Illusions and visual special effects – explanations and tutorials

Optical Illusions

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The vertical/horizontal illusion

December 30th, 2008 by david


This aimiable looking old guy was Austin Crothers, governor of Maryland USA in the first years of the last century, and a notable scourge of deception and corruption.  His top hat however presents one mysterious deception that even he couldn’t unravel.  It looks to me about as wide as it is high. But now look at the measuring rod, first when vertical, and running the full height of the hat.  When horizontal, at the foot of the picture, we can see that the same line stretches only a touch over two thirds of the way across the width of the hat. The hat is MUCH wider than it is high.  It’s an example of the horizontal/vertical illusion – we tend to overestimate height. Check out pictures of the St. Louis Arch, seen from the front, for example.  It’s just as wide as it’s high, but looks higher.

There’s no agreement on why.  There are lots of speculations, for example that the effect arises from some adjustment to allow for the inequality between the width and height of the visual field in normal binocular viewing.

The discovery of the illusion is attributed to J.J.Oppel in 1855.  It’s usually seen in this simplified version.

It works the other way up too, and is sometimes called the T illusion.  It’s one of many illusions for which you’ll find a brilliant interactive demo on Michael Bach’s site.

It’s amazing that we’ve made so little decisive progress with simple illusions like this one, after more than a century.  I can’t think of another area of science in which progress has been quite so hard, except of course some areas of maths.  But with these illusions, the explanations proposed in papers from over a century ago are sometimes much the same as those we are still discussing today.

The photo of Crothers is from the Grantham Bain collection in the Library of Congress and can I believe be used without copyright restrictions. 

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Improved artworks no.2

December 24th, 2008 by david

On the right, with apologies to Eduard Munch, I’d like to propose an improvement to his famous picture The Scream.  In my version, the screamer really does have something to scream about:  he’s holding up a duplicate of his own head for inspection.  But which head is the one that’s attached to the body, and which is being held up for inspection?  You can make it work both ways, with the upper head looking down on the lower, handheld one;  or, as if the whole figure was leaning over to the right,  with the lower head looking up at the handheld upper head.  It’s another example of the effect in the Mask/Skull illusion, and in Improved artworks no. 1.  I think I invented it, with a hint from Picasso (see the Mask/skull post).  But I’ll be delighted if you prove me wrong by finding an older version. 

No problem about the title for the improved version, it would have to be The Screams.

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Rotating Heads – Santa turns into Ibsen (more or less)

December 17th, 2008 by david


Here’s a rotating head illusion for Christmas.  I’ve been giving talks about Christmas imagery, and sometimes use old fashioned transparencies.  Recently I glanced at my slide of Santa upside down, and there was the face of the great Norwegian playwrite of a century ago, Henrik Ibsen.  It’s an illusion in the tradition of the one I posted earlier, about two characters called Mr. and Mrs. Turner.  (That post includes an animation). There are lots of other pictures of rotating heads by nineteenth century illusion artists.



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Figure/ground artistic effects

December 12th, 2008 by david

It wouldn’t be easy to find two more different artefacts than these.  On the left is a silk velvet embroidery made in Italy about five hundred and fifty years ago.  On the right is a shield from Koave in Papua New Guinea, made in the last hundred years.  Yet they both use exactly the same graphic device, a figure/ground effect.

Read the rest of this entry »

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Kitten with query bubble

December 4th, 2008 by david

This isn’t an illusion, more a special effect, but I just like bubble pictures.  The question mark soap bubble started out as two photos of real bubbles.  In a later post I’ll go into my the ways I use Photoshop to distort and adapt the bubble images.  For how I take the bubble photos, and who else is doing it, see my earlier posts on bubble pictures, by just clicking on Soap Bubble Pictures in Categories, to the right.