1. A method for specifying via an application programming interface (API) at least one instruction to a pixel shader computational subunit of a graphics subsystem of a computing system, whereby said pixel shader computational subunit performs according to said at least one instruction, the method including: specifying, via the API, at least one instruction that refers to a value of a face storage element of said pixel shader computational subunit, wherein the value of the face storage element represents whether a pixel processed by the pixel shader computational subunit is a forward facing pixel or a backward facing pixel.