Thus the area of SD is obtained by integrating the length of the normal vector r → u × r → v {\displaystyle {\vec {r}}_{u}\times {\vec {r}}_{v}} to the surface over the appropriate region D in the parametric uv plane. The area of the whole surface is then obtained by adding together the areas of the pieces, using additivity of surface area. The main formula can be specialized to different classes of surfaces, giving, in particular, formulas for areas of graphs z = f(x,y) and surfaces of revolution.