1. A method comprising: (i) rendering a captured two-dimensional digital image captured by a non-volumetric imaging camera and defined by a plurality of image pixels as a non-photorealistically rendered two-dimensional NPR image defined by a plurality of NPR image pixels each having an associated NPR transparency value, wherein the transparency value of at least some of the NPR image pixels is determined in dependence upon corresponding captured image pixels; and (ii) rendering a separately derived virtual object behind the NPR image such that at least part of the virtual object is visible.