Fix support preview artifacts (missing layers) (#13310)

Fix support preview artifacts caused by incorrect gap subdivision

Support generation could sometimes split a gap into too many steps,
even when it should fit exactly into a single layer.

This was most noticeable when max_suport_layer_height was equal to
the print layer height (e.g. 0.2 mm).

This resulted in incorrect support layering and visible artifacts
in preview.

The issue was caused by floating-point precision, where values that
should be equal to the configured limit were treated as slightly larger.

Fix by biasing the subdivision calculation with EPSILON so near-equal
values are not split into extra steps.

Applied consistently to:
- SupportMaterial (normal supports)
- TreeSupportCommon (tree stepping)
- TreeSupport (layer creation)

src/libslic3r/Support/SupportMaterial.cpp

@@ -2882,8 +2882,9 @@ SupportGeneratorLayersPtr PrintObjectSupportMaterial::raft_and_intermediate_supp
                 intermediate_layers.push_back(&layer_new);
             }
         } else {
-            // Insert intermediate layers.
-            size_t        n_layers_extra = size_t(ceil(dist / m_slicing_params.max_suport_layer_height)); 
+            // ORCA: Bias by EPSILON so a gap effectively equal to
+            // max_suport_layer_height is not split by floating-point noise.
+            size_t n_layers_extra = size_t(ceil((dist - EPSILON) / m_slicing_params.max_suport_layer_height));
             assert(n_layers_extra > 0);
             coordf_t      step   = dist / coordf_t(n_layers_extra);
             if (extr1 != nullptr && extr1->layer_type == SupporLayerType::TopContact &&
@@ -2898,7 +2899,9 @@ SupportGeneratorLayersPtr PrintObjectSupportMaterial::raft_and_intermediate_supp
                 layer_new.height   = extr1->height;
                 intermediate_layers.push_back(&layer_new);
                 dist = extr2z - extr1z;
-                n_layers_extra = size_t(ceil(dist / m_slicing_params.max_suport_layer_height));
+                // ORCA: Recalculate with the same EPSILON bias after re-anchoring at the top
+                // contact layer so near-equal gaps do not gain an extra split here either.
+                n_layers_extra = size_t(ceil((dist - EPSILON) / m_slicing_params.max_suport_layer_height));
                 if (n_layers_extra == 0)
                     continue;
                 // Continue printing the other layers up to extr2z.

src/libslic3r/Support/TreeSupport.cpp

@@ -1157,12 +1157,17 @@ void TreeSupport::create_tree_support_layers()
 
         // Layers between the raft contacts and bottom of the object.
         double dist_to_go = m_slicing_params.object_print_z_min - raft_print_z;
-        auto nsteps = int(ceil(dist_to_go / m_slicing_params.max_suport_layer_height));
-        double height = dist_to_go / nsteps;
-        for (int i = 0; i < nsteps; ++i) {
-            raft_print_z += height;
-            raft_slice_z = raft_print_z - height / 2;
-            m_object->add_tree_support_layer(layer_id++, height, raft_print_z, raft_slice_z);
+        // ORCA: Guard tiny residual raft-to-object gaps so the EPSILON-biased ceil()
+        // below cannot turn them into zero steps after floating-point accumulation.
+        if (dist_to_go > EPSILON) {
+            // ORCA: Bias by EPSILON so near-equal gaps do not get an extra split from FP noise.
+            auto nsteps = int(ceil((dist_to_go - EPSILON) / m_slicing_params.max_suport_layer_height));
+            double height = dist_to_go / nsteps;
+            for (int i = 0; i < nsteps; ++i) {
+                raft_print_z += height;
+                raft_slice_z = raft_print_z - height / 2;
+                m_object->add_tree_support_layer(layer_id++, height, raft_print_z, raft_slice_z);
+            }
         }
         m_raft_layers = layer_id;
     }

src/libslic3r/Support/TreeSupportCommon.hpp

@@ -343,7 +343,8 @@ public:
             }
             if (double dist_to_go = slicing_params.object_print_z_min - z; dist_to_go > EPSILON) {
                 // Layers between the raft contacts and bottom of the object.
-                auto nsteps = int(ceil(dist_to_go / slicing_params.max_suport_layer_height));
+                // ORCA: Bias by EPSILON so near-equal gaps do not get an extra split from FP noise.
+                auto nsteps = int(ceil((dist_to_go - EPSILON) / slicing_params.max_suport_layer_height));
                 double step = dist_to_go / nsteps;
                 for (int i = 0; i < nsteps; ++ i) {
                     z += step;
@@ -754,4 +755,4 @@ enum class LineStatus
 
 } // namespace Slic3r
 
-#endif // slic3r_TreeSupportCommon_hpp
+#endif // slic3r_TreeSupportCommon_hpp