Merge tiny cracks that is part of a large bottom surface into bottom surface (OrcaSlicer#9332)

* Merge tiny cracks that is part of a large bottom surface into bottom surface (OrcaSlicer#9329)

* Revert bottom offset

* Top surfaces will have perimeters around, which need to be removed from bottom surfaces so bridges not extended to visible areas

Author: Noisyfox

src/libslic3r/PrintObject.cpp

@@ -1309,8 +1309,7 @@ void PrintObject::detect_surfaces_type()
                     Layer       *upper_layer = (idx_layer + 1 < this->layer_count()) ? m_layers[idx_layer + 1] : nullptr;
                     Layer       *lower_layer = (idx_layer > 0) ? m_layers[idx_layer - 1] : nullptr;
                     // collapse very narrow parts (using the safety offset in the diff is not enough)
-                    const float offset_top = layerm->flow(frExternalPerimeter).scaled_width() / 10.f;
-                    const float offset_bottom = layerm->flow(frExternalPerimeter).scaled_width();
+                    const float offset = layerm->flow(frExternalPerimeter).scaled_width() / 10.f;
 
                     ExPolygons     layerm_slices_surfaces = to_expolygons(layerm->slices.surfaces);
                     // no_perimeter_full_bridge allow to put bridges where there are nothing, hence adding area to slice, that's why we need to start from the result of PerimeterGenerator.
@@ -1325,7 +1324,7 @@ void PrintObject::detect_surfaces_type()
                         ExPolygons upper_slices = interface_shells ?
                             diff_ex(layerm_slices_surfaces, upper_layer->m_regions[region_id]->slices.surfaces, ApplySafetyOffset::Yes) :
                             diff_ex(layerm_slices_surfaces, upper_layer->lslices, ApplySafetyOffset::Yes);
-                        surfaces_append(top, opening_ex(upper_slices, offset_top), stTop);
+                        surfaces_append(top, opening_ex(upper_slices, offset), stTop);
                     } else {
                         // if no upper layer, all surfaces of this one are solid
                         // we clone surfaces because we're going to clear the slices collection
@@ -1351,7 +1350,7 @@ void PrintObject::detect_surfaces_type()
                             bottom,
                             opening_ex(
                                 diff_ex(layerm_slices_surfaces, lower_layer->lslices, ApplySafetyOffset::Yes),
-                                offset_bottom),
+                                offset),
                             surface_type_bottom_other);
                         // if user requested internal shells, we need to identify surfaces
                         // lying on other slices not belonging to this region
@@ -1365,7 +1364,7 @@ void PrintObject::detect_surfaces_type()
                                         intersection(layerm_slices_surfaces, lower_layer->lslices), // supported
                                         lower_layer->m_regions[region_id]->slices.surfaces,
                                         ApplySafetyOffset::Yes),
-                                    offset_bottom),
+                                    offset),
                                 stBottom);
                         }
 #endif
@@ -1383,14 +1382,22 @@ void PrintObject::detect_surfaces_type()
                     if (! top.empty() && ! bottom.empty()) {
                         const auto cracks = intersection_ex(top, bottom);
                         if (!cracks.empty()) {
-                            const float small_crack_threshold = -offset_bottom;
+                            const float small_crack_threshold = -layerm->flow(frExternalPerimeter).scaled_width() * 1.5;
 
                             for (const auto& crack : cracks) {
                                 if (offset_ex(crack, small_crack_threshold).empty()) {
+                                    // For small cracks, if it's part of a large bottom surface, then it should be added to bottom as well
+                                    if (std::any_of(bottom.begin(), bottom.end(), [&crack, small_crack_threshold](const Surface& s) {
+                                            const auto& se = s.expolygon;
+                                            return diff_ex(crack, se, ApplySafetyOffset::Yes).empty()
+                                                && se.area() > crack.area() * 2
+                                                && !offset_ex(diff_ex(se, crack), small_crack_threshold).empty();
+                                    })) continue;
+
                                     // Crack too small, leave it as part of the top surface, remove it from bottom surfaces
                                     Surfaces bot_tmp;
                                     for (auto& b : bottom) {
-                                        surfaces_append(bot_tmp, diff_ex(b.expolygon, crack), b.surface_type);
+                                        surfaces_append(bot_tmp, diff_ex(b.expolygon, offset_ex(crack, -small_crack_threshold)), b.surface_type);
                                     }
                                     bottom = std::move(bot_tmp);
                                 }