#206 reverse direction on overhang overhaul:

 - now has a setting & a threshold
 - when reverse_direction trigger on the external loop, it also trigger for all other loops on this side (so, no more U-turn)
 - do not depends on extra_perimeter anymore
 - now extra_perimeters split into extra_perimeters (the old useless one) and extra_perimeters_overhangs
 - reordering settings in the tab.

6 files changed, 71 insertions, 16 deletions

diff --git a/src/libslic3r/Layer.cpp b/src/libslic3r/Layer.cpp
index 0528fdc85..0729f99ae 100644
--- a/src/libslic3r/Layer.cpp
+++ b/src/libslic3r/Layer.cpp
@@ -144,6 +144,7 @@ void Layer::make_perimeters()
                 && config.external_perimeters_nothole == other_config.external_perimeters_nothole
                 && config.external_perimeter_speed  == other_config.external_perimeter_speed
                 && config.extra_perimeters_odd_layers == other_config.extra_perimeters_odd_layers
+                && config.extra_perimeters_overhangs == other_config.extra_perimeters_overhangs
                 && config.gap_fill                  == other_config.gap_fill
                 && config.gap_fill_min_area         == other_config.gap_fill_min_area
                 && config.gap_fill_speed            == other_config.gap_fill_speed
@@ -153,6 +154,8 @@ void Layer::make_perimeters()
                 && config.only_one_perimeter_top    == other_config.only_one_perimeter_top
                 && config.overhangs                 == other_config.overhangs
                 && config.overhangs_width           == other_config.overhangs_width
+                && config.overhangs_reverse         == other_config.overhangs_reverse
+                && config.overhangs_reverse_threshold == other_config.overhangs_reverse_threshold
                 && config.perimeter_extrusion_width == other_config.perimeter_extrusion_width
                 && config.perimeter_loop            == other_config.perimeter_loop
                 && config.perimeter_loop_seam       == other_config.perimeter_loop_seam
diff --git a/src/libslic3r/PerimeterGenerator.cpp b/src/libslic3r/PerimeterGenerator.cpp
index 2ec55961e..66678d72c 100644
--- a/src/libslic3r/PerimeterGenerator.cpp
+++ b/src/libslic3r/PerimeterGenerator.cpp
@@ -295,8 +295,8 @@ void PerimeterGenerator::process()
 
             // Add perimeters on overhangs : initialization
             ExPolygons overhangs_unsupported;
-            if (this->config->extra_perimeters && !last.empty()
-                && this->lower_slices != NULL  && !this->lower_slices->empty()) {
+            if ( (this->config->extra_perimeters_overhangs || (this->config->overhangs_reverse && this->layer_id % 2 == 1))
+                && !last.empty()  && this->lower_slices != NULL  && !this->lower_slices->empty()) {
                 //remove holes from lower layer, we only ant that for overhangs, not bridges!
                 ExPolygons lower_without_holes;
                 for (const ExPolygon &exp : *this->lower_slices)
@@ -330,6 +330,18 @@ void PerimeterGenerator::process()
                     }
                 }
             }
+            bool has_steep_overhang = false;
+            if (this->layer_id % 2 == 1 && this->config->overhangs_reverse //check if my option is set and good layer
+                && !last.empty() && !overhangs_unsupported.empty() //has something to work with
+                ) {
+                coord_t offset = scale_(config->overhangs_reverse_threshold.get_abs_value(this->perimeter_flow.width));
+                //version with °: scale_(std::tan(PI * (0.5f / 90) * config->overhangs_reverse_threshold.value ) * this->layer_height)
+
+                if (offset_ex(overhangs_unsupported, -offset / 2).size() > 0) {
+                    //allow this loop to be printed in reverse
+                    has_steep_overhang = true;
+                }
+            }
 
             // In case no perimeters are to be generated, loop_number will equal to -1.            
             std::vector<PerimeterGeneratorLoops> contours(loop_number+1);    // depth => loops
@@ -341,10 +353,10 @@ void PerimeterGenerator::process()
                 // We can add more perimeters if there are uncovered overhangs
                 // improvement for future: find a way to add perimeters only where it's needed.
                 bool has_overhang = false;
-                if (this->config->extra_perimeters && !last.empty() && !overhangs_unsupported.empty()) {
+                if ( this->config->extra_perimeters_overhangs && !last.empty() && !overhangs_unsupported.empty()) {
                     overhangs_unsupported = intersection_ex(overhangs_unsupported, last, true);
                     if (overhangs_unsupported.size() > 0) {
-                        //add fake perimeters here
+                        //please don't stop adding periemter yet.
                         has_overhang = true;
                     }
                 }
@@ -512,11 +524,11 @@ void PerimeterGenerator::process()
 
                 for (const ExPolygon &expolygon : next_onion) {
                     //TODO: add width here to allow variable width (if we want to extrude a sightly bigger perimeter, see thin wall)
-                    contours[i].emplace_back(PerimeterGeneratorLoop(expolygon.contour, i, true, has_overhang));
+                    contours[i].emplace_back(expolygon.contour, i, true, has_steep_overhang);
                     if (! expolygon.holes.empty()) {
                         holes[i].reserve(holes[i].size() + expolygon.holes.size());
                         for (const Polygon &hole : expolygon.holes)
-                            holes[i].emplace_back(PerimeterGeneratorLoop(hole, i, false, has_overhang));
+                            holes[i].emplace_back(hole, i, false, has_steep_overhang);
                     }
                 }
                 last = std::move(next_onion);
@@ -889,7 +901,8 @@ ExtrusionEntityCollection PerimeterGenerator::_traverse_loops(
             ExtrusionLoop *eloop = static_cast<ExtrusionLoop*>(coll.entities[idx.first]);
             coll.entities[idx.first] = nullptr;
             if (loop.is_contour) {
-                if (loop.is_overhang && this->layer_id % 2 == 1)
+                //note: this->layer_id % 2 == 1 already taken into account in the is_steep_overhang compute (to save time).
+                if (loop.is_steep_overhang && this->layer_id % 2 == 1)
                     eloop->make_clockwise();
                 else
                     eloop->make_counter_clockwise();
diff --git a/src/libslic3r/PerimeterGenerator.hpp b/src/libslic3r/PerimeterGenerator.hpp
index a3e5a3c8e..070b03e50 100644
--- a/src/libslic3r/PerimeterGenerator.hpp
+++ b/src/libslic3r/PerimeterGenerator.hpp
@@ -29,7 +29,7 @@ public:
     // Contours are CCW oriented, holes are CW oriented.
     bool is_contour;
     //overhang may need to be reversed
-    bool is_overhang;
+    bool is_steep_overhang;
     // Depth in the hierarchy. External perimeter has depth = 0. An external perimeter could be both a contour and a hole.
     unsigned short depth;
     // Children contour, may be both CCW and CW oriented (outer contours or holes).
@@ -37,9 +37,9 @@ public:
 
 
     PerimeterGeneratorLoop(Polygon polygon, unsigned short depth, bool is_contour) :
-        polygon(polygon), is_contour(is_contour), depth(depth), is_overhang(false) {}
-    PerimeterGeneratorLoop(Polygon polygon, unsigned short depth, bool is_contour, bool is_overhang) :
-        polygon(polygon), is_contour(is_contour), depth(depth), is_overhang(is_overhang) {}
+        polygon(polygon), is_contour(is_contour), depth(depth), is_steep_overhang(false) {}
+    PerimeterGeneratorLoop(Polygon polygon, unsigned short depth, bool is_contour, bool is_steep_overhang) :
+        polygon(polygon), is_contour(is_contour), depth(depth), is_steep_overhang(is_steep_overhang) {}
     // External perimeter. It may be CCW or CW oriented (outer contour or hole contour).
     bool is_external() const { return this->depth == 0; }
     // An island, which may have holes, but it does not have another internal island.
diff --git a/src/libslic3r/PrintConfig.cpp b/src/libslic3r/PrintConfig.cpp
index 04c87fdc6..35d061475 100644
--- a/src/libslic3r/PrintConfig.cpp
+++ b/src/libslic3r/PrintConfig.cpp
@@ -761,12 +761,24 @@ void PrintConfigDef::init_fff_params()
 
     def = this->add("extra_perimeters", coBool);
     def->label = L("filling horizontal gaps on slopes");
-    def->full_label = L("Extra perimeters if needed");
+    def->full_label = L("Extra perimeters (do nothing)");
     def->category = OptionCategory::perimeter;
     def->tooltip = L("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, and keep adding periemter until all overhangs are filled."
-        "\n!! this is a very slow algorithm !!");
+        "is supported."
+        "\nIf you succeed to trigger the algorihtm behind this setting, please send me a message."
+        " Personally, i think it's useless.");
+    def->mode = comExpert;
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("extra_perimeters_overhangs", coBool);
+    def->label = L("on overhangs");
+    def->full_label = L("Extra perimeters in overhangs");
+    def->category = OptionCategory::perimeter;
+    def->tooltip = L("Add more perimeters when needed for avoiding gaps in sloping walls. "
+        "Slic3r keeps adding periemter until all overhangs are filled."
+        "\n!! this is a very slow algorithm !!"
+        "\nIf you use this setting, consider strongly using also overhangs_reverse.");
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionBool(false));
 
@@ -2030,13 +2042,31 @@ void PrintConfigDef::init_fff_params()
     def->set_default_value(new ConfigOptionBool(true));
 
     def = this->add("overhangs_width", coFloatOrPercent);
-    def->label = L("Threshold");
-    def->full_label = L("Overhang Threshold");
+    def->label = L("As bridge threshold");
+    def->full_label = L("Overhang bridge threshold");
     def->category = OptionCategory::perimeter;
     def->tooltip = L("Minimum unsupported width for an extrusion to be considered an overhang. Can be in mm or in a % of the nozzle diameter.");
     def->mode = comExpert;
     def->set_default_value(new ConfigOptionFloatOrPercent(50, true));
 
+    def = this->add("overhangs_reverse", coBool);
+    def->label = L("Reverse on odd");
+    def->full_label = L("Overhang reversal");
+    def->category = OptionCategory::perimeter;
+    def->tooltip = L("Extrude perimeters that have a part over an overhang in the reverse direction in odd layers. That alternating pattern can drastically improve steep overhang."
+        "\n!! this is a very slow algorithm (it uses the same results as extra_perimeters_overhangs) !!");
+    def->mode = comAdvanced;
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("overhangs_reverse_threshold", coFloatOrPercent);
+    def->label = L("Reverse threshold");
+    def->full_label = L("Overhang reversal threshold");
+    def->category = OptionCategory::perimeter;
+    def->tooltip = L("Number of mm the overhang need to be for the reversal to be considered useful. Can be a % of the periemter width.");
+    def->min = 0;
+    def->mode = comAdvanced;
+    def->set_default_value(new ConfigOptionFloatOrPercent(250, true));
+
     def = this->add("no_perimeter_unsupported_algo", coEnum);
     def->label = L("No perimeters on bridge areas");
     def->category = OptionCategory::perimeter;
diff --git a/src/libslic3r/PrintConfig.hpp b/src/libslic3r/PrintConfig.hpp
index c2a3951a4..49c13b9f1 100644
--- a/src/libslic3r/PrintConfig.hpp
+++ b/src/libslic3r/PrintConfig.hpp
@@ -597,6 +597,7 @@ public:
     ConfigOptionBool                external_perimeters_hole;
     ConfigOptionBool                extra_perimeters;
     ConfigOptionBool                extra_perimeters_odd_layers;
+    ConfigOptionBool                extra_perimeters_overhangs;
     ConfigOptionBool                only_one_perimeter_top;
     ConfigOptionFloat               fill_angle;
     ConfigOptionPercent             fill_density;
@@ -620,6 +621,8 @@ public:
     // Detect bridging perimeters
     ConfigOptionBool                overhangs;
     ConfigOptionFloatOrPercent      overhangs_width;
+    ConfigOptionBool                overhangs_reverse;
+    ConfigOptionFloatOrPercent      overhangs_reverse_threshold;
     ConfigOptionEnum<NoPerimeterUnsupportedAlgo> no_perimeter_unsupported_algo;
     ConfigOptionInt                 perimeter_extruder;
     ConfigOptionFloatOrPercent      perimeter_extrusion_width;
@@ -675,6 +678,7 @@ protected:
         OPT_PTR(external_perimeters_hole);
         OPT_PTR(extra_perimeters);
         OPT_PTR(extra_perimeters_odd_layers);
+        OPT_PTR(extra_perimeters_overhangs);
         OPT_PTR(only_one_perimeter_top);
         OPT_PTR(fill_angle);
         OPT_PTR(fill_density);
@@ -697,6 +701,8 @@ protected:
         OPT_PTR(infill_first);
         OPT_PTR(overhangs);
         OPT_PTR(overhangs_width);
+        OPT_PTR(overhangs_reverse);
+        OPT_PTR(overhangs_reverse_threshold);
 		OPT_PTR(no_perimeter_unsupported_algo);
         OPT_PTR(perimeter_extruder);
         OPT_PTR(perimeter_extrusion_width);
diff --git a/src/libslic3r/PrintObject.cpp b/src/libslic3r/PrintObject.cpp
index 16923e46b..3e4eb9844 100644
--- a/src/libslic3r/PrintObject.cpp
+++ b/src/libslic3r/PrintObject.cpp
@@ -598,6 +598,9 @@ bool PrintObject::invalidate_state_by_config_options(const std::vector<t_config_
             || opt_key == "gap_fill"
             || opt_key == "gap_fill_speed"
             || opt_key == "overhangs"
+            || opt_key == "overhangs_width"
+            || opt_key == "overhangs_reverse"
+            || opt_key == "overhangs_reverse_threshold"
             || opt_key == "first_layer_extrusion_width"
             || opt_key == "perimeter_extrusion_width"
             || opt_key == "infill_overlap"