cancel top of purge tower early if no extra parts to print

src/libslic3r/Print.cpp

@@ -4345,6 +4345,27 @@ void Print::_plan_belt_purge()
     const bool  use_flush_matrix = m_config.purge_in_prime_tower && m_config.single_extruder_multi_material;
     const float flush_multiplier = (float) m_config.flush_multiplier.get_at(0);
 
+    // Cancel the purge prism early: pre-scan the tool ordering for the highest
+    // print_z that actually has a toolchange, then drop the prism's layers above
+    // it so the tower stops at the last color swap (saves filament/time). This
+    // MUST happen before the marking loop below: ensure_perimeters_infills_order
+    // force-overrides the prism's extrusions on every layer (it is a dedicated
+    // flush object), so truncating afterwards would leave dangling overrides
+    // pointing into deleted layers.
+    {
+        double       last_tc_z   = -1.;
+        unsigned int cur_ext     = m_wipe_tower_data.tool_ordering.first_extruder();
+        for (const auto &lt : m_wipe_tower_data.tool_ordering.layer_tools())
+            for (const unsigned int e : lt.extruders)
+                if (e != cur_ext) { last_tc_z = lt.print_z; cur_ext = e; }
+        if (last_tc_z >= 0.)
+            for (PrintObject *po : m_objects)
+                if (po->config().belt_purge_tower_object.value) {
+                    po->belt_truncate_layers_above(last_tc_z);
+                    break;
+                }
+    }
+
     // Diagnostic: the prism only absorbs purge at toolchange layers whose
     // print_z coincides with one of its own layers. Compare the prism's layer
     // print_z range to the toolchange print_z range and count how many

src/libslic3r/Print.hpp

@@ -562,6 +562,9 @@ private:
     // clipping shift) by delta, used by Print::_align_belt_purge_layers() to
     // snap the purge prism's layers onto the printed objects' layer grid.
     void belt_shift_layer_grid(double delta);
+    // Belt mode: remove layers above z (cancel the purge prism past the last
+    // toolchange). Returns how many layers were dropped.
+    size_t belt_truncate_layers_above(coordf_t z);
     //BBS
     ExPolygons _shrink_contour_holes(double contour_delta, double hole_delta, const ExPolygons& polys) const;
     // BBS

src/libslic3r/PrintObjectSlice.cpp

@@ -1413,6 +1413,31 @@ void PrintObject::belt_shift_layer_grid(double delta)
         << " first_layer.print_z=" << (m_layers.empty() ? 0. : m_layers.front()->print_z);
 }
 
+// Belt mode: drop layers strictly above z (used to cancel the purge prism early
+// once there are no more toolchanges above z, so the tower stops at the last
+// color swap instead of wasting filament up the rest of the belt). Each layer's
+// cross-section is already sliced, so removing upper layers does not affect the
+// last toolchange's coverage. Deletes the Layer objects and clears the new top
+// layer's upper-layer link. Returns the number of layers removed.
+size_t PrintObject::belt_truncate_layers_above(coordf_t z)
+{
+    size_t keep = m_layers.size();
+    while (keep > 0 && m_layers[keep - 1]->print_z > z + EPSILON)
+        --keep;
+    if (keep >= m_layers.size())
+        return 0;
+    const size_t removed = m_layers.size() - keep;
+    for (size_t i = keep; i < m_layers.size(); ++i)
+        delete m_layers[i];
+    m_layers.resize(keep);
+    if (!m_layers.empty())
+        m_layers.back()->upper_layer = nullptr;
+    BOOST_LOG_TRIVIAL(debug) << "[BELT-DEBUG] truncate purge prism above print_z=" << z
+        << " kept=" << keep << " removed=" << removed
+        << " new_top=" << (m_layers.empty() ? 0. : m_layers.back()->print_z);
+    return removed;
+}
+
 // 1) Decides Z positions of the layers,
 // 2) Initializes layers and their regions
 // 3) Slices the object meshes