The notion of quantitative invisibility and the machine rendering of solids

Line drawings are the most common type of rendering used to convey geometrical description. This is due to the economy of preparing such drawings and the great information density obtainable. On a pure line drawing, that is where no attempt is made to specify or suggest shadows, tone or color, the lines rendered are either the intersection curves of surfaces or the contour curves of surfaces. The nature of these curves are adequately discussed in the literature 1 and in a previous report.2 In order to convey a realistic impression of an object or an assembly of objects, the segments of lines which cannot be seen by an observer are not drawn or are drawn dashed. Without specification of visibility a drawing is ambiguous. This paper presents a recently developed scheme for the determination of visibility in a line drawing which enables comparitively high speed calculation and excellent resolution.