Category Archives: Optical Illusions

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.  I’ll be posting later about what we think our illusion shows that’s new.

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.  Have you got the maths ability to crack it?

 

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 http://jamiefobertarchitects.com/work/burlington-arcade/

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, http://www.moorparkgc.co.uk/

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.

William Wollaston’s Gaze Illusion

The eyes in each pair of pictures don’t change at all, and yet in one picture in each pair they seem to look directly at us, whilst in the other they have rotated downwards. (featured: a Sportive Lemur; & a young man by German 16th century sculptor Michel Erhart in the Victoria and Albert Museum, London).

William Wollaston published this illusion nearly 200 years ago. He claimed that our brains seem set up to judge the direction in which the eyes are looking in relation to the face from the position of the pupils in relation to the whites of the eyes, but that direction of gaze depends on head orientation. You can read his original paper in the Philosophical Transactions of the Royal Society online. But here’s an animation of the original illustrations:

The original drawings were done for Wollaston by the leading portrait painter of his day, Sir Thomas Lawrence.  They are now in the Royal Society in London, and there’s a movie about them you can watch on Youtube.

A variant on the “Witch Ring” Illusion

 

Here’s another version of the illusion I’ve been working on with my colleague Priscilla Heard.  The bright V shaped zones imposed on the faces look like they are expanding or contracting sideways, especially near the sharp end of the Vs at the bottom.  It can be hard to see that it’s an illusion, because the faces really are expanding.  But to the right an isolated bright zone shows that objectively the V tracks are quite static.  You may still see a bit of illusory lateral expansion even in this isolated set of tracks, but note that the top right corner of the track is quite static in relation to the corner of the movie.

It’s a version of the Witch Ring Illusion.   I posted about that back in 2011, noting a plan to take a look at it.  This year Priscilla and I published a paper about it in the journal Perception.

Best Illusion of the Year Competition

Here’s a copy with slight variations of a stunning new animation of the Ebbinghaus Illusion, by Christopher Blair, Gideon Caplovitz and Ryan E.B. Mruczek.  Their version won the Best Illusion of the Year Competition in 2014, a few weeks ago.  It’s a brilliant competition whose lead organiser is Susanna Martinez-Conde, and is accumulating a fascinating illusion resource as the ten finalists are added each year.

In the movie, as the figure moves up and down the screen, all the circles seem to change size.  Yet objectively only the outer ring of circles do so:  the central circle remains exactly the same size throughout.  It’s so vivid it’s hard to believe, but I’ve just added some yellow rails as a track for the central circle.  You can see that the circle always just fits the rails – and they don’t change size.

For more info and links on the Ebbinghaus illusion (aka Titchener Circles) see our earlier post on the traditional, static version.