//black knob border
cairo_set_line_width (cr, border_width);
cairo_set_source_rgba (cr, 0,0,0, 1 );
cairo_arc (cr, 0, 0, center_radius, 0, 2.0*G_PI);
cairo_stroke (cr);
//black knob border
cairo_set_line_width (cr, border_width);
cairo_set_source_rgba (cr, 0,0,0, 1 );
cairo_arc (cr, 0, 0, center_radius, 0, 2.0*G_PI);
cairo_stroke (cr);