Automatic detection of support material threshold angle

README.markdown

@@ -189,7 +189,8 @@ The author of the Silk icon set is Mark James.
        Support material options:
         --support-material  Generate support material for overhangs
         --support-material-threshold
-                            Overhang threshold angle (range: 0-90, default: 45)
+                            Overhang threshold angle (range: 0-90, set 0 for automatic detection,
+                            default: 0)
         --support-material-pattern
                             Pattern to use for support material (default: rectilinear)
         --support-material-spacing

lib/Slic3r/Config.pm

@@ -536,11 +536,11 @@ our $Options = {
     },
     'support_material_threshold' => {
         label   => 'Overhang threshold',
-        tooltip => 'Support material will not generated for overhangs whose slope angle is above the given threshold.',
+        tooltip => 'Support material will not generated for overhangs whose slope angle is above the given threshold. Set to zero for automatic detection.',
         sidetext => '°',
         cli     => 'support-material-threshold=i',
         type    => 'i',
-        default => 45,
+        default => 0,
     },
     'support_material_pattern' => {
         label   => 'Pattern',

lib/Slic3r/Print/Object.pm

@@ -2,7 +2,7 @@ package Slic3r::Print::Object;
 use Moo;
 
 use Slic3r::ExtrusionPath ':roles';
-use Slic3r::Geometry qw(Z scale unscale deg2rad scaled_epsilon);
+use Slic3r::Geometry qw(Z PI scale unscale deg2rad rad2deg scaled_epsilon);
 use Slic3r::Geometry::Clipper qw(diff_ex intersection_ex union_ex);
 use Slic3r::Surface ':types';
 
@@ -531,8 +531,12 @@ sub combine_infill {
 sub generate_support_material {
     my $self = shift;
     
+    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 $flow                    = $self->print->support_material_flow;
-    my $threshold_rad           = deg2rad($Slic3r::Config->support_material_threshold + 1);   # +1 makes the threshold inclusive
     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.pl

@@ -237,7 +237,8 @@ $j
    Support material options:
     --support-material  Generate support material for overhangs
     --support-material-threshold
-                        Overhang threshold angle (range: 0-90, default: $config->{support_material_threshold})
+                        Overhang threshold angle (range: 0-90, set 0 for automatic detection,
+                        default: $config->{support_material_threshold})
     --support-material-pattern
                         Pattern to use for support material (default: $config->{support_material_pattern})
     --support-material-spacing