The movie in the middle is a clip from the one shown (larger) in the previous 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 – no illusion. Centre left is the movie with no sky background – slightly reduced illusion, in our judgment. Lower left the clouds are not moving – no illusion, we reckon. Then top right, clouds moving in the opposite direction to the figure in the movie in the V-shape – no illusion; centre right, no squeeze and stretch distortion of the movie in the V-shape – no illusion; & bottom right, a more complicated one: 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.
We’ll post again on this illusion when we (or someone else) puzzles out what’s going on. But don’t hold your breath ….