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!

We also produced tried the illusion with different backgrounds and characteristics, to see how they affect its strength.

The movie in the middle is a clip from the one shown (larger) at the start of the post.  The illusion is that the objectively static sloping sides of the V shape seem to move sideways horizontally.  All around it are some of the ways of reducing or cancelling the illusion, hopefully offering some clues about what is causing it (which for now is a mystery). Top left is an empty triangle against the clouds – there’s no illusion. Centre left is the movie with no sky background – there’s slightly reduced illusion, in our judgment. Lower left the clouds are not moving, giving no illusion, we reckon. Then top right, with clouds moving in the opposite direction to the figure in the movie in the V-shape, again there’s no illusion;  centre right, with no squeeze and stretch distortion of the movie in the V-shape, again there’s no illusion.  at lower right a the figure in the V shape accelerates as he moves upwards, getting bigger and moving faster as he rises, so that the figure seems to loom out towards us as he rise. The expansion illusion is replaced with an impression of depth.