ENH: fix extrusion of interface on the first layer

Signed-off-by: qing.zhang <[email protected]>
Change-Id: I08a3599e349f2137a53737010220209dc2a54d6a

src/libslic3r/SupportMaterial.cpp

@@ -4517,9 +4517,13 @@ void PrintObjectSupportMaterial::generate_toolpaths(
                     (m_object_config->support_interface_bottom_layers == 0 && &layer_ex == &bottom_contact_layer);
                 //FIXME Bottom interfaces are extruded with the briding flow. Some bridging layers have its height slightly reduced, therefore
                 // the bridging flow does not quite apply. Reduce the flow to area of an ellipse? (A = pi * a * b)
-                auto interface_flow = layer_ex.layer->bridging ?
-                    Flow::bridging_flow(layer_ex.layer->height, m_support_params.support_material_bottom_interface_flow.nozzle_diameter()) :
-                    (interface_as_base ? &m_support_params.support_material_flow : &m_support_params.support_material_interface_flow)->with_height(float(layer_ex.layer->height));
+                Flow interface_flow;
+                if (layer_ex.layer->bridging)
+                    interface_flow = Flow::bridging_flow(layer_ex.layer->height, m_support_params.support_material_bottom_interface_flow.nozzle_diameter());
+                else if (layer_ex.layer->bottom_z < EPSILON) {
+                    interface_flow = m_support_params.first_layer_flow;
+                }else
+                    interface_flow = (interface_as_base ? &m_support_params.support_material_flow : &m_support_params.support_material_interface_flow)->with_height(float(layer_ex.layer->height));
                 filler_interface->angle = interface_as_base ?
                         // If zero interface layers are configured, use the same angle as for the base layers.
                         angles[support_layer_id % angles.size()] :