Sometimes Andrea Mantegna was just showing off. For the Palazzo Ducale in Mantua, he painted a false ceiling above the Camera degli Sposi. Around a great trompe l’oeil oculus, apparently open to the sky, assorted gawpers and cherubs lean nosily over the parapet: ‘What’s going on down there, then?’ Only the Duke and Duchess of Gonzaga entertaining their friends from Ferrara.