VirtualPath: Export tangent from findClosestPointTo()

The tangent vector is important for the move parallel tool where we want
to calculate the distance of the cursor from the tangent line touching
the path in question.

Author: lpechacek

src/core/objects/object.cpp

@@ -1160,7 +1160,7 @@ ClosestPathCoord PathObject::findClosestPointTo(
 
 	using distance_type = decltype(ClosestPathCoord::distance_squared);
 	return std::accumulate(begin(path_parts), end(path_parts),
-	                       ClosestPathCoord { {}, std::numeric_limits<distance_type>::max() },
+	                       ClosestPathCoord { {}, {}, std::numeric_limits<distance_type>::max() },
 	                       op);
 }
 

src/core/path_coord.h

@@ -163,6 +163,7 @@ class PathCoord
 struct ClosestPathCoord
 {
 	PathCoord path_coord;
+	MapCoordF tangent;
 	double distance_squared;
 };
 

src/core/virtual_path.cpp

@@ -347,9 +347,11 @@ ClosestPathCoord VirtualPath::findClosestPointTo(
         size_type start_index,
         size_type end_index ) const
 {
-	Q_ASSERT(!path_coords.empty());
+	Q_ASSERT(path_coords.size() > 1);
 
-	auto result = ClosestPathCoord { path_coords.front(), distance_bound_squared };
+	auto result = ClosestPathCoord { path_coords.front(), 
+	              path_coords[1].pos - path_coords[0].pos,
+	              distance_bound_squared };
 
 	// Find upper bound for distance.
 	for (const auto& path_coord : path_coords)
@@ -364,6 +366,27 @@ ClosestPathCoord VirtualPath::findClosestPointTo(
 		if (dist_sq < result.distance_squared)
 		{
 			result.distance_squared = dist_sq;
+			// Here we create predictable behavior for corners and handle 
+			// the ends. Although corners do not have a tangent in 
+			// the mathematical sense, the humans intuitively expect the 
+			// "tangent" to go across the corner and divide the plane into the
+			// inner and outer parts.
+			bool in_ok = false;
+			bool out_ok = false;
+			auto const incoming = calculateIncomingTangent(path_coord.index, in_ok);
+			auto const outgoing = calculateOutgoingTangent(path_coord.index, out_ok);
+			if (in_ok && out_ok)
+			{
+				result.tangent = incoming + outgoing;
+			}
+			else if (in_ok)
+			{
+				result.tangent = incoming;
+			}
+			else if (out_ok)
+			{
+				result.tangent = outgoing;
+			}
 			result.path_coord = path_coord;
 		}
 	}
@@ -391,6 +414,7 @@ ClosestPathCoord VirtualPath::findClosestPointTo(
 			if (to_coord.lengthSquared() < result.distance_squared)
 			{
 				result.distance_squared = to_coord.lengthSquared();
+				result.tangent = tangent;
 				result.path_coord = *pc;
 			}
 			continue;
@@ -402,6 +426,7 @@ ClosestPathCoord VirtualPath::findClosestPointTo(
 			if (coord.distanceSquaredTo(next_pos) < result.distance_squared)
 			{
 				result.distance_squared = coord.distanceSquaredTo(next_pos);
+				result.tangent = tangent;
 				result.path_coord = *next_pc;
 			}
 			continue;
@@ -425,13 +450,16 @@ ClosestPathCoord VirtualPath::findClosestPointTo(
 			if (coords.flags[result.path_coord.index].isCurveStart())
 			{
 				MapCoordF unused;
+				MapCoordF curve2_control_point1;
 				PathCoord::splitBezierCurve(MapCoordF(coords.flags[result.path_coord.index]), MapCoordF(coords.flags[result.path_coord.index+1]),
 											MapCoordF(coords.flags[result.path_coord.index+2]), MapCoordF(coords.flags[result.path_coord.index+3]),
-											result.path_coord.param, unused, unused, result.path_coord.pos, unused, unused);
+											result.path_coord.param, unused, unused, result.path_coord.pos, curve2_control_point1, unused);
+				result.tangent = curve2_control_point1 - result.path_coord.pos;
 			}
 			else
 			{
-				result.path_coord.pos = pos + (next_pos - pos) * double(factor);
+				result.tangent = next_pos - pos;
+				result.path_coord.pos = pos + result.tangent * double(factor);
 			}
 		}
 	}