Image space shading of three-dimensional objects.

Two-dimensional images of 3-D objects need shading to give depth. Traditional (object space) shading methods require extra data (normal vectors) to be stored with the object description, but when representations are obtained directly from measured data, these normal vectors may be expensive to compute and if the object is modified interactively they must be recomputed frequently. To simplify, a shading method is devised which uses only information available in image space, after coordinates have been transformed, hidden surfaces removed, and a complete pre-image of all objects has been assembled. This involves using both the distance from the light source and the surface orientation as the basis of shading. This theory and its implementation are discussed and examples of shaded images presented.-from Authors