A data processor of a processing system, such as a graphics processing system, converts an input data value into an output data value by approximating a function which maps input values to output values. The data processor approximates the function using first and second predetermined ranges of values which are quantized into plural corresponding pairs of range sections, a predetermined gradient for each pair of range sections, and predetermined section end values for each pair of range sections. By using these predetermined parameters, the approximation of the function can be implemented efficiently by the data processor of the processing system.