Shadow mural

While staring listlessly at my bedroom wall once, I noticed quite a striking effect. It normally looks like a plain, white painted wall (which it is), but as the sun went down, for a few minutes it was shining directly along the surface of the wall. The angle of the light caused every tiny dip or bump in the wall to cast a shadow, so every previously undetectable feature became highly visible. It was also quite a fetching fiery red colour, but I digress...

My proposal is to use this effect to make a mural which is only visible at sunset. At most times of day, the wall would look like a perfectly ordinary wall. But it is covered with tiles which have had a picture printed on them with tiny dots of enamel. As the sun goes down, for a few minutes every evening the dots are thrown into sharp relief and the picture becomes visible.