optimize the brdige over infill by extractng only the sparse infill lines from previous layer

src/libslic3r/Fill/Fill.cpp

@@ -643,6 +643,94 @@ void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive:
 #endif
 }
 
+Polylines Layer::generate_sparse_infill_polylines_for_anchoring() const
+{
+    std::vector<SurfaceFill>  surface_fills = group_fills(*this);
+    const Slic3r::BoundingBox bbox          = this->object()->bounding_box();
+    const auto                resolution    = this->object()->print()->config().gcode_resolution.value;
+
+    Polylines sparse_infill_polylines{};
+
+    for (SurfaceFill &surface_fill : surface_fills) {
+        // skip patterns for which additional input is nullptr
+        switch (surface_fill.params.pattern) {
+        case ipLightning: continue; break;
+        case ipAdaptiveCubic: continue; break;
+        case ipSupportCubic: continue; break;
+        case ipCount: continue; break;
+        case ipSupportBase: continue; break;
+        case ipEnsuring: continue; break;
+        case ipRectilinear:
+        case ipMonotonic:
+        case ipMonotonicLines:
+        case ipAlignedRectilinear:
+        case ipGrid:
+        case ipTriangles:
+        case ipStars:
+        case ipCubic:
+        case ipLine:
+        case ipConcentric:
+        case ipHoneycomb:
+        case ip3DHoneycomb:
+        case ipGyroid:
+        case ipHilbertCurve:
+        case ipArchimedeanChords:
+        case ipOctagramSpiral: break;
+        }
+
+        // Create the filler object.
+        std::unique_ptr<Fill> f = std::unique_ptr<Fill>(Fill::new_from_type(surface_fill.params.pattern));
+        f->set_bounding_box(bbox);
+        f->layer_id = this->id();
+        f->z        = this->print_z;
+        f->angle    = surface_fill.params.angle;
+        // f->adapt_fill_octree   = (surface_fill.params.pattern == ipSupportCubic) ? support_fill_octree : adaptive_fill_octree;
+        f->print_config        = &this->object()->print()->config();
+        f->print_object_config = &this->object()->config();
+
+        // calculate flow spacing for infill pattern generation
+        double link_max_length = 0.;
+        if (!surface_fill.params.bridge) {
+#if 0
+            link_max_length = layerm.region()->config().get_abs_value(surface.is_external() ? "external_fill_link_max_length" : "fill_link_max_length", flow.spacing());
+//            printf("flow spacing: %f,  is_external: %d, link_max_length: %lf\n", flow.spacing(), int(surface.is_external()), link_max_length);
+#else
+            if (surface_fill.params.density > 80.) // 80%
+                link_max_length = 3. * f->spacing;
+#endif
+        }
+
+        // Maximum length of the perimeter segment linking two infill lines.
+        f->link_max_length = (coord_t) scale_(link_max_length);
+        // Used by the concentric infill pattern to clip the loops to create extrusion paths.
+        f->loop_clipping = coord_t(scale_(surface_fill.params.flow.nozzle_diameter()) * LOOP_CLIPPING_LENGTH_OVER_NOZZLE_DIAMETER);
+
+        LayerRegion &layerm = *m_regions[surface_fill.region_id];
+
+        // apply half spacing using this flow's own spacing and generate infill
+        FillParams params;
+        params.density           = float(0.01 * surface_fill.params.density);
+        params.dont_adjust       = false; //  surface_fill.params.dont_adjust;
+        params.anchor_length     = surface_fill.params.anchor_length;
+        params.anchor_length_max = surface_fill.params.anchor_length_max;
+        params.resolution        = resolution;
+        params.use_arachne       = false;
+        params.layer_height      = layerm.layer()->height;
+
+        for (ExPolygon &expoly : surface_fill.expolygons) {
+            // Spacing is modified by the filler to indicate adjustments. Reset it for each expolygon.
+            f->spacing                     = surface_fill.params.spacing;
+            surface_fill.surface.expolygon = std::move(expoly);
+            try {
+                Polylines polylines = f->fill_surface(&surface_fill.surface, params);
+                sparse_infill_polylines.insert(sparse_infill_polylines.end(), polylines.begin(), polylines.end());
+            } catch (InfillFailedException &) {}
+        }
+    }
+
+    return sparse_infill_polylines;
+}
+
 // Create ironing extrusions over top surfaces.
 void Layer::make_ironing()
 {