On the left is Edward Adelson’s famous Checker Shadow Illusion, published in 1995.  Square B looks much darker than D.  However they are actually the same uniform grey, as can be seen by comparison with the bar overlaying them in the middle image.

What’s going on?  The brain has strategies for keeping the relationships between the tones and colours of objects in a scene – for example between the light and dark rectangles here – consistent, in spite of big variations in illumination in different parts of the scene.  The shadow of the cylinder gives the variation in illumination in this scene.

That makes the middle picture visually paradoxical.  C looks much lighter than A, and yet we can see that it is the same tone as D and the overlaying grey bar, and therefore B as well.  But B looks the same tone as A, even though A looks, paradoxically, darker than C.

To my eye, the paradox is even more mysterious in the right hand version.  At first glance, A, B, C and D all look to me the same tone – and yet if I concentrate just on A and C, C tends still to look lighter.

The key is that Adelson’s illusion only works when we see objects as part of a three-dimensional scene, complete with illumination.  The introduction of the grey bar in the middle image breaks the spell, for the squares it overlays.  In the right hand image, the left hand side looks three dimensional, and shows the illusion.  But the two squares sticking out to the right no longer seem so much a part of the scene, and we see them in their true colours.  It is remarkable that there can be such variations in appearance in different parts of the visual field, and from moment to moment with shifts in attention.

For Adelson’s own, more technical (and authoritative) explanation, go to:

https://bcs.mit.edu/directory/edward-adelson  and click on Research

The snail appears to inflate, and yet its shape does not really change at all.  Nor does the lighting change.  All that changes is the character of the surface of the shell.  It transforms from very shiny to completely matt and unreflective. Surfaces don’t change like that in the real world, but it’s easily achieved in 3D animation software, such as Blender, which I use.

But why should that make the shell seem to inflate?  The size of the shell is just as well specified when shiny as when matt.  I think it may have to do with the movement of the blurry shadow edge as it sweeps outward towards the edges of the shell.   We transmit blurry, so-called low spatial frequency, soft edges along different brain pathways from the ones that transmit the sharp, so-called high spatial frequency edges, which we see in highly reflective surfaces.  Soft edges indicate the overall roundness of objects.  And soft edges moving symmetrically over a surface towards the object edges would probably only appear, in the real world, in the case of an expansion.  However, the effect  also depends in a way I don’t understand on the shape of the object.  Not any old shape gives the expansion effect with the same treatment – I ended up with this one by trial and error.

This is an opportunity to say how terrific the Blender package is.  It’s free for anyone to download and use and the developers and the Blender community have done a really fantastic job in making it available to anyone, anywhere with internet access.  It takes a bit of learning (I don’t begin to do it justice) but it’s now comparable to high end professional animation software.

(Post updated at 31/8/19). This is a 3D movie, which I showed recently at a vision science meeting, (ECVP in Leuven, Belgium).  You will only be able to view the movie in 3D if you have learned how to fuse the two images into one without a viewer, by so-called cross-eyed viewing.  If you don’t have the technique and want to learn it, one of the best Youtube guides is this Youtube tutorial.  (But best to give the technique a miss if you have any eye adjustment problems).

Alternatively, if you have a pair of red/cyan movie glasses, view this next version (ideally full-screen – the larger the better):

The movies are based on one of René Magritte’s most haunting paintings, Carte Blanche, or The Blank Cheque, which you can see at:

https://www.nga.gov/collection/art-object-page.66422.html

Magritte  brilliantly baffles our expectations about spatial organisation, just by bringing into the foreground, in front of the horse and rider, one strip of landscape background, and one tree-trunk.  I wanted to see how the effect would work as a fly-by, 3D animation.

That required a change of motif.  For the animated version the central motif must not touch the ground.  So instead of the horse and rider in the original painting, I chose another Magritte theme: a floating pipe.

The 3D versions set up a competition, between two processes usually in perfect agreement:  on the one hand our judgments of depth based on binocular disparity – the slightly different viewpoint of each eye;  and on the other hand depth arrangements indicated by the occlusion (or masking) of more distant objects by nearer ones.

Different observers see the result in different ways.  Most commonly, when the pipe is interrupted because a tree trunk or background appears in front of it, the different parts of the pipe seem to be at different distances from us.  It can even seem to weave in and out of the tree-trunks, and they can bend around it.  It seems that the occlusion (masking) cues are winning out over the disparity cues.

A few weeks back, with collaborators Priscilla Heard and Christopher Tyler, I won second prize with this movie in the annual Best Visual Illusion of the Year Contest.  I think we were lucky!  As ever the competition showcased some fascinating, quite new illusions, and this year’s were a particularly strong bunch.

Extra – July 2019 – Earlier this year we published a paper analysing the illusion  in the online, open access journal i-Perception.

Last year the competition organisers published a beautiful book, showing some of the illusions that won prizes in the years since the competition started in 2005.  You can see details on this site.