external_perimeter_cut_corners setting

it reduce the flow around corners, depending of the angle.
experimental status! not tested yet!

resources/ui_layout/print.ui

@@ -235,6 +235,7 @@ group:Flow
 		setting:fill_top_flow_ratio
 		setting:first_layer_flow_ratio
 	end_line
+	setting:external_perimeter_cut_corners
 
 page:Multiple extruders:funnel
 group:Extruders

src/libslic3r/GCode.cpp

@@ -3559,17 +3559,75 @@ std::string GCode::_extrude(const ExtrusionPath &path, const std::string &descri
         * this->config().print_extrusion_multiplier.get_abs_value(1);
     if (this->m_layer_index <= 0) e_per_mm *= this->config().first_layer_flow_ratio.get_abs_value(1);
     if (m_writer.extrusion_axis().empty()) e_per_mm = 0;
-    double path_length = 0.;
+    if (path.polyline.lines().size() > 0) {
+        //get last direction //TODO: save it
         {
             std::string comment = m_config.gcode_comments ? description : "";
+            if (path.role() != erExternalPerimeter || config().external_perimeter_cut_corners.value == 0) {
+                // normal & legacy pathcode
                 for (const Line& line : path.polyline.lines()) {
-            const double line_length = line.length() * SCALING_FACTOR;
-            path_length += line_length;
                     gcode += m_writer.extrude_to_xy(
                         this->point_to_gcode(line.b),
-                e_per_mm * line_length,
+                        e_per_mm * unscaled(line.length()),
                         comment);
                 }
+            } else {
+                // external_perimeter_cut_corners pathcode
+                Point last_pos = path.polyline.lines()[0].a;
+                for (const Line& line : path.polyline.lines()) {
+                    //check the angle
+                    double angle = line.a == last_pos ? PI : line.a.ccw_angle(last_pos, line.b);
+                    if (angle > 1) angle = 2 * PI - angle;
+                    double coeff = std::cos(angle) + 1;
+                    //Create extrusion mult
+                    double mult = config().external_perimeter_cut_corners.get_abs_value(0.05375) * coeff; // 0.94625 = 3.78 / 4 = (4 - ((4 - pi) / 4)) / 4  // 4 = carre, 3.14 = disk
+                    double length1 = scale_(path.width) / 4;
+
+                    //extrude in three steps: one short with big mult, one of nozzle size with the rest and then the normal one.
+                    //it's a very rough approx of a cos.
+                    length1 /= (2.1 - coeff);
+                    if (line.length() > length1 && mult > 0.001) {
+                        //Create a point
+                        Point inter_point1 = line.point_at(length1);
+                        //extrude very reduced
+                        gcode += m_writer.extrude_to_xy(
+                            this->point_to_gcode(inter_point1),
+                            e_per_mm * unscaled(length1) * (1 - mult * 2.5),
+                            comment);
+
+                        double length2 = scale_(path.width);
+                        length2 /= (2.1 - coeff);
+                        if (line.length() > length2) {
+                            Point inter_point2 = line.point_at(length2);
+                            //extrude reduced
+                            gcode += m_writer.extrude_to_xy(
+                                this->point_to_gcode(inter_point2),
+                                e_per_mm * unscaled(length2 - length1) * (1 - mult / 2),
+                                comment);
+
+                            //extrude normal
+                            gcode += m_writer.extrude_to_xy(
+                                this->point_to_gcode(line.b),
+                                e_per_mm * unscaled(line.length() - length2),
+                                comment);
+                        } else {
+                            gcode += m_writer.extrude_to_xy(
+                                this->point_to_gcode(line.b),
+                                e_per_mm * unscaled(line.length() - length1) * (1 - mult / 2),
+                                comment);
+                        }
+                    } else {
+                        gcode += m_writer.extrude_to_xy(
+                            this->point_to_gcode(line.b),
+                            e_per_mm * unscaled(line.length() - length1) * (1 - mult * path.width / unscaled(line.length())),
+                            comment);
+                    }
+
+                    //relance
+                    last_pos = line.a;
+                }
+            }
+        }
     }
     gcode += this->_after_extrude(path);
 

src/libslic3r/Print.cpp

@@ -86,6 +86,7 @@ bool Print::invalidate_state_by_config_options(const std::vector<t_config_option
         "duplicate_distance",
         "end_gcode",
         "end_filament_gcode",
+        "external_perimeter_cut_corners",
         "extrusion_axis",
         "extruder_clearance_height",
         "extruder_clearance_radius",

src/libslic3r/PrintConfig.cpp

@@ -684,6 +684,16 @@ void PrintConfigDef::init_fff_params()
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionFloatOrPercent(0, false));
 
+    def = this->add("external_perimeter_cut_corners", coPercent);
+    def->label = L("Cutting corners");
+    def->full_label = L("Ext. peri. cut corners");
+    def->category = OptionCategory::width;
+    def->tooltip = L("Activate this option to modify the flow to acknoledge that the nozzle is round and the corners will have a round shape, and so change the flow to realized that and avoid over-extrusion."
+        " 100% is activated, 0% is deactivated and 50% is half-activated");
+    def->sidetext = L("%");
+    def->mode = comExpert;
+    def->set_default_value(new ConfigOptionPercent(0));
+
     def = this->add("external_perimeter_speed", coFloatOrPercent);
     def->label = L("External");
     def->full_label = L("External perimeters speed");

src/libslic3r/PrintConfig.hpp

@@ -466,6 +466,7 @@ public:
     ConfigOptionBool                clip_multipart_objects;
     ConfigOptionBool                dont_support_bridges;
     ConfigOptionFloat               elefant_foot_compensation;
+    ConfigOptionPercent             external_perimeter_cut_corners;
     ConfigOptionBool                exact_last_layer_height;
     ConfigOptionFloatOrPercent      extrusion_width;
     ConfigOptionFloatOrPercent      first_layer_height;
@@ -525,6 +526,7 @@ protected:
         OPT_PTR(clip_multipart_objects);
         OPT_PTR(dont_support_bridges);
         OPT_PTR(elefant_foot_compensation);
+        OPT_PTR(external_perimeter_cut_corners);
         OPT_PTR(exact_last_layer_height);
         OPT_PTR(extrusion_width);
         OPT_PTR(first_layer_height);

src/slic3r/GUI/Preset.cpp

@@ -520,7 +520,8 @@ const std::vector<std::string>& Preset::print_options()
         , "curve_smoothing_cutoff_dist"
         , "curve_smoothing_angle_convex"
         , "curve_smoothing_angle_concave",
-        "print_extrusion_multiplier"
+        "print_extrusion_multiplier",
+        "external_perimeter_cut_corners"
     };
     return s_opts;
 }