Add method to find distance from a point to a poylgon's boundary

python/core/geometry/qgspolygon.sip

@@ -35,4 +35,12 @@ class QgsPolygonV2: public QgsCurvePolygon
     virtual void setExteriorRing( QgsCurve* ring /Transfer/ );
 
     virtual QgsAbstractGeometry* boundary() const /Factory/;
+
+    /**
+     * Returns the distance from a point to the boundary of the polygon (either the
+     * exterior ring or any closer interior rings). The returned distance will be
+     * negative if the point lies outside the polygon.
+     * @note added in QGIS 3.0
+     */
+    double pointDistanceToBoundary( double x, double y ) const;
 };

src/core/geometry/qgspolygon.cpp

@@ -261,6 +261,40 @@ QgsAbstractGeometry* QgsPolygonV2::boundary() const
   }
 }
 
+double QgsPolygonV2::pointDistanceToBoundary( double x, double y ) const
+{
+  if ( !mExteriorRing )
+    return std::numeric_limits< double >::quiet_NaN();
+
+  bool inside = false;
+  double minimumDistance = DBL_MAX;
+  double minDistX = 0.0;
+  double minDistY = 0.0;
+
+  int numRings = mInteriorRings.size() + 1;
+  for ( int ringIndex = 0; ringIndex < numRings; ++ringIndex )
+  {
+    const QgsLineString* ring = static_cast< const QgsLineString* >( ringIndex == 0 ? mExteriorRing : mInteriorRings.at( ringIndex - 1 ) );
+
+    int len = ring->numPoints() - 1; //assume closed
+    for ( int i = 0, j = len - 1; i < len; j = i++ )
+    {
+      double aX = ring->xAt( i );
+      double aY = ring->yAt( i );
+      double bX = ring->xAt( j );
+      double bY = ring->yAt( j );
+
+      if ((( aY > y ) != ( bY > y ) ) &&
+          ( x < ( bX - aX ) * ( y - aY ) / ( bY - aY ) + aX ) )
+        inside = !inside;
+
+      minimumDistance = qMin( minimumDistance, QgsGeometryUtils::sqrDistToLine( x, y, aX, aY, bX, bY, minDistX, minDistY, 4 * DBL_EPSILON ) );
+    }
+  }
+
+  return ( inside ? 1 : -1 ) * sqrt( minimumDistance );
+}
+
 QgsPolygonV2* QgsPolygonV2::surfaceToPolygon() const
 {
   return clone();

src/core/geometry/qgspolygon.h

@@ -60,5 +60,13 @@ class CORE_EXPORT QgsPolygonV2: public QgsCurvePolygon
 
     virtual QgsAbstractGeometry* boundary() const override;
 
+    /**
+     * Returns the distance from a point to the boundary of the polygon (either the
+     * exterior ring or any closer interior rings). The returned distance will be
+     * negative if the point lies outside the polygon.
+     * @note added in QGIS 3.0
+     */
+    double pointDistanceToBoundary( double x, double y ) const;
+
 };
 #endif // QGSPOLYGONV2_H

tests/src/core/testqgsgeometry.cpp

@@ -3026,6 +3026,29 @@ void TestQgsGeometry::polygon()
   QCOMPARE( lineBoundary->zAt( 3 ), 10.0 );
   delete boundary;
 
+  // point distance to boundary
+
+  QgsLineString pd1;
+  pd1.setPoints( QList<QgsPointV2>() << QgsPointV2( 0, 0 ) << QgsPointV2( 1, 0 ) << QgsPointV2( 1, 1 ) << QgsPointV2( 0, 1 ) << QgsPointV2( 0, 0 ) );
+  QgsPolygonV2 pd;
+  // no meaning, but let's not crash
+  ( void )pd.pointDistanceToBoundary( 0, 0 );
+
+  pd.setExteriorRing( pd1.clone() );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0, 0.5 ), 0.0, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0.1, 0.5 ), 0.1, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( -0.1, 0.5 ), -0.1, 0.0000000001 );
+  // with a ring
+  QgsLineString pdRing1;
+  pdRing1.setPoints( QList<QgsPointV2>() << QgsPointV2( 0.1, 0.1 ) << QgsPointV2( 0.2, 0.1 ) << QgsPointV2( 0.2, 0.6 )  << QgsPointV2( 0.1, 0.6 ) << QgsPointV2( 0.1, 0.1 ) );
+  pd.setInteriorRings( QList< QgsCurve* >() << pdRing1.clone() );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0, 0.5 ), 0.0, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0.1, 0.5 ), 0.0, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0.01, 0.5 ), 0.01, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0.08, 0.5 ), 0.02, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( 0.12, 0.5 ), -0.02, 0.0000000001 );
+  QGSCOMPARENEAR( pd.pointDistanceToBoundary( -0.1, 0.5 ), -0.1, 0.0000000001 );
+
 }
 
 void TestQgsGeometry::multiPoint()