Fix of Can't convert polyline with more than two points to a line (#6933)

Sometimes Clipper produces a polyline with more than 2 points when
clipping a line with a polygon or a set of polygons. We hope the intermediate
points are collinear with the line, so we may just ignore them.

src/libslic3r/ClipperUtils.cpp

@@ -647,7 +647,9 @@ Lines _clipper_ln(ClipperLib::ClipType clipType, const Lines &subject, const Pol
     // convert Polylines to Lines
     Lines retval;
     for (Polylines::const_iterator polyline = polylines.begin(); polyline != polylines.end(); ++polyline)
-        retval.emplace_back(polyline->line());
+        if (polyline->size() >= 2)
+            //FIXME It may happen, that Clipper produced a polyline with more than 2 collinear points by clipping a single line with polygons. It is a very rare issue, but it happens, see GH #6933.
+            retval.push_back({ polyline->front(), polyline->back() });
     return retval;
 }
 

src/libslic3r/Fill/FillAdaptive.cpp

@@ -928,7 +928,9 @@ static Polylines connect_lines_using_hooks(Polylines &&lines, const ExPolygon &b
                             Linef l { { bg::get<0, 0>(seg), bg::get<0, 1>(seg) }, { bg::get<1, 0>(seg), bg::get<1, 1>(seg) } };
                             assert(line_alg::distance_to_squared(l, Vec2d(pt.cast<double>())) > 1000 * 1000);
     #endif // NDEBUG
-                        } else if (pl.line().distance_to_squared(pt) <= 1000 * 1000)
+                        } else if (pl.size() >= 2 && 
+                            //FIXME Hoping that pl is really a line, trimmed by a polygon using ClipperUtils. Sometimes Clipper leaves some additional collinear points on the polyline, let's hope it is all right.
+                            Line{ pl.front(), pl.back() }.distance_to_squared(pt) <= 1000 * 1000)
                             out = closest.front().second;
                     }
                     return out;

src/libslic3r/MultiPoint.hpp

@@ -33,7 +33,9 @@ public:
     void rotate(double angle, const Point &center);
     void reverse() { std::reverse(this->points.begin(), this->points.end()); }
 
-    const Point& first_point() const { return this->points.front(); }
+    const Point& front() const { return this->points.front(); }
+    const Point& back() const { return this->points.back(); }
+    const Point& first_point() const { return this->front(); }
     virtual const Point& last_point() const = 0;
     virtual Lines lines() const = 0;
     size_t size() const { return points.size(); }

src/libslic3r/Polyline.cpp

@@ -10,13 +10,6 @@
 
 namespace Slic3r {
 
-Line Polyline::line() const
-{
-    if (this->points.size() > 2) 
-        throw Slic3r::InvalidArgument("Can't convert polyline with more than two points to a line");
-    return Line(this->points.front(), this->points.back());
-}
-
 const Point& Polyline::leftmost_point() const
 {
     const Point *p = &this->points.front();

src/libslic3r/Polyline.hpp

@@ -62,7 +62,6 @@ public:
   
     const Point& last_point() const override { return this->points.back(); }
     const Point& leftmost_point() const;
-    Line line() const;
     Lines lines() const override;
 
     void clip_end(double distance);