Almost anything can become a touchpad with some spray paint

Electric field tomography.

“It’s as simple as adding a conductive coating, such as special paint, to an object and then putting sensing electrodes around the edges of the area you want to make interactive, say the researchers. You can even add a protective coating. To sense the placement of touch, they use a technique called electric field tomography.”

https://www.cnet.com/news/electrick-touchpad-spray-paint-carnegie-mellon/