 0 )
 }
 
+QgsGeometry*QgsGeometry::buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit )
+{
 #if defined(GEOS_VERSION_MAJOR) && defined(GEOS_VERSION_MINOR) && \
  ((GEOS_VERSION_MAJOR>3) || ((GEOS_VERSION_MAJOR==3) && (GEOS_VERSION_MINOR>=3)))
+  if ( mDirtyGeos )
+    exportWkbToGeos();
+
+  if ( !mGeos )
+    return 0;
+
+  try
+  {
+    return fromGeosGeom( GEOSBufferWithStyle( mGeos, distance, segments, endCapStyle, joinStyle, mitreLimit ) );
+  }
+  CATCH_GEOS( 0 )
+#else
+  return 0;
+#endif
+}
+
 QgsGeometry* QgsGeometry::offsetCurve( double distance, int segments, int joinStyle, double mitreLimit )
 {
+#if defined(GEOS_VERSION_MAJOR) && defined(GEOS_VERSION_MINOR) && \
+ ((GEOS_VERSION_MAJOR>3) || ((GEOS_VERSION_MAJOR==3) && (GEOS_VERSION_MINOR>=3)))
   if ( mDirtyGeos )
     exportWkbToGeos();
 
@@ -5609,8 +5629,10 @@ QgsGeometry* QgsGeometry::offsetCurve( double distance, int segments, int joinSt
     return fromGeosGeom( GEOSOffsetCurve( mGeos, distance, segments, joinStyle, mitreLimit ) );
   }
   CATCH_GEOS( 0 )
-}
+#else
+  return 0;
 #endif
+}
 
 QgsGeometry* QgsGeometry::simplify( double tolerance )
 {

src/core/qgsgeometry.h

@@ -355,14 +355,22 @@ class CORE_EXPORT QgsGeometry
         of segments used to approximate curves */
     QgsGeometry* buffer( double distance, int segments );
 
-#if defined(GEOS_VERSION_MAJOR) && defined(GEOS_VERSION_MINOR) && \
+    /** Returns a buffer region around the geometry, with additional style options.
- ((GEOS_VERSION_MAJOR>3) || ((GEOS_VERSION_MAJOR==3) && (GEOS_VERSION_MINOR>=3)))
+     * @param segments    For round joins, number of segments to approximate quarter-circle
-    /** Returns an offset line at a given distance and side from an input line (uses GEOS)
+     * @param endCapStyle Round (1) / Flat (2) / Square (3) end cap style
-     @note added in 2.3
+     * @param joinStyle   Round (1) / Mitre (2) / Bevel (3) join style
-     @note only available with GEOS >= 3.3
+     * @param mitreLimit  Limit on the mitre ratio used for very sharp corners
-      */
+     * @note added in 2.4
+     * @note needs GEOS >= 3.3 - otherwise always returns 0
+     */
+    QgsGeometry* buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit );
+
+    /** Returns an offset line at a given distance and side from an input line.
+     * See buffer() method for details on parameters.
+     * @note added in 2.4
+     * @note needs GEOS >= 3.3 - otherwise always returns 0
+     */
     QgsGeometry* offsetCurve( double distance, int segments, int joinStyle, double mitreLimit );
-#endif
 
     /** Returns a simplified version of this geometry using a specified tolerance value */
     QgsGeometry* simplify( double tolerance );

src/core/symbology-ng/qgssymbollayerv2utils.cpp

@@ -708,7 +708,13 @@ QList<QPolygonF> offsetLine( QPolygonF polyline, double dist, QGis::GeometryType
   QgsGeometry* tempGeometry = ( geometryType == QGis::Polygon ) ? QgsGeometry::fromPolygon( QgsPolygon() << tempPolyline ) : QgsGeometry::fromPolyline( tempPolyline );
   if ( tempGeometry )
   {
-    QgsGeometry* offsetGeom = ( geometryType == QGis::Polygon ) ? tempGeometry->buffer( -dist, 8 /*quadSegments*/ ) : tempGeometry->offsetCurve( dist, 8 /*quadSegments*/, 0 /*joinStyle*/, 5.0 /*mitreLimit*/ );
+    int quadSegments = 0; // we want mitre joins, not round joins
+    double mitreLimit = 2.0; // the default value in GEOS (5.0) allows for fairly sharp endings
+    QgsGeometry* offsetGeom = 0;
+    if ( geometryType == QGis::Polygon )
+      offsetGeom = tempGeometry->buffer( -dist, quadSegments, GEOSBUF_CAP_FLAT, GEOSBUF_JOIN_MITRE, mitreLimit );
+    else
+      offsetGeom = tempGeometry->offsetCurve( dist, quadSegments, GEOSBUF_JOIN_MITRE, mitreLimit );
 
     if ( offsetGeom )
     {