Like those ridged images that appear to animate as you tilt them back and forth — on photo postcards, bookmarks, rulers, books, toys, and more — watch as this cake icing seems to change colors as it rotates.
You take your two different images and load them into a computer graphics program. The program cuts each image into dozens of thin strips and weaves them together so the strips from the first image alternate with the strips from the second. This process is called interlacing. If you look at the doubled-up image printed this way, it’s just a horribly confusing mess, but not for long! Next, you place a transparent plastic layer on top of the doubled-up image. This is made of dozens of separate ridges called lenticles. Half of them lean to the left and half of them lean to the right, but they’re arranged so they alternate: left, right, left, right. That means, when you look at the image from the left, the left-leaning lenticles show you only half the printed strips—the ones directly underneath them—so you see only the first of the two images. When you look from the right, you can’t see through these left-leaning lenticles at all. Instead, you see only the strips under the right-leaning lenticles—and the second image. Move your head back and forth and the image flips back and forth too like a kind of “visual see-saw”.
…And on a cake, the trick is in how the carefully-made sharp peaks in the buttercream icing are airbrushed. Here’s the color changing cake tutorial from YouTuber Charlotte Sometimes, who made the cake above: