1. A method of operating a graphics processing system that includes a vertex shading stage which processes values of input attributes to the vertex shading stage to generate values of output attributes of the vertex shading stage for use by a subsequent stage or stages of the graphics processing system, the method comprising: the graphics processing system associating with one or more of the input and output attributes of the vertex shading stage, meta-information to be used when loading or storing values of the one or more input and output attributes, wherein the meta-information indicates a technique to use for, and any data necessary for, deriving from indices associated with values of the one or more input and output attributes, derived indices to be used to load or store the values of the one or more input and output attributes; and a graphics processing stage of the graphics processing system, when it is to load or store a value of an attribute of the one or more input and output attributes: reading the meta-information associated with the attribute, wherein the meta-information indicates an attribute type, wherein the attribute type is a linear, divisor or modulus attribute; determining a technique to use for, and any data necessary for, deriving from an index associated with the value of the attribute, a derived index to be used to load or store the value of the attribute using the read meta-information; deriving from the index associated with the value of the attribute, the derived index to be used to load or store the value of the attribute using the determined technique and any data; and loading or storing the value of the attribute using the derived index.