The precautions required for drawing multiple spheres, particularly when they intersect or obscure one another, depends somewhat on the technique by which the surfaces are represented. In the point representation, the only decision necessary concerns the visibility of the point, but in the line representation, the two endpoints of the line segment may have different visibilities.
Letting one endpoint determine the visibility of the whole line is only satisfactory when the line segments are very short; otherwise linear interpolation of the visibility criterion greatly improves the appearance of the image. The subroutine which performs this service is pltil.
