From f682ab5cd3939ed65c5ff043dba780b8da29d828 Mon Sep 17 00:00:00 2001
From: Tommaso Bianchi <tommaso.b.bianchi@gmail.com>
Date: Wed, 10 Jun 2026 21:17:47 +0200
Subject: [PATCH] belt: replace height-check skip with a belt-correct
 vertical-clearance check

The original PR skipped the max-print-height check entirely on belt printers
because the sliced (virtual) Z is belt travel, not build height. As the reviewer
noted, that removed the only working height guard. Restore a correct guard:

- Print::validate: on belt printers, compare the upright object height
  (max over instances of the scene-space bbox) against printable_height directly.
  printable_height is the usable VERTICAL clearance above the belt: the gantry
  travels up the tilted plane (reach = height/cos(tilt)) and its axis range is
  sized for that (IR3 V2: ~354 mm gantry travel = 250 mm vertical at 45deg, and
  printable_height = 250). Hardware-confirmed 250 mm vertical clearance, so no
  cos(tilt) factor is applied.
- BuildVolume::set_belt_printer: drop the diagonal Z scaling; the build-volume Z
  already equals printable_height, keeping the live 'outside build volume'
  highlight in agreement with validate().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 src/libslic3r/BuildVolume.cpp | 11 +++++------
 src/libslic3r/Print.cpp       | 22 ++++++++++++++++++++++
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/src/libslic3r/BuildVolume.cpp b/src/libslic3r/BuildVolume.cpp
index 2e194bdd7e..c9e779f9e7 100644
--- a/src/libslic3r/BuildVolume.cpp
+++ b/src/libslic3r/BuildVolume.cpp
@@ -193,12 +193,11 @@ void BuildVolume::set_belt_printer(bool enabled, double angle_deg, bool infinite
             // Extend the Y bound to a very large value for infinite belt.
             m_bboxf.max.y() = 100000.;
         }
-        // Adjust max print height to diagonal reach: printable_height / sin(belt_angle).
-        if (angle_deg > 0. && angle_deg < 90.) {
-            double sin_a = std::sin(angle_deg * M_PI / 180.0);
-            if (sin_a > 0.)
-                m_bboxf.max.z() = m_max_print_height / sin_a;
-        }
+        // Belt printer: the Z extent already equals printable_height (set above), which
+        // is the usable vertical clearance above the belt. The gantry's axis range is
+        // sized to reach height/cos(tilt), so no diagonal scaling is applied here — this
+        // keeps the live "outside build volume" highlight in agreement with Print::validate().
+        (void) angle_deg;
     }
 }
 
diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index b511898ae8..257aa19f1d 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -1406,6 +1406,28 @@ StringObjectException Print::validate(StringObjectException *warning, Polygons*
     // Checks that the print does not exceed the max print height
     for (size_t print_object_idx = 0; print_object_idx < m_objects.size(); ++ print_object_idx) {
         const PrintObject &print_object = *m_objects[print_object_idx];
+        // Belt printer: the sliced (virtual) Z is belt travel along the conveyor, not
+        // build height, so the loop below (which measures the sliced layer stack) is
+        // meaningless here. The real ceiling is the usable vertical clearance above the
+        // belt, which printable_height holds directly: the gantry travels up the tilted
+        // plane, so a part of height z needs gantry reach z / cos(tilt), and the machine's
+        // gantry-axis range is sized to accommodate that (e.g. IdeaFormer IR3 V2: ~354 mm
+        // of gantry travel = 250 mm vertical at 45°, and printable_height = 250). So compare
+        // the upright object height against printable_height directly.
+        if (m_config.belt_printer.value) {
+            const double max_h = m_config.printable_height.value;
+            double obj_top = 0.0;
+            const ModelObject *mo = print_object.model_object();
+            for (const ModelInstance *mi : mo->instances)
+                obj_top = std::max(obj_top, mo->instance_bounding_box(*mi).max.z());
+            if (obj_top > max_h + EPSILON)
+                return StringObjectException{
+                    Slic3r::format(_u8L("The object %1% exceeds the maximum printable height "
+                        "above the belt (%2% mm)."),
+                        print_object.model_object()->name, max_h),
+                    print_object.model_object(), "" };
+            continue;
+        }
         //FIXME It is quite expensive to generate object layers just to get the print height!
         if (auto layers = generate_object_layers(print_object.slicing_parameters(), layer_height_profile(print_object_idx), print_object.config().precise_z_height.value);
             !layers.empty()) {