Category Archives: Optical Illusions

The Breathing Golden Snail Illusion

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.

Celebrating Magritte

(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:

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.

Fly By Night

This movie was suggested to me by my colleague Christopher Tyler, and we developed it together.  When the movie isn’t playing, the picture looks like a flat field of dots.  But as soon as you run the movie, the scene springs into depth, as we appear to fly past high-rise and other city centre buildings.

It’s known as kinetic depth, or structure from motion, and it shows how attuned our brains are to extract information from optic flows, as they stream across the retina, just as much as from images on the retina of static objects.  There are particularly beautiful examples of the human figure in motion and of rotating globes on the website of UK researcher George Mather.  But no one as far as I know has done a night-time city flyby.  We used the 3D animation software Blender.

A prize surprise

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.


A Funny Turn

It may take a bit of practice, but you’ll be able to see the silhouette dinosaur rotating either way around.  One way round, it’s  rotating just like the top right hand of the four coloured dinos.  To see it rotate the other way round, start with the head facing left.  Then try imagining that the head is getting nearer to you as it goes lower.  Once you’ve seen the rotation go both ways, it tends to change spontaneously from one to the other.

I’m not great at imagining 3D shapes moving in space. When I set this up, I assumed that when the silhouette rotates the way around that doesn’t match the top right hand coloured dinosaur, it  will instead match one of the two left hand dinos, but half a rotation out of step.  The top left one is just a mirror reflection of the top right coloured one, and the lower left one a time-reversed version of it.  But I wasn’t sure which of the those left hand dinos it would match.  (The lower right coloured dino is a reflection plus a time-reversal of the top right one.  That switches the rotation twice – back to matching the way around the top right dino goes).

But the silhouette dino, when seen as rotating opposed to the top right dino, isn’t like either of the left hand versions!  In those, the head is always nearest to us when it’s at its highest, and furthest from us when it is lower down.  With the silhouette view clockwise, it’s the other way round:  the head is nearest to us when its low down.  I think it’s an impossible view, invented by the brain.  There’s no way I can get the real dino to give me that view.  (Actually – full disclosure – I didn’t film a real dinosaur.  It’s a model).

I haven’t quite puzzled out why the views work like this.

Extra:  18/2/20

I found the explanation in a chapter in the wonderful Oxford Compendium of Visual Illusions,  Nicolaus F. Troye “The Kayahara silhouette illusion” pp 582-585.

The key is that the sihouette version image is not just rotationally ambiguous (it rotates clockwise or anti-clockwise), even as a static image it’s also depth ambiguous.  So the silhouette above can either appear just like the upper left dino in the picture, head nearest to us, or a bit like the lower centre dino, head furthest away.  Both ambiguities – rotation and depth – can only flip at the same time.

But note that the silhouette and the picture of the dino with head more distant are not quite the same.  That’s because the dino extends so far into depth that it shows perspective effects.  First, with reversal our viewpoint slightly changs.  But more strikingly, when we see the silhouette dino with its head furthest from us, the head looks far too large.  That’s because of the size constancy effect – we tend perceptually to enlarge the size of more distant objects.  So here, when the head is seen as more distant, it also appears far too large, and the tail, conversely, too small.

That also applies with the moving version, when the silhouette dino is seen as revolving anti-clockwise:  its head seems to shrink as it rotates towards us, and to expand as it rotates away.


Watch your step!

 We’ve posted on illusions in architecture before, but they’ve been historic ones, in Roman mosaic floors and in so-called trompe-l’oeil ceilings.  Here’s a brilliant recent example, (2015) from Jamie Fobert Architects, for London’s luxury shopping Burlington Arcade.  The arcade was opened in 1819, when modern shopping was just beginning to be a boom activity.  For a description of the new floor project by the architects, click on

A friend just told me there are also eye-popping illusion floors in some of the bedrooms of the imaginatively named Seaside Park hotel in Leipzig.

A Shrink and Swell Illusion

We  just got into the final top ten entries in the annual Best Visual Illusion of the Year contest.  We didn’t get in the top three, (in fact, we seem to have come last … ) but it’s great to have made the top ten.  The movie here is a different version of our competition entry.

The illusion is that the objectively static sides of a V-shaped window appear either to expand or to contract horizontally. Figures within the window, expanded at the top and squashed at the bottom into the V-shape, rise or fall at constant speed.

At the end of this version, we show that we’ve actually found three fairly different ways of producing this illusion.  We found them by studying the reflections seen in novelty illusion rings called Witch Rings as they rotate.  We posted about using animations to imitate effects seen in the reflections, in 2013, and then in 2015 and 2016.  But now we think the illusion we’ve found in our animations, though an interesting discovery, only makes a small contribution to the very vivid illusion seen in the rings in the real world – the secret of the rings remains mysterious!

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The Oxford Compendium of Visual Illusions

I’ve copied this beautiful demo (with small changes) from one by Stuart Anstis, who is one of the world’s leading and most prolific researchers into illusions.  His website includes a page of great movies, including this one.  Whilst the yellow circles are visible, we tend to focus locally on the pairs of spheres, each pair orbiting a central point.  But without the circles, loosely fixate the central blob, and though the movement of the spheres remains just the same, they appear to re-group into a more global view, of two pulsating, intersecting circles of spheres.

I came across Stuart’s movie amongst the many web pages of figures and demonstrations that accompany a once-in-a-generation, landmark publication, the Oxford Compendium of Visual Illusions.  (It’s not cheap – check the price before ordering!).  But that’s because it’s HUGE, with some 800 pages.  Almost all the leading researchers in the field worldwide have contributed, with essays on the history of visual illusions, up-to-the-minute, detailed discussions of a comprehensive range of illusions and effects, and philosophical essays on whether the word illusion is really the right term to describe them.


Johann Joseph Who?

This may not look a dramatic illusion by contemporary standards: the top and bottom lines are each divided into three equal segments, but the middle segment appears longer than the flanking segments in the top line, and vice versa below. Yet it’s a really remarkable figure. It’s one of sixteen in a pioneering paper of 1855, the first ever study of illusions of this geometric kind.  Even more remarkably, as an illusion it remains to this day wholly unexplained, as baffling as it is simple.

The paper was by a Frankfurt schoolmaster called Johann Joseph  Oppel.  In it he named his illusions as geometric-optical illusions, to distinguish them from illusions observed in the natural environment such as the Moon illusion. Oppel seems to have observed the misjudgments that these new geometric illusions give rise to in math lessons, when his pupils were drawing or judging figures on the blackboard.  I suspect he was probably an unforgettable but demanding teacher.  He was for sure a real one-off – a tirelessly curious observer, leading the way into acoustical and language research, as well as geometric illusions, with fearless independence.

Four of us, Nicholas Wade, Dejan Todorovic, Bernd Lingelbach, and I, have just collaborated on the first ever translation of Oppel’s 1855 paper into English, with a commentary, freely available in the online journal iPerception.
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Trompe (or Trump?) l’oeil

In the era of Trump it’s got harder to tell reality from illusion, but here’s a ceiling from about 300 hundred years ago that shows a real magician in the illusion line at work.  The inside of the dome  – everything inside the gold circle – looks as if it extends upwards, but it is just painting on a flat canvas.  It’s an example of so-called trompe l’oeil, French for deceive the eye.  I don’t know why we use a French term – it was never a French speciality.

Some time back I posted about the brilliant trompe-l’oeil by Andrea Pozzo on the ceiling of a church in Vienna, painted a little over three hundred years ago. The photos here show a ceiling in the UK painted thirty years later, by an artist following where Pozzo led the way.  It’s in the main entrance hall of a mansion called Moor Park, in the West London suburb of Ruislip, painted in the 1730s. The mansion is now a posh golf club, so it’s not usually open to non-members, but it’s visitable by arrangement,

The painting of the hallway was done by a trio of Italian specialists in architectural decoration – Giacomo Amiconi, Gaetano Brunetti and Francesco Sleter.   I’m not sure whether Brunetti or Sleter painted the illusory dome.  They’re not well known.  Painting this kind of thing was quite lucrative, but didn’t make you a star in the art world of the time.

Like the dome painted by Pozzo, the illusion in Moor Park is amazingly successful – but only from one viewpoint.  As you can see in the lower photo, from everywhere else the dome looks wonky.