1. A method comprising performing a selected transform equation having a plurality of sub-transform equations, using pre-selected integer values in place of the transform constants, to produce a result; and modifying at least one of the result of the transform equation and a comparison value to be compared with the result, using a pre-selected value, wherein each constant of a sub-transform approximates to a function of the pre-selected integer used in its place and a second value which is common to each transform constant in the sub-transform, and wherein the pre-selected value is a function of second values associated with each sub-transform.