# Third set of optical illusion pictures

Here is another set of small optical illusion and visual perception related images you can link to from your own website / profile page.

In case you missed them you can still see the second perception pictures as well as the first set of illusion pictures.

# Lamp falling over!

Only joking, but the right hand picture does seem to show this old lamp near me leaning over more than it does in the left hand picture.  But now check out the two pictures.  They’re identical!  It’s an illusion only recently reported by Frederick Kingdom and colleagues in McGill University (scroll down that link for their bit).  It’s yet another demonstration of the strength of the size-constancy effect.  (See my earlier posts on the wonky window and paradoxical size-constancy).  What’s remarkable about the McGill report is that it shows size-constancy coming into play even across the gap between two clearly separate pictures. I guess it means that at an early stage in trying to make sense of the visual scene, the brain just accepts the consistent depth cues in the two pictures as signalling that they are both part of one spatial scheme.

As Kingdom and colleagues demonstrated, the effect also works horizontally.  Here’s a demo:

This time the platform looks pretty much the same in the two pictures (as it is – the pictures are of course once again identical). But rails to the right, seen at a more oblique angle, seem to point a bit more towards vertical in the right hand image than in the left hand one.

# The twisted castle

This is another transformation based on one in my illusion cartoon story.  I did it to see whether I could devise an adventure, set in graphic world, with transformations that are forbidden in our everyday world as the events that take the story forward.  Most of the transformations, like this one, offer an escape route for a character being pursued.  Not very imaginative.  I did also have animation in mind, and working out this particular 3D morph would be no joke.  It’s based on the four sided version of the famous impossible triangle, with added extensions for the towers, as above.  For the classic impossible tribar, see my earlier post on Escher’s Waterfall Explained.  For impossible figures with four sides and more, check out Gershon Elber’s site.

# Tonal contrast

The local tones and colours we see in a scene or image are hugely affected both by the tonal and colour balance of the scene as a whole, and also by the tones and colours immediately adjacent to the patch we are focussed on.  The effects can be subtle.  Here’s a demo of one, called the Munker-White illusion.  The bat (well, it’s a sort of bat shape), in the left hand fractal pattern (it’s a Peano-Gosper curve), looks pale, and the one in the right hand pattern looks dark.  And yet the mid tone components of the bat in the left hand image are in fact exactly the same tone and colour as the mid-toned components of the bat to the right.

# 3D picture pairs

We all know why stereo picture pairs give rise to a vivid illusion of 3D, don’t we?  It’s because they imitate the way that our brains, if we are lucky enough to have normal vision, take advantage of the slight differences in viewpoint from our two eyes.  We grow up hard wired with brain cells that look out for similarities in patches in the field of view of each eye that are just slightly displaced from one another.  So if you rewired the brain, so that the left eye sees what the the right eye should, and vice versa, you’d see all the depth effects reversed, wouldn’t you?  Well, no, in fact, not necessarily.  Look at these picture pairs of a canal lock, near where I live, just north of London in the UK.

If you haven’t yet aquired the knack of viewing stereo picture pairs without a viewer, try this tutorial.  You’ll easily find other guides by searching on “viewing 3d pictures” and similar phrases.

These pairs show a scene with the viewpoint from right and left preserved (lower pair) and reversed (upper pair).  And yet both pairs give (for me) a fairly normal illusion of depth.  One pair works better than the other.  Usually, when viewing stereo picture pairs without a special viewer, the arrangement as in the upper pair above works best.  (More on that below).  So which pair looks better can depend on which technique you have used to view the pictures.  But the fact that both pairs give an illusion of depth at all is remarkable.