Concave or convex?

Facebook Frome is an exhibition created by the Belgian artist, Hans Borgonjon, who is creating a wall of porcelain faces at the Silk Mill Studios, life cast from a cross-section of the people of his home town of Frome, which is part of the 2012 Frome Festival. This particular section of faces created a very clever optical illusion - are they concave or convex? Go large to see if you can work it out!
More photos here.