New option to only infill where strictly needed for supporting ceilings

README.markdown

@@ -197,6 +197,8 @@ The author of the Silk icon set is Mark James.
         --solid-infill-below-area
                             Force solid infill when a region has a smaller area than this threshold
                             (mm^2, default: 70)
+        --infill-only-where-needed
+                            Only infill under ceilings (default: no)
       
        Support material options:
         --support-material  Generate support material for overhangs

lib/Slic3r/Config.pm

@@ -380,6 +380,13 @@ our $Options = {
         min     => 0,
         default => 0,
     },
+    'infill_only_where_needed' => {
+        label   => 'Only infill where needed',
+        tooltip => 'This option will limit infill to the areas actually needed for supporting ceilings (it will act as internal support material).',
+        cli     => 'infill-only-where-needed!',
+        type    => 'bool',
+        default => 0,
+    },
     
     # flow options
     'extrusion_width' => {

lib/Slic3r/GUI/Tab.pm

@@ -417,7 +417,8 @@ sub build {
         },
         {
             title => 'Advanced',
-            options => [qw(infill_every_layers solid_infill_every_layers fill_angle solid_infill_below_area only_retract_when_crossing_perimeters)],
+            options => [qw(infill_every_layers infill_only_where_needed solid_infill_every_layers fill_angle
+                solid_infill_below_area only_retract_when_crossing_perimeters)],
         },
     ]);
     

lib/Slic3r/Layer/Region.pm

@@ -2,7 +2,7 @@ package Slic3r::Layer::Region;
 use Moo;
 
 use Slic3r::ExtrusionPath ':roles';
-use Slic3r::Geometry qw(scale chained_path_items);
+use Slic3r::Geometry qw(PI scale chained_path_items);
 use Slic3r::Geometry::Clipper qw(safety_offset union_ex diff_ex intersection_ex);
 use Slic3r::Surface ':types';
 
@@ -16,6 +16,7 @@ has 'layer' => (
 has 'region'            => (is => 'ro', required => 1);
 has 'perimeter_flow'    => (is => 'rw');
 has 'infill_flow'       => (is => 'rw');
+has 'overhang_width'    => (is => 'lazy');
 
 # collection of spare segments generated by slicing the original geometry;
 # these need to be merged in continuos (closed) polylines
@@ -64,6 +65,12 @@ sub _update_flows {
         : $self->region->flows->{infill});
 }
 
+sub _build_overhang_width {
+    my $self = shift;
+    my $threshold_rad = PI/2 - atan2($self->perimeter_flow->width / $self->height / 2, 1);
+    return scale($self->height * ((cos $threshold_rad) / (sin $threshold_rad)));
+}
+
 # build polylines from lines
 sub make_surfaces {
     my $self = shift;

lib/Slic3r/Print.pm

@@ -367,6 +367,7 @@ sub export_gcode {
     # they will be split in internal and internal-solid surfaces
     $status_cb->(60, "Generating horizontal shells");
     $_->discover_horizontal_shells for @{$self->objects};
+    $_->clip_fill_surfaces for @{$self->objects};
     
     # combine fill surfaces to honor the "infill every N layers" option
     $status_cb->(70, "Combining infill");

lib/Slic3r/Print/Object.pm

@@ -375,6 +375,53 @@ sub detect_surfaces_type {
     }
 }
 
+sub clip_fill_surfaces {
+    my $self = shift;
+    return unless $Slic3r::Config->infill_only_where_needed;
+    
+    # We only want infill under ceilings; this is almost like an
+    # internal support material.
+    
+    my $additional_margin = scale 3;
+    
+    my @overhangs = ();
+    for my $layer_id (reverse 0..$#{$self->layers}) {
+        my $layer = $self->layers->[$layer_id];
+        
+        # clip this layer's internal surfaces to @overhangs
+        foreach my $layerm (@{$layer->regions}) {
+            my @new_internal = map Slic3r::Surface->new(
+                    expolygon       => $_,
+                    surface_type    => S_TYPE_INTERNAL,
+                ),
+                @{intersection_ex(
+                    [ map @$_, @overhangs ],
+                    [ map @{$_->expolygon}, grep $_->surface_type == S_TYPE_INTERNAL, @{$layerm->fill_surfaces} ],
+                )};
+            @{$layerm->fill_surfaces} = (
+                @new_internal,
+                (grep $_->surface_type != S_TYPE_INTERNAL, @{$layerm->fill_surfaces}),
+            );
+        }
+        
+        # get this layer's overhangs
+        if ($layer_id > 0) {
+            my $lower_layer = $self->layers->[$layer_id-1];
+            # loop through layer regions so that we can use each region's
+            # specific overhang width
+            foreach my $layerm (@{$layer->regions}) {
+                my $overhang_width = $layerm->overhang_width;
+                # we want to support any solid surface, not just tops
+                # (internal solids might have been generated)
+                push @overhangs, map $_->offset_ex($additional_margin), @{intersection_ex(
+                    [ map @{$_->expolygon}, grep $_->surface_type != S_TYPE_INTERNAL, @{$layerm->fill_surfaces} ],
+                    [ map @$_, map $_->offset_ex(-$overhang_width), @{$lower_layer->slices} ],
+                )};
+            }
+        }
+    }
+}
+
 sub discover_horizontal_shells {
     my $self = shift;
     
@@ -580,13 +627,15 @@ sub generate_support_material {
     my $self = shift;
     return if $self->layer_count < 2;
     
-    my $threshold_rad = $Slic3r::Config->support_material_threshold
-                        ? deg2rad($Slic3r::Config->support_material_threshold + 1)  # +1 makes the threshold inclusive
-                        : PI/2 - atan2($self->layers->[1]->regions->[0]->perimeter_flow->width/$Slic3r::Config->layer_height/2, 1);
-    Slic3r::debugf "Threshold angle = %d°\n", rad2deg($threshold_rad);
-    
+    my $overhang_width;
+    if ($Slic3r::Config->support_material_threshold) {
+        my $threshold_rad = deg2rad($Slic3r::Config->support_material_threshold + 1);  # +1 makes the threshold inclusive
+        Slic3r::debugf "Threshold angle = %d°\n", rad2deg($threshold_rad);
+        $overhang_width = scale $Slic3r::Config->layer_height * ((cos $threshold_rad) / (sin $threshold_rad));
+    } else {
+        $overhang_width = $self->layers->[1]->regions->[0]->overhang_width;
+    }
     my $flow                    = $self->print->support_material_flow;
-    my $overhang_width          = $threshold_rad == 0 ? undef : scale $Slic3r::Config->layer_height * ((cos $threshold_rad) / (sin $threshold_rad));
     my $distance_from_object    = 1.5 * $flow->scaled_width;
     my $pattern_spacing = ($Slic3r::Config->support_material_spacing > $flow->spacing)
         ? $Slic3r::Config->support_material_spacing

slic3r.pl

@@ -245,6 +245,8 @@ $j
     --solid-infill-below-area
                         Force solid infill when a region has a smaller area than this threshold
                         (mm^2, default: $config->{solid_infill_below_area})
+    --infill-only-where-needed
+                        Only infill under ceilings (default: no)
   
    Support material options:
     --support-material  Generate support material for overhangs