Cut up line segments that are partially within the roofing area

CURA-11129

src/FffGcodeWriter.cpp

@@ -2613,7 +2613,7 @@ bool FffGcodeWriter::processInsets(
                     roofing_mask.add(layer_part.outline);
                 }
             }
-            return roofing_mask.offset(100);
+            return roofing_mask;
         }();
 
         gcode_layer.setRoofingMask(roofing_mask);

src/LayerPlan.cpp

@@ -642,6 +642,8 @@ void LayerPlan::addPolygonsByOptimizer(
 
 static constexpr double max_non_bridge_line_volume = MM2INT(100); // limit to accumulated "volume" of non-bridge lines which is proportional to distance x extrusion rate
 
+static int i = 0;
+
 void LayerPlan::addWallLine(
     const Point2LL& p0,
     const Point2LL& p1,
@@ -773,14 +775,54 @@ void LayerPlan::addWallLine(
 
     if (use_roofing_config)
     {
-        addExtrusionMove(
-            p1,
-            roofing_config,
-            SpaceFillType::Polygons,
-            flow,
-            width_factor,
-            spiralize,
-            1.0_r);
+        // The line segment is wholly or partially in the roofing area. The line is intersected
+        // with the roofing area into line segments. Each line segment left in this intersection
+        // will be printed using the roofing config, all removed segments will be printed using
+        // the non_bridge_config. Since the original line segment was straight we can simply print
+        // to the first and last point of the intersected line segments alternating between
+        // roofing and non_bridge_config's.
+        Polygons line_polys;
+        line_polys.addLine(p0, p1);
+        constexpr bool restitch = false; // only a single line doesn't need stitching
+        auto has_area_above_poly_lines = roofing_mask_.intersectionPolyLines(line_polys, restitch);
+
+        if (has_area_above_poly_lines.empty())
+        {
+            addExtrusionMove(p1, roofing_config, SpaceFillType::Polygons, flow, width_factor, spiralize, 1.0_r);
+        }
+        else
+        {
+            // reorder all the line segments so all lines start at p0 and end at p1
+            for (auto& line_poly : has_area_above_poly_lines)
+            {
+                const Point2LL& line_p0 = line_poly.front();
+                const Point2LL& line_p1 = line_poly.back();
+                if (vSize2(line_p1 - p0) < vSize2(line_p0 - p0))
+                {
+                    std::reverse(line_poly.begin(), line_poly.end());
+                }
+            }
+            std::sort(has_area_above_poly_lines.begin(), has_area_above_poly_lines.end(), [&](auto& a, auto& b) { return vSize2(a.front() - p0) < vSize2(b.front() - p0); });
+
+            // add intersected line segments, alternating between roofing and non_bridge_config
+            for (const auto& line_poly : has_area_above_poly_lines)
+            {
+                // This is only relevant for the very fist iteration of the loop
+                // if the start of the line segment is already the same as p0 then no move is required
+                if (line_poly.front() != p0)
+                {
+                    addExtrusionMove(line_poly.front(), roofing_config, SpaceFillType::Polygons, flow, width_factor, spiralize, 1.0_r);
+                }
+
+                addExtrusionMove(line_poly.back(), non_bridge_config, SpaceFillType::Polygons, flow, width_factor, spiralize, 1.0_r);
+            }
+
+            // if the last point is not yet at p1 then add a move to p1
+            if (has_area_above_poly_lines.back().back() != p1)
+            {
+                addExtrusionMove(p1, roofing_config, SpaceFillType::Polygons, flow, width_factor, spiralize, 1.0_r);
+            }
+        }
     }
     else if (bridge_wall_mask_.empty())
     {