An efficient way to determine which objects in a 3D image are to be displayed and which are not because they are obscured by other displayed objects. Displayable elements are assigned depth values defining their relative perceived nearness to the viewer of the image. A comparison of depth values determines which elements are to be displayed and which are not to be displayed because they are obscured by displayed elements. Rather than comparing the depth value of every pixel in a displayable object to determine whether it is to be displayed, the invention compares groups of pixels defined by spans. Minimum and maximum depth values are determined for each span so that depth variations within a span can be accommodated. Masks are used when only partial spans are to be considered because some pixels in a span are outside the pixel boundaries being considered in a particular comparison.