Suppose you have a curved surface, but the boundary of
this surface is not curved, as shown in the above diagram.
The rectangle is a window through which the curved surface is seen. Since the
window is at the front, the front view of the curved surface essentially appears to be a rectangle (Shaded region). The curved
surface is completely plain. Should our eye still see 3D? Try out this experiment yourself! Anyway here’s the conclusion:
You don’t see 3D even with both your eyes wide open! Here’s why. Our eye needs two clues to determine the shape
of objects. Either it should be able to focus properly, which requires that the surface have recognizable contrast, which
in turn is used to correlate the images from both the eyes to perceive 3D, or it should be able to see the object boundary,
which in turn helps it to make some assumptions about the 3D shape of the object. In the above case we have deprived it of
both!
