(re-draft August 2016) The right hand upper window is leaning the wrong way, which is wonky for a start, but it’s not quite as wonky as it looks. It’s really identical to the window on the left and only seems to lean over more.  What’s going on?

Thanks to perspective, if two objects of the same size are at different distances from the eye, the further one produces an image on the retina that is smaller.  However, that’s not always how we see it.  The brain tends to reduce the effect, so that to the mind’s eye the more distant object is not reduced in size so much.  We tend to see the Moon, for example, as larger than its tiny image on the retina would imply, expecially when it appears near the horizon.  The effect is called size-constancy.

It can produce some remarkable illusions in pictures that present (a) very strong perspective cues to depth, like the dramatically converging edges of the house in the Wonky Window picture; and (b) things at different distances in the picture that are not reduced in size by perspective as they should be, but are identical.  The two yellow lines in this picture are an example.  They are objectively identical in length, but thanks the size-constancy effect  we see the lower one as longer.  The upper windows are also identical, but the effect has expanded the apparent horizontal extent of their lower edges, so that the right hand window looks like it’s leaning over.