I’m not sure why it never occurred to me to do this before. I was watching Mallrats for the 20th time (or so), and I paused it when Willem is trying the magic eye poster. The poster is supposed to show a sailboat or schooner, but when you fuse the images, it is not at all a sailboat. It’s just a bunch of geometric shapes. There is a row of cones at the bottom and then two rows of what look like flowers (but not enough like flowers to be flowers). No boat - that is for sure.
I hadn’t really thought about the fact that there was supposed to be just one image. These posters rarely have just one image, because it is impossible to fuse an image that large. You would have to stand too far away, and then you would lose perception of the depth that makes them seem 3d. They usually have repeating images, spaced about 10 cm apart, which is about the distance between your eyes.