Methods and apparatus, including computer program products, for modelling a non-linear transfer function with a power law function. A transfer function is received. Iteratively, until a termination flag is set, a first power law function is received, an auxiliary function is generated from the transfer function and local differences between the transfer function and the first power law function, a second power law function is fitted to the auxiliary function, a modelling error is calculated from the second power law function and the transfer function, and the termination flag is set when the modelling error is less than a predetermined value.