1. A method for specifying a shader program via an application programming interface (API) of a computing system and for transmitting the shader program to a procedural shader computational subunit of a graphics subsystem of the computing system, whereby said procedural shader computational subunit becomes programmed to perform according to said shader program, the method including: specifying, via the API, at least one instruction of said shader program that computes a rate of change of at least one value processed by the procedural shader computational subunit when the shader program is executed by the procedural shader computational subunit.