1. A computer-implemented method for specifying a shader program via an application programming interface (API) of a computing system and for programming a pixel shader of a procedural shader computational subunit of a graphics subsystem to perform according to said shader program, the method including: the computing system transmitting the specified shader program to the procedural shader computational subunit; the computing system specifying, via the API, at least one instruction of said specified shader program that is executed by the procedural shader computational subunit to compute a rate of change of at least one pixel value in image space relative to a screen display processed by the procedural shader computational subunit; and programming, via the API, the pixel shader of the procedural shader computational subunit to perform according to said at least one instruction of said specified shader program.