ENH: optimize area of top surface

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: Ia3d1be6ad3b9f55ee1acda14d0be955055f6dfcb
(cherry picked from commit 448fd3043950613a4d91df256fa9ee52a57fcaca)

src/libslic3r/Layer.cpp

@@ -350,6 +350,50 @@ void Layer::export_region_fill_surfaces_to_svg_debug(const char *name) const
     this->export_region_fill_surfaces_to_svg(debug_out_path("Layer-fill_surfaces-%s-%d.svg", name, idx ++).c_str());
 }
 
+coordf_t Layer::get_sparse_infill_max_void_area()
+{
+    double max_void_area = 0.;
+    for (auto layerm : m_regions) {
+        Flow flow = layerm->flow(frInfill);
+        float density = layerm->region().config().sparse_infill_density;
+        InfillPattern pattern = layerm->region().config().sparse_infill_pattern;
+        if (density == 0.)
+            return -1;
+
+        //BBS: rough estimation and need to be optimized
+        double spacing = flow.scaled_spacing() * (100 - density) / density;
+        switch (pattern) {
+            case ipConcentric:
+            case ipRectilinear:
+            case ipLine:
+            case ipGyroid:
+            case ipAlignedRectilinear:
+            case ipOctagramSpiral:
+            case ipHilbertCurve:
+            case ip3DHoneycomb:
+            case ipArchimedeanChords:
+                max_void_area = std::max(max_void_area, spacing * spacing);
+                break;
+            case ipGrid:
+            case ipHoneycomb:
+            case ipLightning:
+                max_void_area = std::max(max_void_area, 4.0 * spacing * spacing);
+                break;
+            case ipCubic:
+            case ipAdaptiveCubic:
+            case ipTriangles:
+            case ipStars:
+            case ipSupportCubic:
+                max_void_area = std::max(max_void_area, 4.5 * spacing * spacing);
+                break;
+            default:
+                max_void_area = std::max(max_void_area, spacing * spacing);
+                break;
+        }
+    };
+    return max_void_area;
+}
+
 BoundingBox get_extents(const LayerRegion &layer_region)
 {
     BoundingBox bbox;

src/libslic3r/Layer.hpp

@@ -184,6 +184,8 @@ public:
 
     //BBS
     void simplify_extrusion_path() { for (auto layerm : m_regions) layerm->simplify_extrusion_entity();}
+    //BBS: this function calculate the maximum void grid area of sparse infill of this layer. Just estimated value
+    coordf_t get_sparse_infill_max_void_area();
 
 protected:
     friend class PrintObject;

src/libslic3r/LayerRegion.cpp

@@ -152,11 +152,20 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
     {
         // Voids are sparse infills if infill rate is zero.
         Polygons voids;
+
+        double max_grid_area = -1;
+        if (this->layer()->lower_layer != nullptr)
+            max_grid_area = this->layer()->lower_layer->get_sparse_infill_max_void_area();
         for (const Surface &surface : this->fill_surfaces.surfaces) {
             if (surface.is_top()) {
                 // Collect the top surfaces, inflate them and trim them by the bottom surfaces.
                 // This gives the priority to bottom surfaces.
+                if (max_grid_area < 0 || surface.expolygon.area() < max_grid_area)
                     surfaces_append(top, offset_ex(surface.expolygon, margin, EXTERNAL_SURFACES_OFFSET_PARAMETERS), surface);
+                else
+                    //BBS: Don't need to expand too much in this situation. Expand 3mm to eliminate hole and 1mm for contour
+                    surfaces_append(top, intersection_ex(offset(surface.expolygon.contour, margin / 3.0, EXTERNAL_SURFACES_OFFSET_PARAMETERS),
+                                                         offset_ex(surface.expolygon, margin, EXTERNAL_SURFACES_OFFSET_PARAMETERS)), surface);
             } else if (surface.surface_type == stBottom || (surface.surface_type == stBottomBridge && lower_layer == nullptr)) {
                 // Grown by 3mm.
                 surfaces_append(bottom, offset_ex(surface.expolygon, margin, EXTERNAL_SURFACES_OFFSET_PARAMETERS), surface);