diff --git a/README.markdown b/README.markdown
index 8fe787b70..ee3d088ff 100644
--- a/README.markdown
+++ b/README.markdown
@@ -28,20 +28,26 @@ Also, http://xkcd.com/224/
 
 ## What's its current status?
 
-Slic3r can now successfully parse and analyze an STL file by slicing it in
-layers and representing internally the following features:
+Slic3r is able to:
 
-* holes in surfaces;
-* external top/bottom surfaces.
+* read binary and ASCII STL files;
+* generate multiple perimeters (skins);
+* generate rectilinear feed (100% solid for external surfaces or with customizable less density for inner surfaces);
+* use relative or absolute extrusion commands;
+* center print around bed center point;
+* output relevant GCODE.
 
-This kind of abstraction will allow to implement particular logic and allow the
-user to specify custom options.
+Roadmap include the following goals:
 
-It is also able to generate perimeters and to produce working GCODE.
-To reach a minimum level of usability, I need to implement an algorithm to generate 
-surface fill.
-
-Future goals include support material, options to control bridges, skirt, cool.
+* set up a command line interface and hide debug messages;
+* output some statistics;
+* allow the user to customize initial and final GCODE commands;
+* option for filling multiple solid layers near external surfaces;
+* support material for internal perimeters;
+* ability to infill in the direction of bridges;
+* skirt;
+* cool;
+* nice packaging for cross-platform deployment.
 
 ## Is it usable already?
 
diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index f72dd1eab..b0eaddb5c 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -4,6 +4,7 @@ use strict;
 use warnings;
 
 use Slic3r::ExtrusionPath;
+use Slic3r::Fill;
 use Slic3r::Layer;
 use Slic3r::Line;
 use Slic3r::Perimeter;
@@ -17,6 +18,7 @@ use Slic3r::Surface;
 our $layer_height       = 0.4;
 our $resolution         = 0.1;
 our $perimeter_offsets  = 3;
+our $fill_density       = 0.2;  # 1 = 100%
 our $flow_width         = 0.4;  # TODO: verify this is a multiple of $resolution
 our $temperature        = 195;
 
diff --git a/lib/Slic3r/Fill.pm b/lib/Slic3r/Fill.pm
new file mode 100644
index 000000000..7e01f0304
--- /dev/null
+++ b/lib/Slic3r/Fill.pm
@@ -0,0 +1,6 @@
+package Slic3r::Fill;
+use Moose;
+
+use Slic3r::Fill::Rectilinear;
+
+1;
diff --git a/lib/Slic3r/Fill/Rectilinear.pm b/lib/Slic3r/Fill/Rectilinear.pm
new file mode 100644
index 000000000..4e3be8072
--- /dev/null
+++ b/lib/Slic3r/Fill/Rectilinear.pm
@@ -0,0 +1,221 @@
+package Slic3r::Fill::Rectilinear;
+use Moose;
+
+use constant epsilon => 1E-10;
+use constant PI => 4 * atan2(1, 1);
+use constant X1 => 0;
+use constant Y1 => 1;
+use constant X2 => 2;
+use constant Y2 => 3;
+
+use Math::Geometry::Planar;
+use XXX;
+
+sub make_fill {
+    my $self = shift;
+    my ($print, $layer) = @_;
+    printf "Filling layer %d:\n", $layer->id;
+    
+    # let's alternate fill direction
+    my @axes = $layer->id % 2 == 0 ? (0,1) : (1,0);
+    printf "  primary axis: %d\n", $axes[0];
+    
+    foreach my $surface (@{ $layer->fill_surfaces }) {
+        printf " Processing surface %s:\n", $surface->id;
+        my $polygon = $surface->mgp_polygon;
+        
+        # rotate surface as needed
+        if ($axes[0] == 1) {
+            $polygon = $polygon->rotate(PI/2)->move($print->x_length, $print->y_length);
+        }
+        
+        # force 100% density for external surfaces
+        my $density = $surface->surface_type eq 'internal' ? $Slic3r::fill_density : 1;
+        my $distance_between_lines = $Slic3r::flow_width / $Slic3r::resolution / $density;
+        my $number_of_lines = ($axes[0] == 0 ? $print->x_length : $print->y_length) / $distance_between_lines;
+        
+        #printf "distance_between_lines = %f\n", $distance_between_lines;
+        #printf "number_of_lines = %d\n", $number_of_lines;
+        #printf "axes = %d, %d\n", @axes;
+        
+        # this arrayref will hold intersection points of the fill grid with surface segments
+        my $points = [ map [], 0..$number_of_lines-1 ];
+        foreach my $line (map $self->_lines_from_mgp_points($_), @{ $polygon->polygons }) {
+        
+            # for a possible implementation of "infill in direction of bridges"
+            # we should rotate $line so that primary axis is in detected direction;
+            # then, generated extrusion paths should be rotated back to the original
+            # coordinate system
+            
+            # find out the coordinates
+            my @coordinates = map @$_, @$line;
+            printf "Segment %d,%d - %d,%d\n", @coordinates;
+            
+            # get the extents of the segment along the primary axis
+            my @line_c = sort ($coordinates[X1], $coordinates[X2]);
+            
+            for (my $c = $line_c[0]; $c <= $line_c[1]; $c += $distance_between_lines) {
+                my $i = sprintf('%.0f', $c / $distance_between_lines) - 1;
+                
+                # if the segment is parallel to our ray, there will be two intersection points
+                if ($line_c[0] == $line_c[1]) {
+                    printf "  Segment is parallel!\n";
+                    push @{ $points->[$i] }, $coordinates[Y1], $coordinates[Y2];
+                    printf "   intersections at %f (%d) = %f, %f\n", $c, $i, $points->[$i][-2], $points->[$i][-1];
+                } else {
+                    printf "  Segment NOT parallel!\n";
+                    # one point of intersection
+                    push @{ $points->[$i] }, $coordinates[Y1] + ($coordinates[Y2] - $coordinates[Y1])
+                        * ($c - $coordinates[X1]) / ($coordinates[X2] - $coordinates[X1]);
+                    printf "   intersection at %f (%d) = %f\n", $c, $i, $points->[$i][-1];
+                }
+            }
+        }
+        
+        # sort and remove duplicates
+        $points = [
+            map {
+                my %h = map { sprintf("%.0f", $_) => 1 } @$_;
+                [ sort keys %h ];
+            } @$points
+        ];
+        
+        # generate extrusion paths
+        my (@paths, @path_points) = ();
+        my $direction = 0;
+        
+        my $stop_path = sub {
+            # defensive programming
+            if (@path_points == 1) {
+                YYY \@path_points;
+                die "There shouldn't be only one point in the current path";
+            }
+                
+            # if we were constructing a path, stop it
+            push @paths, [ @path_points ] if @path_points;
+            @path_points = ();
+        };
+        
+        # loop until we have spare points
+        while (map @$_, @$points) {
+        
+            # loop through rows
+            ROW: for (my $i = 0; $i < $number_of_lines; $i++) {
+                my $row = $points->[$i];
+                printf "Processing row %d...\n", $i;
+                if (!@$row) {
+                    printf "  no points\n";
+                    $stop_path->();
+                    next ROW;
+                }
+                printf "  points = %s\n", join ', ', @$row;
+                
+                # coordinate of current row
+                my $c = ($i + 1) * $distance_between_lines;
+                
+                # need to start a path?
+                if (!@path_points) {
+                    push @path_points, [ $c, shift @$row ];
+                }
+                
+                my @connectable_points = $self->find_connectable_points($polygon, $path_points[-1], $c, $row);
+                @connectable_points = reverse @connectable_points if $direction == 1;
+                printf "  found %d connectable points = %s\n", scalar(@connectable_points),
+                    join ', ', @connectable_points;
+                
+                if (!@connectable_points && @path_points && $path_points[-1][0] != $c) {
+                    # no connectable in this row
+                    $stop_path->();
+                }
+                
+                foreach my $p (@connectable_points) {
+                    push @path_points, [ $c, $p ];
+                    @$row = grep $_ != $p, @$row;  # remove point from row
+                }
+                
+                # invert direction
+                $direction = $direction ? 0 : 1;
+            }
+            $stop_path->() if @path_points;
+        }
+        
+        # paths must be rotated back
+        if ($axes[0] == 1) {
+            @paths = map $self->_mgp_from_points_ref($_)->move(-$print->x_length, -$print->y_length)->rotate(-PI()/2)->points, @paths;
+        }
+        
+        # save into layer
+        push @{ $layer->fills }, map Slic3r::ExtrusionPath->new_from_points(@$_), @paths;
+    }
+}
+
+# this function will select the first contiguous block of 
+# points connectable to a given one
+sub find_connectable_points {
+    my $self = shift;
+    my ($polygon, $point, $c, $points) = @_;
+    
+    my @connectable_points = ();
+    foreach my $p (@$points) {
+        push @connectable_points, $p 
+            if $self->can_connect($polygon, $point, [ $c, $p ]);
+    }
+    return @connectable_points;
+}
+
+# this subroutine tries to determine whether two points in a surface
+# are connectable without crossing contour or holes
+sub can_connect {
+    my $self = shift;
+    my ($polygon, $p1, $p2) = @_;
+    
+    # there's room for optimization here
+    
+    # this is not needed since we assume that $p1 and $p2 belong to $polygon
+    ###for ($p1, $p2) {
+        ###return 0 unless $polygon->isinside($_);
+    ###}
+    
+    # check whether the $p1-$p2 segment doesn't intersect any segment
+    # of the contour or of holes
+    foreach my $points (@{ $polygon->polygons }) {
+        foreach my $line ($self->_lines_from_mgp_points($points)) {
+            my $point = SegmentIntersection([$p1, $p2, @$line]);
+            if ($point && !$self->points_coincide($point, $p1) && !$self->points_coincide($point, $p2)) {
+                return 0;
+            }
+        }
+    }
+    
+    return 1;
+}
+
+sub points_coincide {
+    my $self = shift;
+    my ($p1, $p2) = @_;
+    return 0 if $p2->[0] - $p1->[0] < epsilon && $p2->[1] - $p1->[1] < epsilon;
+    return 1;
+}
+
+sub _lines_from_mgp_points {
+    my $self = shift;
+    my ($points) = @_;
+    
+    my @lines = ();
+    my $last_point = $points->[-1];
+    foreach my $point (@$points) {
+        push @lines, [ $last_point, $point ];
+        $last_point = $point;
+    }
+    return @lines;
+}
+
+sub _mgp_from_points_ref {
+    my $self = shift;
+    my ($points) = @_;
+    my $p = Math::Geometry::Planar->new;
+    $p->points($points);
+    return $p;
+}
+
+1;
diff --git a/lib/Slic3r/Layer.pm b/lib/Slic3r/Layer.pm
index 933c5e6b7..7f84105a3 100644
--- a/lib/Slic3r/Layer.pm
+++ b/lib/Slic3r/Layer.pm
@@ -55,6 +55,13 @@ has 'fill_surfaces' => (
     default => sub { [] },
 );
 
+# ordered collection of extrusion paths to fill surfaces
+has 'fills' => (
+    is      => 'rw',
+    isa     => 'ArrayRef[Slic3r::ExtrusionPath]',
+    default => sub { [] },
+);
+
 sub z {
     my $self = shift;
     return $self->id * $Slic3r::layer_height / $Slic3r::resolution;
diff --git a/lib/Slic3r/Line.pm b/lib/Slic3r/Line.pm
index 147e5635e..2cdead36b 100644
--- a/lib/Slic3r/Line.pm
+++ b/lib/Slic3r/Line.pm
@@ -45,6 +45,11 @@ sub id {
     return $self->a->id . "-" . $self->b->id;
 }
 
+sub coordinates {
+    my $self = shift;
+    return ($self->a->coordinates, $self->b->coordinates);
+}
+
 sub coincides_with {
     my $self = shift;
     my ($line) = @_;
@@ -55,7 +60,7 @@ sub coincides_with {
 
 sub has_endpoint {
     my $self = shift;
-    my ($point) = @_;#printf "    %s has endpoint %s: %s\n", $self->id, $point->id, ($point eq $self->a || $point eq $self->b);
+    my ($point) = @_;
     return $point->coincides_with($self->a) || $point->coincides_with($self->b);
 }
 
diff --git a/lib/Slic3r/Perimeter.pm b/lib/Slic3r/Perimeter.pm
index ce6a84099..6879864ad 100644
--- a/lib/Slic3r/Perimeter.pm
+++ b/lib/Slic3r/Perimeter.pm
@@ -44,7 +44,8 @@ sub make_perimeter {
         
         # create one more offset to be used as boundary for fill
         push @{ $layer->fill_surfaces }, 
-            map Slic3r::Surface->new_from_mgp($_), $self->offset_polygon($perimeters[-1]);
+            map Slic3r::Surface->new_from_mgp($_, surface_type => $surface->surface_type), 
+                $self->offset_polygon($perimeters[-1]);
     }
     
     # generate paths for holes
diff --git a/lib/Slic3r/Point.pm b/lib/Slic3r/Point.pm
index f0f8354c6..2f93a7471 100644
--- a/lib/Slic3r/Point.pm
+++ b/lib/Slic3r/Point.pm
@@ -31,6 +31,11 @@ sub id {
     return $self->x . "," . $self->y; #;;
 }
 
+sub coordinates {
+    my $self = shift;
+    return ($self->x, $self->y); #))
+}
+
 sub coincides_with {
     my $self = shift;
     my ($point) = @_;
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index 599d71a99..c67e6ad42 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -55,6 +55,19 @@ sub extrude_perimeters {
     }
 }
 
+sub extrude_fills {
+    my $self = shift;
+    
+    my $fill_extruder = Slic3r::Fill::Rectilinear->new;
+    
+    foreach my $layer (@{ $self->layers }) {
+        $fill_extruder->make_fill($self, $layer);
+        printf "  generated %d paths: %s\n",
+            scalar @{ $layer->fills },
+            join '  ', map $_->id, @{ $layer->fills };
+    }
+}
+
 sub export_gcode {
     my $self = shift;
     my ($file) = @_;
@@ -107,9 +120,32 @@ sub export_gcode {
         print  $fh "\n";
     };
     
+    my $z;
+    my $Extrude = sub {
+        my ($path, $description) = @_;
+        
+        # reset extrusion distance counter
+        my $extrusion_distance = 0;
+        if (!$Slic3r::use_relative_e_distances) {
+            print $fh "G92 E0 ; reset extrusion distance\n";
+        }
+        
+        # go to first point (without extruding)
+        $G1->($path->lines->[0]->a, $z, 0, "move to first $description point");
+        
+        # extrude while going to next points
+        foreach my $line (@{ $path->lines }) {
+            $extrusion_distance = 0 if $Slic3r::use_relative_e_distances;
+            $extrusion_distance += $line->a->distance_to($line->b);
+            $G1->($line->b, $z, $extrusion_distance, $description);
+        }
+        
+        # TODO: retraction
+    };
+    
     # write gcode commands layer by layer
     foreach my $layer (@{ $self->layers }) {
-        my $z = ($layer->z * $Slic3r::resolution);
+        $z = ($layer->z * $Slic3r::resolution);
         
         # go to layer
         # TODO: retraction
@@ -117,24 +153,10 @@ sub export_gcode {
             $z, $travel_feed_rate;
         
         # extrude perimeters
-        foreach my $perimeter (@{ $layer->perimeters }) {
-            
-            # reset extrusion distance counter
-            my $extrusion_distance = 0;
-            if (!$Slic3r::use_relative_e_distances) {
-                print $fh "G92 E0 ; reset extrusion distance\n";
-            }
-            
-            # go to first point (without extruding)
-            $G1->($perimeter->lines->[0]->a, $z, 0, 'move to first perimeter point');
-            
-            # extrude while going to next points
-            foreach my $line (@{ $perimeter->lines }) {
-                $extrusion_distance = 0 if $Slic3r::use_relative_e_distances;
-                $extrusion_distance += $line->a->distance_to($line->b);
-                $G1->($line->b, $z, $extrusion_distance, 'perimeter');
-            }
-        }
+        $Extrude->($_, 'perimeter') for @{ $layer->perimeters };
+        
+        # extrude fills
+        $Extrude->($_, 'fill') for @{ $layer->fills };
     }
     
     # write end commands to file
diff --git a/lib/Slic3r/Surface.pm b/lib/Slic3r/Surface.pm
index 5333a7eb6..60b81eb1d 100644
--- a/lib/Slic3r/Surface.pm
+++ b/lib/Slic3r/Surface.pm
@@ -20,6 +20,8 @@ has 'holes' => (
     },
 );
 
+# TODO: to allow for multiple solid skins to be filled near external
+# surfaces, a new type should be defined: internal-solid
 has 'surface_type' => (
     is      => 'rw',
     isa     => enum([qw(internal bottom top)]),
@@ -44,7 +46,7 @@ sub BUILD {
 
 sub new_from_mgp {
     my $self = shift;
-    my ($polygon) = @_;
+    my ($polygon, %params) = @_;
     
     my ($contour_p, @holes_p) = @{ $polygon->polygons };
     
@@ -53,6 +55,7 @@ sub new_from_mgp {
         holes   => [
             map Slic3r::Polyline::Closed->new_from_points(@$_), @holes_p
         ],
+        %params,
     );
 }
 
@@ -78,4 +81,9 @@ sub mgp_polygon {
     return $p;
 }
 
+sub lines {
+    my $self = shift;
+    return @{ $self->contour->lines }, map @{ $_->lines }, @{ $self->holes };
+}
+
 1;
diff --git a/slic3r.pl b/slic3r.pl
index 9f8ade592..a76d64c3a 100644
--- a/slic3r.pl
+++ b/slic3r.pl
@@ -15,6 +15,7 @@ my $stl_parser = Slic3r::STL->new;
 my $print = $stl_parser->parse_file("testcube20mm.stl");
 
 $print->extrude_perimeters;
+$print->extrude_fills;
 
 $print->export_gcode("testcube20mm.gcode");