1) Take any 3-d object.
2) Look at this object without moving the object or yourself.
3) Pretend that this object is now a 2-d painting. Look at the very edge of the object where it meets the space around it. (without moving the object or yourself).
4) Note that there’s so much detail in that very edge of the object. Right at the very edge of that object there’s all the information about that side condensed in such a small section of your perspective.
5) Now go back to treating it as a 3-d object and turn the object around to view the side. See, now all that stuff on the edge before… it’s all right there. All that stuff on the side was crammed into the very outline of the object.
I realized this while doing some photoshop work on a seashell. The background was being trimmed out and also part of the seashell where it was the very edge. It made me a little sad to realize that all that information about the side of the object was being lost. Such fine subtlties.