Don't put supports under bridges and potential bridges

README.md

@@ -257,6 +257,8 @@ The author of the Silk icon set is Mark James.
         --support-material-enforce-layers
                             Enforce support material on the specified number of layers from bottom,
                             regardless of --support-material and threshold (0+, default: 0)
+        --dont-support-bridges
+                            Experimental option for preventing support material from being generated under bridged areas (default: yes)
     
        Retraction options:
         --retract-length    Length of retraction in mm when pausing extrusion (default: 1)

lib/Slic3r/GUI/Tab.pm

@@ -493,7 +493,8 @@ sub build {
         {
             title => 'Options for support material and raft',
             options => [qw(support_material_pattern support_material_spacing support_material_angle
-                support_material_interface_layers support_material_interface_spacing)],
+                support_material_interface_layers support_material_interface_spacing
+                dont_support_bridges)],
         },
     ]);
     

lib/Slic3r/Layer/BridgeDetector.pm

@@ -3,7 +3,7 @@ use Moo;
 
 use List::Util qw(first sum max min);
 use Slic3r::Geometry qw(PI unscale scaled_epsilon rad2deg epsilon);
-use Slic3r::Geometry::Clipper qw(intersection_pl intersection_ex union);
+use Slic3r::Geometry::Clipper qw(intersection_pl intersection_ex union offset diff_pl union_ex);
 
 has 'expolygon'         => (is => 'ro', required => 1);
 has 'lower_slices'      => (is => 'rw', required => 1);  # ExPolygons or ExPolygonCollection
@@ -213,4 +213,41 @@ sub coverage {
     return $coverage;
 }
 
+# this method returns the bridge edges (as polylines) that are not supported
+# but would allow the entire bridge area to be bridged with detected angle
+# if supported too
+sub unsupported_edges {
+    my ($self, $angle) = @_;
+    
+    if (!defined $angle) {
+        return [] if !defined($angle = $self->angle);
+    }
+    
+    # get bridge edges (both contour and holes)
+    my @bridge_edges = map $_->split_at_first_point, @{$self->expolygon};
+    $_->[0]->translate(1,0) for @bridge_edges;  # workaround for Clipper bug, see comments in Slic3r::Polygon::clip_as_polyline()
+    
+    # get unsupported edges
+    my $grown_lower = offset([ map @$_, @{$self->lower_slices} ], +$self->extrusion_width);
+    my $unsupported = diff_pl(
+        \@bridge_edges,
+        $grown_lower,
+    );
+    
+    if (0) {
+        require "Slic3r/SVG.pm";
+        Slic3r::SVG::output(
+            "unsupported_" . rad2deg($angle) . ".svg",
+            expolygons          => [$self->expolygon],
+            green_expolygons    => $self->_anchors,
+            red_expolygons      => union_ex($grown_lower),
+            no_arrows           => 1,
+            polylines           => [ map $_->split_at_first_point, @{$self->expolygon} ],
+            red_polylines       => $unsupported,
+        );
+    }
+    
+    return $unsupported;
+}
+
 1;

lib/Slic3r/Layer/Region.pm

@@ -33,6 +33,9 @@ has 'fill_surfaces' => (is => 'rw', default => sub { Slic3r::Surface::Collection
 # collection of expolygons representing the bridged areas (thus not needing support material)
 has 'bridged' => (is => 'rw', default => sub { Slic3r::ExPolygon::Collection->new });
 
+# collection of polylines representing the unsupported bridge edges
+has 'unsupported_bridge_edges' => (is => 'rw', default => sub { Slic3r::Polyline::Collection->new });
+
 # ordered collection of extrusion paths/loops to build all perimeters
 has 'perimeters' => (is => 'rw', default => sub { Slic3r::ExtrusionPath::Collection->new });
 
@@ -418,6 +421,7 @@ sub process_external_surfaces {
             
             if (defined $angle && $self->object->config->support_material) {
                 $self->bridged->append(@{ $bridge_detector->coverage($angle) });
+                $self->unsupported_bridge_edges->append(@{ $bridge_detector->unsupported_edges }); 
             }
         }
         

lib/Slic3r/Print/SupportMaterial.pm

@@ -158,13 +158,34 @@ sub contact_area {
                     # Void $diff means that there's no upper perimeter whose centerline is
                     # outside the lower slice boundary, thus no overhang
                 }
-            
+                
-                # remove bridged areas
+                if ($self->object_config->dont_support_bridges) {
-                $diff = diff(
+                    if (1) {
-                    $diff,
+                        # remove the entire bridges and only support the unsupported edges
-                    [ map @$_, @{$layerm->bridged} ],
+                        my @bridges = map $_->expolygon,
-                    1,
+                            grep $_->bridge_angle != -1,
-                );
+                            @{$layerm->fill_surfaces->filter_by_type(S_TYPE_BOTTOMBRIDGE)};
+                    
+                        $diff = diff(
+                            $diff,
+                            [ map @$_, @bridges ],
+                            1,
+                        );
+                    
+                        push @$diff, @{intersection(
+                            [ map @{$_->grow(+scale MARGIN)}, @{$layerm->unsupported_bridge_edges} ],
+                            [ map @$_, @bridges ],
+                        )}
+                    
+                    } else {
+                        # just remove bridged areas
+                        $diff = diff(
+                            $diff,
+                            [ map @$_, @{$layerm->bridged} ],
+                            1,
+                        );
+                    }
+                }
                 
                 next if !@$diff;
                 push @overhang, @$diff;  # NOTE: this is not the full overhang as it misses the outermost half of the perimeter width!

slic3r.pl

@@ -381,6 +381,8 @@ $j
     --support-material-enforce-layers
                         Enforce support material on the specified number of layers from bottom,
                         regardless of --support-material and threshold (0+, default: $config->{support_material_enforce_layers})
+    --dont-support-bridges
+                        Experimental option for preventing support material from being generated under bridged areas (default: yes)
   
    Retraction options:
     --retract-length    Length of retraction in mm when pausing extrusion (default: $config->{retract_length}[0])

xs/src/PrintConfig.hpp

@@ -139,6 +139,11 @@ class PrintConfigDef
         Options["disable_fan_first_layers"].cli = "disable-fan-first-layers=i";
         Options["disable_fan_first_layers"].max = 1000;
 
+        Options["dont_support_bridges"].type = coBool;
+        Options["dont_support_bridges"].label = "Don't support bridges";
+        Options["dont_support_bridges"].tooltip = "Experimental option for preventing support material from being generated under bridged areas.";
+        Options["dont_support_bridges"].cli = "dont-support-bridges!";
+
         Options["duplicate_distance"].type = coFloat;
         Options["duplicate_distance"].label = "Distance between copies";
         Options["duplicate_distance"].tooltip = "Distance used for the auto-arrange feature of the plater.";
@@ -968,6 +973,7 @@ class StaticPrintConfig : public virtual StaticConfig
 class PrintObjectConfig : public virtual StaticPrintConfig
 {
     public:
+    ConfigOptionBool                dont_support_bridges;
     ConfigOptionFloatOrPercent      extrusion_width;
     ConfigOptionFloatOrPercent      first_layer_height;
     ConfigOptionBool                infill_only_where_needed;
@@ -988,6 +994,7 @@ class PrintObjectConfig : public virtual StaticPrintConfig
     ConfigOptionInt                 support_material_threshold;
     
     PrintObjectConfig() : StaticPrintConfig() {
+        this->dont_support_bridges.value                         = true;
         this->extrusion_width.value                              = 0;
         this->extrusion_width.percent                            = false;
         this->first_layer_height.value                           = 0.35;
@@ -1012,6 +1019,7 @@ class PrintObjectConfig : public virtual StaticPrintConfig
     };
     
     ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+        if (opt_key == "dont_support_bridges")                       return &this->dont_support_bridges;
         if (opt_key == "extrusion_width")                            return &this->extrusion_width;
         if (opt_key == "first_layer_height")                         return &this->first_layer_height;
         if (opt_key == "infill_only_where_needed")                   return &this->infill_only_where_needed;