[johncolanduoni]

You mean the piano and math pictures? The finish on those surfaces makes both of those easily possible (I also don't think that countertop is marble). You can see the reflection of a white light on a finished marble countertop. Black under ambient light != always black, even under a directed light source.

[TheOtherHobbes]

The finish is the problem. Glass is a clean, smooth surface for touch. Walls and wood aren't. The odd tap is fine, but any app that relies on a lot of dragging is going to feel unpleasant, for at least some users.