Prevent extra wall generation when sparse infill is zero (#3775)

Co-authored-by: SoftFever <[email protected]>

src/libslic3r/PerimeterGenerator.cpp

@@ -1505,8 +1505,9 @@ void PerimeterGenerator::process_classic()
     for (size_t order_idx = 0; order_idx < surface_order.size(); order_idx++) {
         const Surface &surface = this->slices->surfaces[surface_order[order_idx]];
         // detect how many perimeters must be generated for this island
-        int        loop_number = this->config->wall_loops + surface.extra_perimeters - 1;  // 0-indexed loops
-        if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase) // add alternating extra wall
+        int loop_number = this->config->wall_loops + surface.extra_perimeters - 1;  // 0-indexed loops
+        int sparse_infill_density = this->config->sparse_infill_density.value;
+        if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase && sparse_infill_density > 0) // add alternating extra wall
             loop_number++;
         if (this->layer_id == 0 && this->config->only_one_wall_first_layer)
             loop_number = 0;
@@ -1939,8 +1940,9 @@ void PerimeterGenerator::process_arachne()
     for (const Surface& surface : this->slices->surfaces) {
         coord_t bead_width_0 = ext_perimeter_spacing;
         // detect how many perimeters must be generated for this island
-        int        loop_number = this->config->wall_loops + surface.extra_perimeters - 1; // 0-indexed loops
-        if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase) // add alternating extra wall
+        int loop_number = this->config->wall_loops + surface.extra_perimeters - 1; // 0-indexed loops
+        int sparse_infill_density = this->config->sparse_infill_density.value;
+        if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase && sparse_infill_density > 0) // add alternating extra wall
             loop_number++;
 
         // Set the bottommost layer to be one wall