optimize cross hatch infill to enchance strength (#5495)

Optimize the cross-hatch infill pattern to improve strength when low infill density is used.

src/libslic3r/Fill/FillCrossHatch.cpp

@@ -1,6 +1,7 @@
 #include "../ClipperUtils.hpp"
 #include "../ShortestPath.hpp"
 #include "../Surface.hpp"
+#include <cmath>
 
 #include "FillCrossHatch.hpp"
 
@@ -193,7 +194,12 @@ void FillCrossHatch ::_fill_surface_single(
     bb.merge(align_to_grid(bb.min, Point(line_spacing * 4, line_spacing * 4)));
 
     // generate pattern
-    Polylines polylines = generate_infill_layers(scale_(this->z), 1, line_spacing, bb.size()(0), bb.size()(1));
+    //Orca: optimize the cross-hatch infill pattern to improve strength when low infill density is used.
+    double repeat_ratio = 1.0;
+    if (params.density < 0.3)
+        repeat_ratio = std::clamp(1.0 - std::exp(-5 * params.density), 0.2, 1.0);
+
+    Polylines polylines = generate_infill_layers(scale_(this->z), repeat_ratio, line_spacing, bb.size()(0), bb.size()(1));
 
     // shift the pattern to the actual space
     for (Polyline &pl : polylines) { pl.translate(bb.min); }