From 45bfa50cbacb823901651679a98df9368da7fe1a Mon Sep 17 00:00:00 2001
From: Denis Rouzaud <denis.rouzaud@gmail.com>
Date: Sat, 17 Feb 2018 13:28:10 -0400
Subject: [PATCH] [offset tool] fix #18115 improved offset calculation

* use QgsGeometry::contains for polygons (contains => negative offset)
* use the geometry part and not the whole geometry to find closest segment
---
 src/app/qgsmaptooloffsetcurve.cpp | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/app/qgsmaptooloffsetcurve.cpp b/src/app/qgsmaptooloffsetcurve.cpp
index 30038a509f7..c3e6744fa67 100644
--- a/src/app/qgsmaptooloffsetcurve.cpp
+++ b/src/app/qgsmaptooloffsetcurve.cpp
@@ -367,8 +367,15 @@ double QgsMapToolOffsetCurve::calculateOffset( QgsPointXY mapPoint )
     int beforeVertex;
     int leftOf = 0;
 
-    offset = std::sqrt( mOriginalGeometry.closestSegmentWithContext( layerCoords, minDistPoint, beforeVertex, &leftOf ) );
-    offset = leftOf < 0 ? offset : -offset;
+    offset = std::sqrt( mManipulatedGeometry.closestSegmentWithContext( layerCoords, minDistPoint, beforeVertex, &leftOf ) );
+    if ( QgsWkbTypes::geometryType( mManipulatedGeometry.wkbType() ) == QgsWkbTypes::LineGeometry )
+    {
+      offset = leftOf < 0 ? offset : -offset;
+    }
+    else
+    {
+      offset = mManipulatedGeometry.contains( &layerCoords ) ? -offset : offset;
+    }
   }
   return offset;
 }
@@ -573,7 +580,7 @@ void QgsMapToolOffsetCurve::updateGeometryAndRubberBand( double offset )
   }
   else
   {
-    offsetGeom = mManipulatedGeometry.buffer( -offset, quadSegments, capStyle, joinStyle, miterLimit );
+    offsetGeom = mManipulatedGeometry.buffer( offset, quadSegments, capStyle, joinStyle, miterLimit );
   }
 
   if ( !offsetGeom )