Created a total_lengt() free function for Polygons and Polylines.

Modified the "extra perimeters when needed" function to use the new free functions,
extended the tooltip of the "extra perimeters when needed" feature.

xs/src/libslic3r/Polygon.hpp

@@ -59,6 +59,13 @@ extern BoundingBox get_extents_rotated(const Polygon &poly, double angle);
 extern BoundingBox get_extents_rotated(const Polygons &polygons, double angle);
 extern std::vector<BoundingBox> get_extents_vector(const Polygons &polygons);
 
+inline double total_length(const Polygons &polylines) {
+    double total = 0;
+    for (Polygons::const_iterator it = polylines.begin(); it != polylines.end(); ++it)
+        total += it->length();
+    return total;
+}
+
 // Remove sticks (tentacles with zero area) from the polygon.
 extern bool        remove_sticks(Polygon &poly);
 extern bool        remove_sticks(Polygons &polys);

xs/src/libslic3r/Polyline.hpp

@@ -69,6 +69,13 @@ public:
 extern BoundingBox get_extents(const Polyline &polyline);
 extern BoundingBox get_extents(const Polylines &polylines);
 
+inline double total_length(const Polylines &polylines) {
+    double total = 0;
+    for (Polylines::const_iterator it = polylines.begin(); it != polylines.end(); ++it)
+        total += it->length();
+    return total;
+}
+
 inline Lines to_lines(const Polyline &poly) 
 {
     Lines lines;

xs/src/libslic3r/PrintConfig.cpp

@@ -204,7 +204,7 @@ PrintConfigDef::PrintConfigDef()
     def = this->add("extra_perimeters", coBool);
     def->label = "Extra perimeters if needed";
     def->category = "Layers and Perimeters";
-    def->tooltip = "Add more perimeters when needed for avoiding gaps in sloping walls.";
+    def->tooltip = "Add more perimeters when needed for avoiding gaps in sloping walls. Slic3r keeps adding perimeters, until more than 70% of the loop immediately above is supported.";
     def->cli = "extra-perimeters!";
     def->default_value = new ConfigOptionBool(true);
 

xs/src/libslic3r/PrintObject.cpp

@@ -1168,10 +1168,7 @@ PrintObject::_make_perimeters()
             
             // Filter upper layer polygons in intersection_ppl by their bounding boxes?
             // my $upper_layerm_poly_bboxes= [ map $_->bounding_box, @{$upper_layerm_polygons} ];
-            double total_loop_length = 0;
+            const double total_loop_length      = total_length(upper_layerm_polygons);
-            for (Polygons::const_iterator it = upper_layerm_polygons.begin(); it != upper_layerm_polygons.end(); ++it)
-                total_loop_length += it->length();
-            
             const coord_t perimeter_spacing     = layerm.flow(frPerimeter).scaled_spacing();
             const Flow ext_perimeter_flow       = layerm.flow(frExternalPerimeter);
             const coord_t ext_perimeter_width   = ext_perimeter_flow.scaled_width();
@@ -1199,12 +1196,8 @@ PrintObject::_make_perimeters()
                     );
                     
                     // only add an additional loop if at least 30% of the slice loop would benefit from it
-                    {
+                    if (total_length(intersection) <=  total_loop_length*0.3)
-                        double total_intersection_length = 0;
+                        break;
-                        for (Polylines::const_iterator it = intersection.begin(); it != intersection.end(); ++it)
-                            total_intersection_length += it->length();
-                        if (total_intersection_length <= total_loop_length*0.3) break;
-                    }
                     
                     /*
                     if (0) {