Editing 3D projection (section)

===Limitations of parallel projection===
{{See also|Impossible object}}
{{Multiple image
| width= 200
| image1= IsometricFlaw 2.svg
| caption1= An example of the limitations of isometric projection. The height difference between the red and blue balls cannot be determined locally.
| image2= Impossible staircase.svg
| caption2= The [[Penrose stairs]] depicts a staircase which seems to ascend (anticlockwise) or descend (clockwise) yet forms a continuous loop.
}}

Objects drawn with parallel projection do not appear larger or smaller as they extend closer to or away from the viewer. While advantageous for [[architectural drawing]]s, where measurements must be taken directly from the image, the result is a perceived distortion, since unlike [[perspective projection]], this is not how our eyes or photography normally work. It also can easily result in situations where depth and altitude are difficult to gauge, as is shown in the illustration to the right.

In this isometric drawing, the blue sphere is two units higher than the red one. However, this difference in elevation is not apparent if one covers the right half of the picture, as the boxes (which serve as clues suggesting height) are then obscured.

This visual ambiguity has been exploited in [[op art]], as well as "impossible object" drawings. [[M. C. Escher]]'s ''[[Waterfall (M. C. Escher)|Waterfall]]'' (1961), while not strictly utilizing parallel projection, is a well-known example, in which a channel of water seems to travel unaided along a downward path, only to then paradoxically fall once again as it returns to its source. The water thus appears to disobey the [[conservation of energy|law of conservation of energy]]. An extreme example is depicted in the film ''[[Inception]]'', where by a [[forced perspective]] trick an immobile stairway changes its connectivity. The video game ''[[Fez (video game)|Fez]]'' uses tricks of perspective to determine where a player can and cannot move in a puzzle-like fashion.