From 6f3f0b1d0db8da748b464e96d19ae187ebfd7955 Mon Sep 17 00:00:00 2001
From: Kacper Donat <kadet1090@gmail.com>
Date: Tue, 15 Jul 2025 00:14:39 +0200
Subject: [PATCH] Attacher: Orient normals correctly for Midpoint

Co-authored-by: Max Wilfinger <max@wilfinger.de>
---
 src/Mod/Part/App/Attacher.cpp | 34 ++++++++++++++++++++--------------
 src/Mod/Part/App/Geometry.cpp |  9 ++++++++-
 src/Mod/Part/App/Geometry.h   |  2 ++
 3 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/src/Mod/Part/App/Attacher.cpp b/src/Mod/Part/App/Attacher.cpp
index 4ef29ccbe7..4f1e041de9 100644
--- a/src/Mod/Part/App/Attacher.cpp
+++ b/src/Mod/Part/App/Attacher.cpp
@@ -1969,32 +1969,38 @@ AttachEngine3D::_calculateAttachedPlacement(const std::vector<App::DocumentObjec
                 case TopAbs_FACE: {
                     auto surface = freecad_cast<GeomSurface*>(geom.get());
 
+                    auto face = TopoDS::Face(shape->getShape());
+                    auto adaptorSurface = BRepAdaptor_Surface(face, true);
+
+                    auto u1 = adaptorSurface.FirstUParameter();
+                    auto u2 = adaptorSurface.LastUParameter();
+                    auto v1 = adaptorSurface.FirstVParameter();
+                    auto v2 = adaptorSurface.LastVParameter();
+
+                    auto midU = (u1 + u2) / 2;
+                    auto midV = (v1 + v2) / 2;
+
                     if (auto sphere = freecad_cast<GeomSphere*>(geom.get())) {
                         placement.setPosition(sphere->getLocation());
                     } else if (auto cone = freecad_cast<GeomCone*>(geom.get())) {
                         placement.setPosition(cone->getApex());
+                    } else if (auto point = surface->point(midU, midV)) {
+                        placement.setPosition(*point);
                     } else if (auto com = shape->centerOfGravity()) {
                         placement.setPosition(*com);
                     } else {
                         placement.setPosition(shape->getBoundBox().GetCenter());
                     }
 
-                    if (auto rotation = surface->getRotation()) {
-                        placement.setRotation(*rotation);
-                    } else {
-                        auto adaptorSurface = BRepAdaptor_Surface(TopoDS::Face(shape->getShape()), true);
+                    // calculate the normal at midpoint of the surface and use it as Z axis
+                    gp_Dir dir;
+                    surface->normal(midU, midV, dir);
 
-                        auto u1 = adaptorSurface.FirstUParameter();
-                        auto u2 = adaptorSurface.LastUParameter();
-                        auto v1 = adaptorSurface.FirstVParameter();
-                        auto v2 = adaptorSurface.LastVParameter();
-
-                        // calculate the normal at midpoint of the surface and use it as Z axis
-                        gp_Dir dir;
-                        surface->normal((u1 + u2) / 2, (v1 + v2) / 2, dir);
-
-                        placement.setRotation(Base::Rotation::fromNormalVector(Base::convertTo<Base::Vector3d>(-dir)));
+                    if (face.Orientation() == TopAbs_REVERSED) {
+                        dir = -dir;
                     }
+
+                    placement.setRotation(Base::Rotation::fromNormalVector(Base::convertTo<Base::Vector3d>(dir)));
                 }
                 break;
 
diff --git a/src/Mod/Part/App/Geometry.cpp b/src/Mod/Part/App/Geometry.cpp
index 626f288eda..552022e147 100644
--- a/src/Mod/Part/App/Geometry.cpp
+++ b/src/Mod/Part/App/Geometry.cpp
@@ -4884,12 +4884,19 @@ bool GeomSurface::normal(double u, double v, gp_Dir& dir) const
 
     Tools::getNormal(s, u, v, Precision::Confusion(), dir, done);
 
-    if (done)
+    if (done) {
         return true;
+    }
 
     return false;
 }
 
+std::optional<Base::Vector3d> GeomSurface::point(double u, double v) const
+{
+    Handle(Geom_Surface) s = Handle(Geom_Surface)::DownCast(handle());
+    return Base::convertTo<Base::Vector3d>(s->Value(u, v));
+}
+
 gp_Vec GeomSurface::getDN(double u, double v, int Nu, int Nv) const
 {
     Handle(Geom_Surface) s = Handle(Geom_Surface)::DownCast(handle());
diff --git a/src/Mod/Part/App/Geometry.h b/src/Mod/Part/App/Geometry.h
index 2912d30659..d1f5e37288 100644
--- a/src/Mod/Part/App/Geometry.h
+++ b/src/Mod/Part/App/Geometry.h
@@ -889,6 +889,8 @@ public:
     bool tangentU(double u, double v, gp_Dir& dirU) const;
     bool tangentV(double u, double v, gp_Dir& dirV) const;
     bool normal(double u, double v, gp_Dir& dir) const;
+    std::optional<Base::Vector3d> point(double u, double v) const;
+
     /*!
       Computes the derivative of order Nu in the direction U and Nv
       in the direction V at the point P(U, V).