Creating More Complex Man-made Materials - Blender 2.6 Cycles: Materials and Textures - page 202

How it works...
From step 2 to step 6 we built the basic shader of the sponge material and the color.
As you can see in the Rendered camera view, without the bump pattern, there is a
visible artifact in the more distant side of the mesh; this is due to the Smooth shading
we have set at step 13 of the Getting ready section of this recipe; setting the shading
to Flat again will remove the artifact, but will show the blocky faces of the deformed
sponge mesh. By the way, because of the bump pattern and of the fact that the mesh
is subdivided, this is not trivial and anyone of the two solutions (smooth with artifact
or flat but blocky) is fine.
From step 7 to step 20 we built the bump pattern.