Optical illusion

Posted on September 08, 2007 by Steve

Have a look at this pair of photos.

The two images look different, right? The breathless, hyper-linked description makes no effort to explain the effect, and the commenters either fail to see it or misperceive the image as a stereogram (an error I've made myself).

Like many illusions, this one is caused by a difference between the way we perceive real objects and they way they look in flat images. Two plumb buildings standing side by side would not appear to have parallel vertical lines in the real world. At eye level they would, but looking up the lines would appear to incline toward one another, pointing toward a single vanishing point. The vertical lines in the twin images remain parallel all the way to the top, causing a perception that they are leaning away from one another.

Escher rendered straight lines as curves in "Up and Down," creating a scene that looks normal in every detail, but bizarre overall.