Illusions and visual special effects – explanations and tutorials

Optical Illusions

Archives Posts

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.

Archives Posts

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.

Archives Posts

AHA!

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.

Archives Posts

Gustave Verbeek

July 4th, 2012 by david

Gustave Verbeek cartoon

About a hundred years ago one of the most popular newspaper comic-strip artists in America was Gustave Verbeek.  He contrived whole pages of pictures telling cartoon stories, which showed one sequence of scenes when viewed one way up, and the following set when turned upside down.  His best known adventures were those of Lady Lovekins and Old Man Muffaroo, each of them, as in the scenes above, always the inverse of the other.  His stories are so crazy and his drawings so imaginative that it can take a moment to realise one scene really is the exact inverse of the other.   His imagery is surrealist – long before surrealism emerged with artists like Salvador Dali in the establishment art world.

His cartoons have recently been reprinted (not cheap!)

Verbeek was developing an earlier tradition of rotating heads illusions, in which a head has one identity one way up, and another upside down.  See my first earlier post of that, with an animation, and another example with Santa turning into playwrite Henrick Ibsen

Archives Posts

Native Canadian Ambiguous Art

January 31st, 2012 by david

Depending how you see it, this panel either shows two killer whales looking at one another in profile, or a bear looking at you head on – the killer whales’ fins top centre become the bears’ ears.  And then the killer whales’ tails centre bottom become the mouth of a third creature, probably a frog, but the carving was never finished, so only its eyes are carved.

The panel is probably about a hundred years old, and is about five foot wide (1.6 meters or so).  It’s not known why it was never finished, but in the tradition of north west coast Canadian carving, the decoration on the killer whales would have been carved into deeper relief, not just sketched out on the surface.  Nor is it known from just which ethnic group in North West Coast Canada it comes.

However in the mythology of all the groups of that area, such as the Haida and the Kwakuitl, transformations of one creature into another are part of the scheme of things.  That includes transformations by hunting and eating, which were traditionally understood as sacred activities.  So what we see here is not just a visual game, but has spiritual meanings.

For more on that see our earlier post on ambiguous patterns, and other posts in the Illusions and Aesthetics category.

The panel is in the reserve collection of the Manchester Museum.

Archives Posts

Hybrid Portraits

November 25th, 2011 by david

Hybrid portraits superimpose one portrait on top of another, so that one appears with close viewing, and the other emerges with more distant viewing. Or with reduced image size. Or just by taking off your specs, if you are just a little way away and are seriously short-sighted. Whatever works for you, here Charlie Chaplin becomes Queen Victoria. The “top” image is filtered to reduce it so that it resembles an outline drawing, and then made partially transparent (and some viewers might need reading glasses to see it). The ‘back” image is blurred (and may only become apparent when viewed from two or  more meters).

The technique was invented by Aude Oliva, Antonio Torralba and Philippe G. Schyns, and presented in 2006 at the huge annual Siggraph (electronic graphics) conference.  Putting it technically, Charlie here appears with all but his high spatial frequencies filtered out, whilst her late Majesty has had all but her low spatial frequencies filtered away.

But don’t be deterred by the jargon.  If you’d like to try your own hybrid portraits, you can do the filtering very easily if you have a full version of Photoshop to play with, (so long as you’ve got the hang of working with layers).   This is how it works in Photoshop CS2 for Mac.  For the high spatial frequency (outlines) image, with an image file open and the image you want to filter selected, go to filter in the menus at the top of the screen, select other.. (at the very bottom of the list of options) and then high pass.. Then play around! You’ll see how you can transform the image with a slider. For the low spatial frequency (blurry) image, start in a new layer, and once again with the image to filter selected, go to filter again, but this time select blur, and then in the options that open up Gaussian Blur. Once again, then just play around with the slider to explore effects. Then you need to make the top image transparent. Make sure the Layers window is visible (click on layers in the Windows menu at the top of the screen if not). Next make sure the layer holding the top image is selected. Now you can adjust the transparency (they call it opacity) at top right in the layers window.

That’s the easy bit. Once the image pair are more or less presenting the effect OK, adjusting the degree of filtering of the images, their contrast, and then the transparency of the ‘top’ image to find the demon tweak that gives maximum effect will drive you crazy! There is a colossal range of possible combinations.

For some great movies of the effect, try this (MIT) site.

Archives Posts

Hidden Message

August 3rd, 2011 by david

 

Can you read the message encoded in the image?  A few months back I posted about embedding hidden messages in images. Since then I’ve come across a much better way of doing it, using the lettering in the image above.  Lettering? What lettering?  You may not even have been able to spot the lettering yet. I’m not sure who invented it, but it’s brilliant – I just copied the style of lettering from another demo, on the wonderful website of Michael Bach.  It’s clever because to read the message we have not only to achieve a figure/ground reversal, but also because the distracting objects in the picture are seen as if from above (the default view the brain expects), whilst the hidden lettering is seen from below.  So we have to switch two modes of viewing, figure and ground, and also view from above and view from below.  And then I’ve added scene cues to make it even harder.

 

However there is a way of revealing the message easily – just blur the image, as in the version below.  Without all the distracting detail in the sharp edges in the scene, all that’s required is a figure/ground reversal, and recognisable letters become the most salient features in the image for the brain.

 

 

In my first post on this question, I showed a straightforward exchange between the words Truth and Lies with reversal of figure and ground.  I wanted to tweak that, to add the additional reversal between letters seen from above and from below.  To see that ….

Read the rest of this entry »

Archives Posts

Judith with the Head of Holofernes – or is it the other way round?

July 4th, 2011 by david

Who owns the body?   Judith does to start with, but then Holofernes does, and finally, it’s ambiguous.

Here’s a new addition to our series of ambiguous improved artworks.  Apologies this time are due to Rubens.  I got the idea for these illusions from a print by Picasso.

For a downloadable still of the end of our animation …

Read the rest of this entry »

Archives Posts

Arcimboldo exhibition in Milan, Italy

January 14th, 2011 by david

Make whatever sacrifice you have to, but unless you were in Washington last Autumn and caught the National Gallery of Art’s exhibition of the work of Arcimboldo there, seize any chance to travel to Milan, Italy, and see it there from January 27th to May 8th 2011.   It’s coming on at the Palazzo Reale. Arcimboldo was born in Milan, but became an unrivalled magician at ambiguous images, working four hundred years ago, mostly at the dazzling court of the Emperor Rudolph in Prague.  This is his painting of Summer, usually in the Louvre in Paris, and one of a set of paintings of the four seasons.

There’s still a movie about Arcimboldo, available in various formats, to the right on the Washington National Gallery’s website for the show, along with details of a huge sculpture that was in the Washington show, of the painting Winter from the same series of the seasons, by film-maker and sculptor Philip Haas.

We know Arcimboldo didn’t invent this kind of image, if only because of a rather naughty example on a 450 year old dish in the Ashmolean Museum.  But he inspired many of the generations of artists, and later psychologists, working with the ambiguous images that you can see in our Ambiguous Images category.

Archives Posts

Truth Lies Hidden

November 11th, 2010 by david

 

A few weeks back the site got a load of US visitors from a link from a discussion on Reddit.  Someone got caught out illegally downloading at college, and as a punishment had to produce a poster about the evils of pirating.  The poster was to be inspected by the authorities and if approved, prominently displayed.  So the reluctant poster artist put out a query:  how can you do a poster that seems to say one thing, but when you look again says something very different. Whatever your take on piracy, (there is a vigorous exchange on the reddit site), the poster problem is an interesting one.  I mean, any of us might need to do a subversive poster someday.  Someone responded that pixelation might be a way to do it, and linked to our pixelation post.

I reckon that would in practice be a really hard way to do it.  Maybe a better way would be by using word combinations that are figure/ground ambiguous, like the Truth/Lies combination shown.  It would still be mighty risky though, if your effort was to be inspected, because it’s hard to tweak the ambiguity just so, leaving the subversive message hidden – but not invisible.  I tried a couple of tweaks above, trying to get a balanced result at top, to push out Truth in the middle, and Lies below.  I haven’t managed to get the recessive message hidden enough yet – not if my freedom depended on it – but maybe you can do better.

Note added at August 3rd 2o11! I’ve come up with a tweaked version of this word pair.  (Scroll down to the bottom of the link).

There are some clever examples of this kind of ambiguity on John Langdon’s site.

« Previous Entries