1. A computer-implemented method, comprising: performing, by a computer: selectively displaying surfaces of an existing object model of a three-dimensional (3D) physical object, wherein the existing object model comprises data defining a plurality of surfaces of the 3D physical object, wherein said selectively displaying comprises: determining a normal vector to a given surface of the plurality of surfaces included in the existing object model; determining an angle between the normal vector of the given surface and a current viewing direction of the existing object model; and determining whether the angle between the normal vector and the current viewing direction is greater than a threshold value, and displaying the existing object model either without displaying the given surface if the angle is greater than the threshold value or with displaying the given surface if the angle between the normal vector and the current viewing direction is less than the threshold value.