[FEATURE] Alternate extra perimeter

This ports the "Alternate extra wall" feature from [Cura](https://support.makerbot.com/s/article/1837650983225) to PrusaSlicer.

This setting adds an extra wall every other layer. This way the infill gets caught between the walls, resulting in:

* Theoretically, stronger prints due to better infill anchoring.
* Allows for the infill/wall overlap to be set to low values without compromising object strength while maintaining external surface quality.
* It may contribute to improving water tightness of models as the gaps between the top and bottom layers and the walls are overlapping.

src/libslic3r/PerimeterGenerator.cpp

@@ -1024,6 +1024,10 @@ void PerimeterGenerator::process_arachne(
     if (loop_number > 0 && ((params.config.top_one_perimeter_type == TopOnePerimeterType::TopmostOnly && upper_slices == nullptr) || (params.config.only_one_perimeter_first_layer && params.layer_id == 0)))
         loop_number = 0;
 
+    // Extra perimeter on odd layers
+    if (params.config.alternate_extra_perimeter && params.layer_id % 2 == 0)
+        loop_number++;
+
     // Calculate how many inner loops remain when TopSurfaces is selected.
     const int inner_loop_number = (params.config.top_one_perimeter_type == TopOnePerimeterType::TopSurfaces && upper_slices != nullptr) ? loop_number - 1 : -1;
 
@@ -1226,6 +1230,10 @@ void PerimeterGenerator::process_classic(
     // detect how many perimeters must be generated for this island
     int        loop_number = params.config.perimeters + surface.extra_perimeters - 1;  // 0-indexed loops
 
+    // Alternate extra perimeter on odd layers
+    if (params.config.alternate_extra_perimeter && params.layer_id % 2 == 0)
+        loop_number++;
+
     // Set the topmost layer to be one perimeter.
     if (loop_number > 0 && ((params.config.top_one_perimeter_type != TopOnePerimeterType::None && upper_slices == nullptr) || (params.config.only_one_perimeter_first_layer && params.layer_id == 0)))
         loop_number = 0;

src/libslic3r/Preset.cpp

@@ -561,6 +561,8 @@ static std::vector<std::string> s_Preset_print_options {
     "travel_jerk",
     "wipe_tower_accel_to_decel",
     "wipe_tower_jerk",
+
+    "alternate_extra_perimeter", 
 };
 
 static std::vector<std::string> s_Preset_filament_options {

src/libslic3r/PrintConfig.cpp

@@ -1153,6 +1153,16 @@ void PrintConfigDef::init_fff_params()
     def->mode = comExpert;
     def->set_default_value(new ConfigOptionBool(false));
 
+    def = this->add("alternate_extra_perimeter", coBool);
+    def->label = L("Extra perimeter on odd layers");
+    def->category = L("Layers and Perimeters");
+    def->tooltip = L("This setting adds an extra perimeter to every odd layer. This way the "
+                   "infill gets wedged vertically between the perimeters, resulting in stronger "
+                   "prints. Using lightning infill together with this option is not recommended "
+                   "as there is limited infill to anchor the extra perimeters to.");
+    def->set_default_value(new ConfigOptionBool(false));
+
+
     def = this->add("extruder", coInt);
     def->label = L("Extruder");
     def->category = L("Extruders");

src/libslic3r/PrintConfig.hpp

@@ -708,6 +708,7 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionBool,                 external_perimeters_first))
     ((ConfigOptionBool,                 extra_perimeters))
     ((ConfigOptionBool,                 extra_perimeters_on_overhangs))
+    ((ConfigOptionBool,                 alternate_extra_perimeter))
     ((ConfigOptionFloat,                fill_angle))
     ((ConfigOptionPercent,              fill_density))
     ((ConfigOptionEnum<InfillPattern>,  fill_pattern))

src/libslic3r/PrintObject.cpp

@@ -714,8 +714,7 @@ bool PrintObject::invalidate_state_by_config_options(
             || opt_key == "infill_overlap"
             || opt_key == "external_perimeters_first"
             || opt_key == "arc_fitting"
-            || opt_key == "top_one_perimeter_type"
-            || opt_key == "only_one_perimeter_first_layer") {
+            || opt_key == "alternate_extra_perimeter") {
             steps.emplace_back(posPerimeters);
         } else if (
                opt_key == "small_area_infill_flow_compensation"
@@ -1534,7 +1533,10 @@ void PrintObject::discover_vertical_shells()
 	                        ++ i) {
                             at_least_one_top_projected = true;
 	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
-                            if (region_config.ensure_vertical_shell_thickness.value != EnsureVerticalShellThickness::Partial) {
+                            bool combine_ensure_vertical_shell_thickness = region_config.ensure_vertical_shell_thickness.value != EnsureVerticalShellThickness::Partial;
+                            bool combine_alternate_extra_perimeter = !region_config.alternate_extra_perimeter.value || (region_config.alternate_extra_perimeter.value && int(idx_layer) % 2 == 0);
+
+                            if (combine_ensure_vertical_shell_thickness || combine_alternate_extra_perimeter) {
                                 combine_holes(cache.holes);
                             }
 
@@ -1566,7 +1568,10 @@ void PrintObject::discover_vertical_shells()
 	                        -- i) {
                                 at_least_one_bottom_projected = true;
 	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
-                            if (region_config.ensure_vertical_shell_thickness.value != EnsureVerticalShellThickness::Partial) {
+                            bool combine_ensure_vertical_shell_thickness = region_config.ensure_vertical_shell_thickness.value != EnsureVerticalShellThickness::Partial;
+                            bool combine_alternate_extra_perimeter = !region_config.alternate_extra_perimeter.value || (region_config.alternate_extra_perimeter.value && int(idx_layer) % 2 == 0);
+
+                            if (combine_ensure_vertical_shell_thickness || combine_alternate_extra_perimeter) {
                                 combine_holes(cache.holes);
                             }
 

src/slic3r/GUI/ConfigManipulation.cpp

@@ -136,6 +136,7 @@ void ConfigManipulation::update_print_fff_config(DynamicPrintConfig* config, con
 
     if (config->opt_bool("spiral_vase") &&
         ! (config->opt_int("perimeters") == 1 &&
+           ! config->opt_bool("alternate_extra_perimeter") &&
            config->opt_int("top_solid_layers") == 0 &&
            fill_density == 0 &&
            ! config->opt_bool("support_material") &&
@@ -144,6 +145,7 @@ void ConfigManipulation::update_print_fff_config(DynamicPrintConfig* config, con
     {
         wxString msg_text = _(L("The Spiral Vase mode requires:\n"
                                 "- one perimeter\n"
+                                "- no extra perimeter on odd layers\n"
                                 "- no top solid layers\n"
                                 "- 0% fill density\n"
                                 "- no support material\n"
@@ -157,6 +159,7 @@ void ConfigManipulation::update_print_fff_config(DynamicPrintConfig* config, con
         bool support = true;
         if (!is_global_config || answer == wxID_YES) {
             new_conf.set_key_value("perimeters", new ConfigOptionInt(1));
+            new_conf.set_key_value("alternate_extra_perimeter", new ConfigOptionBool(false));
             new_conf.set_key_value("top_solid_layers", new ConfigOptionInt(0));
             new_conf.set_key_value("fill_density", new ConfigOptionPercent(0));
             new_conf.set_key_value("support_material", new ConfigOptionBool(false));

src/slic3r/GUI/GUI_Factories.cpp

@@ -83,7 +83,7 @@ static bool is_improper_category(const std::string& category, const int extruder
 // pt_FFF
 static SettingsFactory::Bundle FREQ_SETTINGS_BUNDLE_FFF =
 {
-    { L("Layers and Perimeters"), { "layer_height" , "perimeters", "top_solid_layers", "bottom_solid_layers" } },
+    { L("Layers and Perimeters"), { "layer_height" , "perimeters", "alternate_extra_perimeter", "top_solid_layers", "bottom_solid_layers" } },
     { L("Infill")               , { "fill_density", "fill_pattern" } },
     { L("Support material")     , { "support_material", "support_material_auto", "support_material_threshold",
                                     "support_material_pattern", "support_material_interface_pattern", "support_material_buildplate_only",

src/slic3r/GUI/Plater.cpp

@@ -633,6 +633,7 @@ Plater::priv::priv(Plater* q, MainFrame* main_frame)
 
         // BOSS
         "init_z_rotate",
+        "alternate_extra_perimeter",
         }))
     , sidebar(new Sidebar(q))
     , notification_manager(std::make_unique<NotificationManager>(q))

src/slic3r/GUI/Tab.cpp

@@ -1450,6 +1450,7 @@ void TabPrint::build()
         optgroup->append_single_option_line("extra_perimeters", category_path + "extra-perimeters-if-needed");
         optgroup->append_single_option_line("extra_perimeters_on_overhangs", category_path + "extra-perimeters-on-overhangs");
         optgroup->append_single_option_line("ensure_vertical_shell_thickness", category_path + "ensure-vertical-shell-thickness");
+        optgroup->append_single_option_line("alternate_extra_perimeter", category_path + "alternate-extra-perimeter");
         optgroup->append_single_option_line("avoid_crossing_curled_overhangs", category_path + "avoid-crossing-curled-overhangs");
         optgroup->append_single_option_line("avoid_crossing_perimeters", category_path + "avoid-crossing-perimeters");
         optgroup->append_single_option_line("avoid_crossing_perimeters_max_detour", category_path + "avoid_crossing_perimeters_max_detour");