The surface area shall be computed by including the entire area within a single, continuous, rectilinear perimeter of not more than eight straight lines, or a circle, or an ellipse, enclosing the extreme limits of the writing, representation, emblem or other display, together with any material or color, forming an integral part of the background of the display or used to differentiate the sign from the backdrop or structure against which it is placed, but not including any supporting framework or bracing that is clearly incidental to the display itself.