This is a custom watchface for the Pebble smart watch. Download it.
It looks like this:
Although it may not be immediately apparent, there are 4 large digits on this face. The background consists of lines going one direction. The foreground, or the digits, has lines in the other direction. The edges of digits appear as corners in these lines, so where ever you see a corner, try to follow it to see the rest of the edge of a digit. Once you can see one, it shouldn't be hard to find the rest.
Thanks to RichardG for the assets. Based on the Kisai Optical Illusion watch from TokyoFlash.