Illusions and visual special effects – explanations and tutorials

Optical Illusions

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Roger Shepard’s Arc de Triomphe revisited

December 29th, 2013 by david

Arc de Triomphe illusion


In 1990 the psychologist and artist Roger Shepard published a cartoon version of this effect, captioned “I stand corrected”, in his book Mindsights (page 91).  I wanted to try a photo processed version of it and here’s my second attempt.  When I tried before, back in 2008, I somehow couldn’t get my mind round what Shepard had done, and produced an even more twisted version.

M.C.Escher’s lithograph Belvedere from 1958 is famous variant on the theme.  Subsequent investigators have presented animated 3D versions of it that help explain the effect.

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Bars and Bands – illusory expansion

August 16th, 2013 by david


When the chevron pattern in the movie is in perspective, so that the bars get thinner and closer together with distance, the bars and the fan of bright bands on them appear to stream past us, as if we were travelling along a tunnel. When the bars are all the same size and equally spaced, so that they don’t show perspective depth cues, the fan of bright bands appears to be expanding. The outer bright bands even look as if they are sliding along the bars.

Thanks to Priscilla Heard for the suggestion that the key to the expansion effect is in the absence of perspective cues. If you’d like more on that ….

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The Barber Pole Illusion

July 3rd, 2013 by david

Go back a couple of centuries and there were no chains of shops or malls. In the high street in the UK you would have found the type of shop you were after by looking out for a sign hanging out.  There were signs for pharmacists, tobacconists, pawnbrokers, whatever.  Nowadays there’s just one traditional sign still sometimes to be seen – the barber’s pole, as left in the animation.

The barber’s sign shows a famous illusion.  The cylinder is rotating horizontally, but the stripes look as if they are rising – which would be impossible, unless you had some long pole sliding through the cylinder.

You can begin to see why in the demo on the right:  focus on the vertical slot and the grating seems to be moving vertically (as in the barber’s pole).  But focus on the horizontal slot and in a moment the grating may seem to move horizontally.  Behind the round hole, for me it tends to look as if moving obliquely.

It’s a demo that depends for a start on a fact I find counter-intuitive:  if a grating of oblique lines passes behind a small hole in a mask, (an extensive grating, so that the edges of the grating never appear in the hole), the lines stay objectively the same as they traverse the hole, whether the grating, at the orientation shown in the animation, is moving horizontally to the left, obliquely up and to the left, or vertically upwards, (or conversely, as not shown here, horizontally to the right, obliquely down and to the right, or downwards).

That leaves the brain struggling to guess which way the grating is actually travelling, and being pretty indecisive about it.  In the animation, when the mask goes a bit transparent, I find that if I focus on the vertical slot the whole grating tends to appear to move vertically.  Focus on the horizontal slot and after a moment the grating moves horizontally.  Behind the circular hole, the grating tends for me to move obliquely.

So behind the illusion first of all is an objective fact, that the stimulus the grating presents as it passes behind the slots remains constant even when its true direction of movement changes.  But then, why does the grating appear to us to move consistently vertically in a vertical slot, and horizontally in a horizontal slot?  The short answer is, nobody is quite sure. For a start, the cell groups which detect movement in the eye only sample small receptive fields, compounding the effects of the mask.  So effects of movement across the visual field, even without the mask in this demo, have to be stitched together in the brain.  In this illusion, the stitching seems to follow signals from the edges of the slots, where the moving ends of the hatched lines do objectively run along the orientation of the slots.  But as ever, not all the effects we see in a careful study follow such a simple rule as that.  Just how the brain finally puts it all together is much, much more subtle.  If you like getting seriously technical, try this paper by Nick Fisher and Johannes Zanker from 2001.

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May 19th, 2013 by david

Rex Whistler heads from AHA!

Some of the best of all illusions in the tradition of rotating heads were designed for advertising in the 1930′s by British artist Rex Whistler – you really have to take a second look to convince yourself the lower faces are just rotations of the upper ones. He was sadly killed in action in World War Two, but the heads were collected in a book of 1979, AHA.  He got the idea from some seventeenth century engravings, (reproduced below), which had first appeared in 1671 in a book by polemicist Pierre Berault. The Western, Christian world at that time was riven with hatred between Catholics and Protestants, and these images are an anti catholic salvo, showing a Pope (left pair of roundels below) and a cardinal (right pair of roundels below) transforming into devils with rotation.

I found these details in two editorials about rotating heads for the journal Perception, by perceptual scientist and artist Nick Wade and colleagues.  Check them out for lots more info and images. One is from 2003, the other from 2005, and they are the most authoritative source of information on rotating heads generally.

17th Century and Roman rotating heads

In the 2003 paper Nick Wade also shows one of the oldest rotating heads we know, a second century AD Roman beaker, shown to the right above. It was spotted by Christine Wade in the Hungarian National  Museum in Budapest.  (Photo © Christine Wade)

There are earlier posts on this site about rotating heads, one with Father Christmas turning into playwright Henrik Ibsen, another about a tale of nightmare in a hotel. And if your appetite for this stuff is insatiable, look at the more recent post on cartoonist Gustave Verbeek.

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Dark Kanizsa Triangle

March 8th, 2013 by david

Here’s a dark-on-light, bubble version of The Kanizsa triangle.  The triangle is usually shown in white against black circles and lines, and can even look slightly brighter than background, though its edges are only indicated by the gaps in the lines and by the segments missing from the circles.   The brain adds the edges and fills in the triangle, as the most probable explanation for what’s missing.  The effect was created by Gaetano Kanizsa, as a demonstration of subjective contours, which in turn were first explored a bit over a century ago, as examples of Gestalt theory.  Bit of a link for enthusiasts that – ditto the following links – but if technical stuff is for you, there’s a great historical survey of the theory.  The theory as then developed is not now accepted, and just how the brain reconstructs the triangle is still debated.

Like many geometric illusions, and like the watercolour illusion (see recent post), the Kanizsa triangle also appears when reversed out as a black shape against bright lines and segments.  So here I’ve recruited some soap bubbles as a background to the effect.

As with other pictures on the site, unless an image is externally sourced, and we indicate that copyright may be asserted, you are welcome to download this images and use it for any private, non-commercial purposes.  But if you might like to buy a professionally produced print, check out our Cafepress site.

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Poggendorff Switch

January 3rd, 2013 by david

If the way I see this animation is how most people do, the strength of the Poggendorff illusion can depend on our patterns of fixation when looking at it.  Adding distracting dots to the figure can attract the eye either obliquely along the parallels or at right angles across them.  To my eye, when the oblique track between the acute angles in the figure is labelled with flashing blobs, the strength of the illusion is reduced.  When the track at right angles across the parallels is labelled, effect is maximised.  The effect doesn’t change instantly for me, but settles down after each track has flashed two or three times.  I get the same effect if I switch fixations every second or so between equivalent blobs in still Poggendorff figures. The effect is strongest, as below, when the blobs are in the acute angles when the parallels are vertical, and across parallels when the parallels are horizontal.  So in the figure below, the illusion is not far off equal strength for me in the bottom pair of figures, but looks maybe a bit stronger at top left, and has almost vanished at top right.

If you’d like more on this, plus some additional demos, check out my site devoted to the Poggendorff illusion.

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The Watercolour Illusion in Reverse

November 30th, 2012 by david

Here’s something I’ve not seen tried yet elsewhere:  a look at how the watercolour illusion (see three posts back) appears when reversed out, black to white.

Update 16/12/12:  My mistake! Here’s a report of an earlier study that does include reversal of the illusion.  It’s by Aula Dostoevsky and Ken Knolbauch.

In the original effect, as seen to left, islands with two-tone outlines seem tinted with the colour of the inner tone, if lighter than the outer, and otherwise appear (as in the lower figure) whiter than the background.

Sure enough, as seen centre, the effect is still there, but a bit weaker, when exactly the same two-tone outlines are placed on black.  The upper interiors, which seemed tinted on white, appear (to my eye) blacker than the black background.  The lower interior, which looked whiter than background on white, now appears tinted.  In other words, on white the haze spreads from the paler outline, when the darker outline encloses it, whereas on black, the haze spreads from the darker outline, and when enclosed by a paler outline.

Consistent with that, to my eye, on the right when the colours of the two-tone outlines are reversed on black, the interior that appeared tinted in the centre lower figure appears blacker than background.  The upper figures, with interiors blacker than background in the centre, look tinted to the right.

The effect is strongest for me in the three lower figures, looking across the whole image.  (Ignoring the Santa figure, added for seasonal effect).  For me the effect also works best slightly larger, so click on the figure to see a larger version.

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Barrier-Grid (or Picket-Fence) Animation

October 30th, 2012 by david

I came across this movie technique on the fun illusion website  It seems to have been around for decades, but I’ve not nailed down its history.  A beautiful recent booklet of barrier-grid animations is Colin Ord’s Magic Moving Images.  Recently a new, more sophisticated version of the technique has been patented by Rufus Butler Seder, called Scanimation.  There’s a Youtube movie of him explaining it, as well as a fascinating, more technical description on the design site DesignBoom.  He’s also published a number of great books for kids using his process.

You’ll often find it stated on the WWW that barrier-grid animations and Seder’s Scanimations are the same process, but, as above, there’s a patented difference.  (And then just when you thought you thought you’d got it all straight, there also used to be an entirely separate early computer animation machine, in the 60′s, called The Scanimate).

In Seder’s and Ord’s books, the illusion happens in the real world, when you pass a real striped acetate mask over the composite base image. My demo is an animation that faithfully follows the real world process, as if a real striped mask was being passed over the base image.  I borrowed the jumping man from this composite photo by nineteenth century movement scientist Etienne Marey.

To make a barrier grid animation, you reduce the subject in each ‘frame’ of the original movie into a black silhouette, and then replace the black infill with a hatching of just a few vertical lines (turning the outline into a subjective contour, if you like a bit of technicality).

The hatched silhouettes are then combined into a composite image, like the one in the animation at the head of the post.  As the striped mask passes over it, only one frame at a time is revealed.  It’s fascinating that whilst the jumper in the final, striped silhouette above is barely recognisable as a figure, we see the jumper much more clearly in the movie. I find it a bit magical the way the jumper quite gradually appears in the movie as the mask begins to move over the composite image from the left. Our brains can discover figures in patterns of amazingly sparse data, if only they move coherently, as when the human body is represented just by dots at the key rotation points (such as knees and elbows).

Want to have a go at making a barrier grid animation yourself?

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How do you see this? (comments would be really helpful)

September 22nd, 2012 by david

How do YOU see the figure above?  It would be really helpful to have your comments, preferably before you read on.

Update 28/Sept/2012  Many thanks for observations – I’ll hold off publishing them to avoid influencing subsequent observers!

So what’s it about?

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The Watercolour Illusion

August 20th, 2012 by david

I’m currently giving a hand with an exhibition on the science and art of illusion, called IllusoriaMente, which will be on in Alghero, Sardinia (italy) in the first week in September, in association with the annual European Conference on Visual Perception.  During the preparations I’ve come across this illusion, which I’d heard of but not checked out.  It’s beautiful and extraordinary.  The areas bounded by three wiggly loops in the picture look different colours, as if just tinted with watercolour, but the effect depends on the brain doing the colouring in.  Each boundary is made up of a dark and a lighter strip.  The infilling always takes its cue from the colour of the lighter strip, and always in the direction from the darker strip towards and beyond the lighter strip.  So in this image, the effect is a bit paradoxical.  The colour appears inside the two islands on the left, in comparison with the white background.  Yet to the right the colouring is outside the island, with the island itself looking a brighter shade of white than the overall background.

Two different processes seem to be co-operating, a colouring effect, and a figure/ground effect, enhancing the separation between the surrounded areas and the background.  If you like a bit more technicality, there’s a Scholarpedia article, with references to academic papers, edited by Baingio Pinna, one of the pioneers researching into the effect (and the organsiser of this year’s ECVP conference in Alghero).

Note added at 10/12/12:  check out our slightly more recent post on the watercolour illusion in reverse.

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