From 4f3a6080090a5eb944d9ea5ca584b9500e872f93 Mon Sep 17 00:00:00 2001
From: Tommaso Bianchi <163908227+tommasobbianchi@users.noreply.github.com>
Date: Thu, 11 Jun 2026 06:54:02 +0200
Subject: [PATCH] belt: don't flag the lead-in as an empty-layer error on belt
 printers (#47)
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collect_layers_to_print() warns (CRITICAL) when an extrusion layer sits above
the previous one with an empty gap below — the fixed-bed assumption that
material with nothing under it is floating and unprintable. On a belt printer a
*leading* empty range (the gap starts at Z=0, no prior extrusion layer) is not
floating: it is the conveyor lead-in, and the part rests on the advancing belt
as the first material is laid down well above Z=0. A part not designed for a
belt (e.g. a flat test model tilted into the belt frame) then trips this as a
false "Object can't be printed for empty layer between 0 and N" error.

Suppress only the leading case (belt_printer && last_extrusion_layer == null);
genuine internal gaps are still flagged, since on a belt those can be an
over-angle overhang printing into air. Non-belt output is unchanged.
---
 src/libslic3r/GCode.cpp | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/libslic3r/GCode.cpp b/src/libslic3r/GCode.cpp
index c25a657953..3f21616bbb 100644
--- a/src/libslic3r/GCode.cpp
+++ b/src/libslic3r/GCode.cpp
@@ -1814,8 +1814,20 @@ std::vector<GCode::LayerToPrint> GCode::collect_layers_to_print(const PrintObjec
                 + std::max(0., extra_gap);
             // Negative support_contact_z is not taken into account, it can result in false positives in cases
 
-            if (has_extrusions && layer_to_print.print_z() > maximal_print_z + 2. * EPSILON)
-                warning_ranges.emplace_back(std::make_pair((last_extrusion_layer ? last_extrusion_layer->print_z() : 0.), layers_to_print.back().print_z()));
+            if (has_extrusions && layer_to_print.print_z() > maximal_print_z + 2. * EPSILON) {
+                // Belt printers: a *leading* empty range (no prior extrusion layer, so the
+                // gap starts at Z=0) is not a floating object — it is just the belt lead-in.
+                // The part rests on the conveyor as it advances, so the first material can
+                // legitimately appear well above Z=0. This empty-layer check assumes a fixed
+                // bed, where material with nothing below it is unprintable; that assumption
+                // does not hold on a belt for the lead-in. Suppress only this leading case,
+                // and keep flagging genuine *internal* gaps (which on a belt may still be an
+                // over-angle overhang that would print into air).
+                const bool belt_leading_gap = object.print()->config().belt_printer.value
+                                           && last_extrusion_layer == nullptr;
+                if (!belt_leading_gap)
+                    warning_ranges.emplace_back(std::make_pair((last_extrusion_layer ? last_extrusion_layer->print_z() : 0.), layers_to_print.back().print_z()));
+            }
         }
         // Remember last layer with extrusions.
         if (has_extrusions)