Hidden Line or Surface Removal:

The problem of hidden lines or surfaces was implicit even in 2-D graphics, but we did not mention it there, because what was intended to be viewed and displayed was already predetermined, and defined. But in 3-D graphics, we present solid objects, and we want to see them from different positions. With each transformation, different portions that were hidden would pop into view, and other portions which were visible would go into hiding. It is indeed a major difficulty.