This cart allows you to create simple "Magic Eye"-style stereograms: 3D images hidden inside a field of seemingly random dots.
Create stereograms by drawing a depth map. This is a picture where different colours signify different distances from the object to the camera. When you're finished, select "make stereogram" to turn your depth map into a stereogram.
These stereograms are designed for parallel viewing. Relax your eyes and allow them to diverge, as if looking through the screen at something much farther away. The two black dots at the bottom of the picture should become three dots, with the middle one in focus. Then the hidden 3d image will emerge. If you're having trouble seeing the pictures, you can search online for "how to view stereograms" for lots of detailed explanations.
The algorithm for creating the stereograms comes from Inglis, S., Thimbleby, H., Witten, I.H. (1994) Displaying 3D Images: Algorithms for Single-Image Random-Dot Stereograms, IEEE Computer, 27(10), pp.38-48.