Implementing a new switch for the shape of support towers:

expanded to a grid (the old way) vs.
snug (like the upstream Slic3r, Cura or Ideamaker).

Snug supports suffered from the degeneracies when merging overhang islands
over a large number of layers when projecting the support towers down.
We borrowed the idea & a bit of code from Cura by simplifying the support
polygons by closing the concave cracks, see the smooth_outward() function
and the MutablePolygon class.

Fixes Support problems with models with hole in the walls. #555
Fixes Support in the Air #740
Fixes [Bug] Supports generated beyond bed edges (X<0 and X>250) and where none are needed. #902
Fixes Unable to remove support material/can't change support "inflation distance" #2708
Fixes FR: support inflation and support conform to boundary #4783
Fixes Support blocker not working on this model #1346
Fixes Unnecessary support material #1993
Fixes support blocker enforcer issue #6240

src/libslic3r/Preset.cpp

@@ -427,7 +427,7 @@ const std::vector<std::string>& Preset::print_options()
         "bridge_acceleration", "first_layer_acceleration", "default_acceleration", "skirts", "skirt_distance", "skirt_height", "draft_shield",
         "min_skirt_length", "brim_width", "brim_offset", "brim_type", "support_material", "support_material_auto", "support_material_threshold", "support_material_enforce_layers",
         "raft_layers", "raft_first_layer_density", "raft_first_layer_expansion", "raft_contact_distance", "raft_expansion",
-        "support_material_pattern", "support_material_with_sheath", "support_material_spacing",
+        "support_material_pattern", "support_material_with_sheath", "support_material_spacing", "support_material_style",
         "support_material_synchronize_layers", "support_material_angle", "support_material_interface_layers", "support_material_bottom_interface_layers",
         "support_material_interface_pattern", "support_material_interface_spacing", "support_material_interface_contact_loops", 
         "support_material_contact_distance", "support_material_bottom_contact_distance",

src/libslic3r/PrintConfig.cpp

@@ -2419,6 +2419,20 @@ void PrintConfigDef::init_fff_params()
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionFloat(60));
 
+    def = this->add("support_material_style", coEnum);
+    def->label = L("Style");
+    def->category = L("Support material");
+    def->tooltip = L("Style and shape of the support towers. Projecting the supports into a regular grid "
+                     "will create more stable supports, while snug support towers will save material and reduce "
+                     "object scarring.");
+    def->enum_keys_map = &ConfigOptionEnum<SupportMaterialStyle>::get_enum_values();
+    def->enum_values.push_back("grid");
+    def->enum_values.push_back("snug");
+    def->enum_labels.push_back(L("Grid"));
+    def->enum_labels.push_back(L("Snug"));
+    def->mode = comAdvanced;
+    def->set_default_value(new ConfigOptionEnum<SupportMaterialStyle>(smsGrid));
+
     def = this->add("support_material_synchronize_layers", coBool);
     def->label = L("Synchronize with object layers");
     def->category = L("Support material");

src/libslic3r/PrintConfig.hpp

@@ -65,6 +65,10 @@ enum SupportMaterialPattern {
     smpRectilinear, smpRectilinearGrid, smpHoneycomb,
 };
 
+enum SupportMaterialStyle {
+    smsGrid, smsSnug,
+};
+
 enum SupportMaterialInterfacePattern {
     smipAuto, smipRectilinear, smipConcentric,
 };
@@ -211,6 +215,15 @@ template<> inline const t_config_enum_values& ConfigOptionEnum<SupportMaterialPa
     return keys_map;
 }
 
+template<> inline const t_config_enum_values& ConfigOptionEnum<SupportMaterialStyle>::get_enum_values() {
+    static t_config_enum_values keys_map;
+    if (keys_map.empty()) {
+        keys_map["grid"]    = smsGrid;
+        keys_map["snug"]    = smsSnug;
+    }
+    return keys_map;
+}
+
 template<> inline const t_config_enum_values& ConfigOptionEnum<SupportMaterialInterfacePattern>::get_enum_values() {
     static t_config_enum_values keys_map;
     if (keys_map.empty()) {
@@ -519,6 +532,7 @@ class PrintObjectConfig : public StaticPrintConfig
     // Spacing between support material lines (the hatching distance).
     ConfigOptionFloat               support_material_spacing;
     ConfigOptionFloat               support_material_speed;
+    ConfigOptionEnum<SupportMaterialStyle> support_material_style;
     ConfigOptionBool                support_material_synchronize_layers;
     // Overhang angle threshold.
     ConfigOptionInt                 support_material_threshold;
@@ -570,6 +584,7 @@ class PrintObjectConfig : public StaticPrintConfig
         OPT_PTR(support_material_interface_pattern);
         OPT_PTR(support_material_spacing);
         OPT_PTR(support_material_speed);
+        OPT_PTR(support_material_style);
         OPT_PTR(support_material_synchronize_layers);
         OPT_PTR(support_material_xy_spacing);
         OPT_PTR(support_material_threshold);

src/libslic3r/PrintObject.cpp

@@ -588,6 +588,7 @@ bool PrintObject::invalidate_state_by_config_options(
             || opt_key == "support_material_interface_extruder"
             || opt_key == "support_material_interface_spacing"
             || opt_key == "support_material_pattern"
+            || opt_key == "support_material_style"
             || opt_key == "support_material_xy_spacing"
             || opt_key == "support_material_spacing"
             || opt_key == "support_material_synchronize_layers"

src/slic3r/GUI/ConfigManipulation.cpp

@@ -278,7 +278,7 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig* config)
     bool have_support_material_auto = have_support_material && config->opt_bool("support_material_auto");
     bool have_support_interface = config->opt_int("support_material_interface_layers") > 0;
     bool have_support_soluble = have_support_material && config->opt_float("support_material_contact_distance") == 0;
-    for (auto el : { "support_material_pattern", "support_material_with_sheath",
+    for (auto el : { "support_material_style", "support_material_pattern", "support_material_with_sheath",
                     "support_material_spacing", "support_material_angle", 
                     "support_material_interface_pattern", "support_material_interface_layers",
                     "dont_support_bridges", "support_material_extrusion_width", "support_material_contact_distance",

src/slic3r/GUI/Field.cpp

@@ -1224,6 +1224,8 @@ boost::any& Choice::get_value()
 			m_value = static_cast<SupportMaterialPattern>(ret_enum);
         else if (m_opt_id.compare("support_material_interface_pattern") == 0)
             m_value = static_cast<SupportMaterialInterfacePattern>(ret_enum);
+        else if (m_opt_id.compare("support_material_style") == 0)
+            m_value = static_cast<SupportMaterialStyle>(ret_enum);
 		else if (m_opt_id.compare("seam_position") == 0)
 			m_value = static_cast<SeamPosition>(ret_enum);
 		else if (m_opt_id.compare("host_type") == 0)

src/slic3r/GUI/GUI.cpp

@@ -198,6 +198,8 @@ void change_opt_value(DynamicPrintConfig& config, const t_config_option_key& opt
 				config.set_key_value(opt_key, new ConfigOptionEnum<SupportMaterialPattern>(boost::any_cast<SupportMaterialPattern>(value)));
 			else if (opt_key.compare("support_material_interface_pattern") == 0)
 				config.set_key_value(opt_key, new ConfigOptionEnum<SupportMaterialInterfacePattern>(boost::any_cast<SupportMaterialInterfacePattern>(value)));
+			else if (opt_key.compare("support_material_style") == 0)
+				config.set_key_value(opt_key, new ConfigOptionEnum<SupportMaterialStyle>(boost::any_cast<SupportMaterialStyle>(value)));
 			else if (opt_key.compare("seam_position") == 0)
 				config.set_key_value(opt_key, new ConfigOptionEnum<SeamPosition>(boost::any_cast<SeamPosition>(value)));
 			else if (opt_key.compare("host_type") == 0)

src/slic3r/GUI/GUI_Factories.cpp

@@ -50,7 +50,7 @@ static SettingsFactory::Bundle FREQ_SETTINGS_BUNDLE_FFF =
     { L("Layers and Perimeters"), { "layer_height" , "perimeters", "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",
+                                    "support_material_pattern", "support_material_pattern", "support_material_interface_pattern", "support_material_buildplate_only",
                                     "support_material_spacing" } },
     { L("Wipe options")         , { "wipe_into_infill", "wipe_into_objects" } }
 };

src/slic3r/GUI/Tab.cpp

@@ -1506,6 +1506,7 @@ void TabPrint::build()
         optgroup->append_single_option_line("raft_expansion");
 
         optgroup = page->new_optgroup(L("Options for support material and raft"));
+        optgroup->append_single_option_line("support_material_style", category_path + "style");
         optgroup->append_single_option_line("support_material_contact_distance", category_path + "contact-z-distance");
         optgroup->append_single_option_line("support_material_bottom_contact_distance", category_path + "contact-z-distance");
         optgroup->append_single_option_line("support_material_pattern", category_path + "pattern");