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.
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?
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!
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.
This is a post to show animations to accompany a poster, which I and my colleague Priscilla Heard presented at ECVP 2016. The poster reported experiments related to the witch ring illusion. The movies below show illusory effects of sideways movement in streaming patterns of dots.
This first movie shows how the static track along which a single file of dots are travelling appears to move sideways when the single track is embedded in a fan-shaped pattern of tracks.
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.
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.
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.
Everyone loves a kaleidoscope, particularly the ones with a lens at the end, so that as you look through them whilst sweeping the kaleidoscope around, the view becomes a dazzling starburst pattern. (I find Nova Magic Marble kaleidoscopes are inexpensive ones for kids that work pretty well). However, real-world kaleidoscopes can only tile the visual field with a limited repertoire of geometric shapes – typically triangles. Digitally we can tile with any shape that will tessellate – that is, fill the plane by repetition without gaps or overlaps. As with real-world kaleidoscopes with a lens at the end, each tile can enclose a streaming segment of a visual scene, if you are handy with graphics and 2D animation packages. If that all sounds a bit puzzling, I think the movie will make it clearer.
But then there’s a surprise! Illusions of movement may appear, dependent on figure/ground effects.
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 ….