The present invention provides methods, apparatus and articles of manufacture for performing a transform equation in which at least two sub-transform equations can be identified. According to the invention, each sub-transform is considered separately and the constants of each sub-transform are replaced with an approximation which is a function of an integer value and a second value, where the second value is common to each constant in a sub-transform. The transform equation is then performed with the integer values in place of the constants. As a result either the input to the transform equation or at least one of the result of the transform and a value to be compared the result, is modified with a value which is a function of the second values for each sub-transform. By considering each sub-transform separately the second values of each sub-transform can be different thus allowing more accurate approximations to be used with minimal degradation in performance.