
[Tom Scott] ran across an interesting visual effect created with Moiré patterns and used for guiding ships but we’re sure it can be adapted for hacks somewhere. Without the aid of any motors or LED animation, the image changes as the user views it from different angles. When viewed straight on, the user sees vertical lines, but from the left they see a right-pointing arrow and from the right, they see a left-pointing arrow. It’s used with shipping to guide ships. For example, one use would be to guide them to the center point of a bridge. When the pilots see straight, vertical lines then they know where to steer the ship.
US patent 4,629,325, Leading mark indicator, explains how it works and how to make one. Two screens are separated from each other. The one in front is vertical but the one behind is split in two and angled. It’s this angle which creates the slants of the arrows when viewed from the left or right. We had to convince ourselves that we understood it correctly and a quick test with two combs showed that we did. See below for the test in action as well as for [Tom’s] video of the real-world shipping one.