1. A computer-implemented method including: enabling a user to specify via an application programming interface (API) of a computing system at least one instruction to a pixel shader computational subunit of a graphics subsystem of the computer system, said pixel shader computational subunit including a face register and a pixel shader; said pixel shader processing said at least one instruction to provide processed graphics data that takes into account whether a pixel being operated upon is part of a primitive facing forward or backward; and providing the processed graphics data to the user through said API, wherein processing said at least one instruction comprises said pixel shader performing the steps of: determining that said at least one instruction refers to a value stored in said face register; and when said at least one instruction refers to the value stored in said face register, reading the value in the face register as indicative of whether the pixel being operated upon by said at least one instruction is part of a forward facing primitive or a backward facing primitive.