The word PUP is easy enough to read even though the letters aren't really there. The only thing that distinguishes the letters from the background is their shadows. You are familiar with the letters so instead of seeing random black shapes your mind fills in the missing edges and you see the letters.
It's amazing how much you rely on shadows to help you see without ever realizing it. Look at the corner of a room where two walls of the same color meet. If one of the walls wasn't slightly darker because it is in shadow you wouldn't be able to tell where one ends and the other starts.
There are two kinds of shapes that shadows can help you identify, two dimensional shapes like circles, triangles and squares, and three dimensional shapes like spheres, pyramids and cubes.
Most often it is three dimensional shapes that shadows help us recognize. If you add shadows to your quilts, you start to give them depth. And that makes them more interesting.
Sometimes shadows can help us recognize two dimensional shapes like walls.
In the example above the shadows do a bit of both. They help us see the letters which are basically flat and they give depth to those letters making them three dimensional.
The letters are from my pieced alphabet Inglewood. To get an idea of how they are pieced go to my 19 April 2010 post.