A method of preprocessing, prior to encoding with an external encoder, image data using a preprocessing network comprising a set of inter-connected learnable weights is provided. At the preprocessing network, image data from one or more images is received. The image data is processed using the preprocessing network to generate an output pixel representation for encoding with the external encoder. The preprocessing network is configured to take as an input encoder configuration data representing one or more configuration settings of the external encoder. The weights of the preprocessing network are dependent upon the one or more configuration settings of the external encoder.