A data processing apparatus is operable to determine non-linear distortion applied to an image in which template signals have been introduced into at least first and second frequency bands of the image respectively, the template signals having a predetermined relationship with each other. The apparatus comprises an image dividing processor operable to generate data representative of selected parts of the image, a transform detection processor operable to generate at least one parameter representative of a linear transformation applied to each of the parts of the image, and a non-linear transform processor operable to estimate parameters of a non-linear transformation representative of the non-linear distortion applied to the image from the linear transform parameters of at least two parts of the image. The data processing apparatus provides a facility for determining parameters of a non-linear transform representative of the distortion applied to the image as a whole. The distortion is therefore evaluated by generating linear transform parameters for different parts of the image and then determining the non-linear transform parameters in accordance with a relationship with the linear transform parameters. The linear transform may be the Affine transform, the parameters determined being Affine transform parameters. The non-linear transform may be the Perspective transform or a Polynomial transform, the non-linear transform parameters being evaluated from the linear transform parameters.