From 4bfa12d86d65accff0a491a9acdc46e5670f4eae Mon Sep 17 00:00:00 2001
From: Abdullah Tahiri <abdullah.tahiri.yo@gmail.com>
Date: Sun, 14 Jan 2018 15:58:45 +0100
Subject: [PATCH] Sketcher: Correction of B spline normals

========================================

When writting solver implementations it has to be taken into account that the normal is not to be provided in the sense of the curvature, but to
the left when walking a curve from start to end.

https://forum.freecadweb.org/viewtopic.php?f=10&t=26312#p209486

This commit has impact on all the previous files using bspline and endpoint to endpoint tangency.

A work-around is to delete the tangency and do it again.
---
 src/Mod/Sketcher/App/planegcs/Geo.cpp | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/src/Mod/Sketcher/App/planegcs/Geo.cpp b/src/Mod/Sketcher/App/planegcs/Geo.cpp
index f226e383f2..5c9bf40175 100644
--- a/src/Mod/Sketcher/App/planegcs/Geo.cpp
+++ b/src/Mod/Sketcher/App/planegcs/Geo.cpp
@@ -613,6 +613,12 @@ DeriVector2 BSpline::CalculateNormal(Point& p, double* derivparam)
     // place holder
     DeriVector2 ret;
     
+    // even if this method is call CalculateNormal, the returned vector is not the normal strictu sensus
+    // but a normal vector, where the vector should point to the left when one walks along the curve from 
+    // start to end.
+    //
+    // https://forum.freecadweb.org/viewtopic.php?f=10&t=26312#p209486
+    
     if (mult[0] > degree && mult[mult.size()-1] > degree) {
     // if endpoints through end poles    
         if(*p.x == *start.x && *p.y == *start.y) {
@@ -620,30 +626,18 @@ DeriVector2 BSpline::CalculateNormal(Point& p, double* derivparam)
             // then tangency is defined by first to second poles
             DeriVector2 endpt(this->poles[1], derivparam);
             DeriVector2 spt(this->poles[0], derivparam);
-            DeriVector2 npt(this->poles[2], derivparam); // next pole to decide normal direction
             
             DeriVector2 tg = endpt.subtr(spt);
-            DeriVector2 nv = npt.subtr(spt);
-            
-            if ( tg.scalarProd(nv) > 0 )
-                ret = tg.rotate90cw();
-            else
-                ret = tg.rotate90ccw();
+            ret = tg.rotate90ccw();
         }
         else if(*p.x == *end.x && *p.y == *end.y) {
             // and you are asking about the normal at end point
             // then tangency is defined by last to last but one poles
             DeriVector2 endpt(this->poles[poles.size()-1], derivparam);
             DeriVector2 spt(this->poles[poles.size()-2], derivparam);
-            DeriVector2 npt(this->poles[poles.size()-3], derivparam); // next pole to decide normal direction
             
             DeriVector2 tg = endpt.subtr(spt);
-            DeriVector2 nv = npt.subtr(spt);
-            
-            if ( tg.scalarProd(nv) > 0 )
-                ret = tg.rotate90ccw();
-            else
-                ret = tg.rotate90cw();
+            ret = tg.rotate90ccw();
         } else {
            // another point and we have no clue until we implement De Boor
             ret = DeriVector2();