I like it!
I have thought for years about coming up with some sort of large dot matrix printer gantry that could paint murals on walls. You have identified a wonderfully simple and inexpensive approach. I wonder how the dry erase markers would work on painted drywall?
Well, unless the "mural" is going to be a single color, single continuous line affair, you need a little more:
- A way to pull the marker off the wall while moving it. A third, "lifting" stepper and line might accomplish this.
- A way to accommodate multiple colors. Mounting them sequentially might do the trick, if reasonable alignment can be achieved.
- Software to parse and scale an image, and translate it to mechanism moves.
While there are lots of ways to do this with more complexity and varying degrees of performance, I applaud the elegant simplicity you describe.