Impossible Box 3D Optical Illusion

What techniques did the artist use to create this optical illusion?

See Another Random Optical Illusion

Explanation

This optical illusion takes advantage of the 3D drawing techniques of overlap and shading to create this impossible cube. If you look carefully, you can see that all of the lines could have been made to end in their normal position, that is, where they would end if we were not creating an optical illusion. Some of them end early, which confuses our eye. Some of the lines are extended beyond their normal position and overlap where they shouldn't which confuses our eye even more.

To compound things, add shading. Shading is used on some parts of the cube which is different than it would be if the box were normal. Now we have a truly impossible box.