Bugfix: a move below z_offset was performed when retract_lift was enabled and lift amount was less than z_offset. Includes regression test. #2349

lib/Slic3r.pm

@@ -165,6 +165,7 @@ sub thread_cleanup {
     no warnings 'redefine';
     *Slic3r::Config::DESTROY                = sub {};
     *Slic3r::Config::Full::DESTROY          = sub {};
+    *Slic3r::Config::GCode::DESTROY         = sub {};
     *Slic3r::Config::Print::DESTROY         = sub {};
     *Slic3r::Config::PrintObject::DESTROY   = sub {};
     *Slic3r::Config::PrintRegion::DESTROY   = sub {};

lib/Slic3r/GCode.pm

@@ -71,6 +71,20 @@ sub init_external_mp {
     $self->_external_mp(Slic3r::MotionPlanner->new($islands));
 }
 
+sub preamble {
+    my ($self) = @_;
+    
+    my $gcode = $self->writer->preamble;
+    
+    # Perform a *silent* move to z_offset: we need this to initialize the Z
+    # position of our writer object so that any initial lift taking place
+    # before the first layer change will raise the extruder from the correct
+    # initial Z instead of 0.
+    $self->writer->travel_to_z($self->config->z_offset, '');
+    
+    return $gcode;
+}
+
 sub change_layer {
     my ($self, $layer) = @_;
     

lib/Slic3r/Print.pm

@@ -546,7 +546,7 @@ sub write_gcode {
     $print_first_layer_temperature->(1);
     
     # set other general things
-    print $fh $gcodegen->writer->preamble;
+    print $fh $gcodegen->preamble;
     
     # initialize a motion planner for object-to-object travel moves
     if ($self->config->avoid_crossing_perimeters) {

t/gcode.t

@@ -1,4 +1,4 @@
-use Test::More tests => 16;
+use Test::More tests => 19;
 use strict;
 use warnings;
 
@@ -41,6 +41,34 @@ use Slic3r::Test;
     ok !defined (first { abs($_ - $config->retract_speed->[0]*60) < 5 } @retract_speeds), 'wipe moves don\'t retract faster than configured speed';
 }
 
+{
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('z_offset', 5);
+    $config->set('start_gcode', '');
+    
+    my $test = sub {
+        my ($comment) = @_;
+        my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
+        my $moves_below_z_offset = 0;
+        Slic3r::GCode::Reader->new->parse(Slic3r::Test::gcode($print), sub {
+            my ($self, $cmd, $args, $info) = @_;
+            
+            if ($info->{travel} && exists $args->{Z}) {
+                $moves_below_z_offset++ if $args->{Z} < $config->z_offset;
+            }
+        });
+        is $moves_below_z_offset, 0, "no Z moves below Z offset ($comment)";
+    };
+    
+    $test->("no lift");
+    
+    $config->set('retract_lift', [3]);
+    $test->("lift < z_offset");
+    
+    $config->set('retract_lift', [6]);
+    $test->("lift > z_offset");
+}
+
 {
     # This tests the following behavior:
     # - complete objects does not crash