1. A system for assessing image quality of a distorted image relative to a reference image, the system comprising: a convolutional neural network that accepts as an input the distorted image and the reference image, and provides as an output a metric of image quality; wherein the convolutional neural network comprises a plurality of layers comprising: an input layer configured to apply a normalizing function to image patches making up each of the distorted image and the reference image to provide a normalized distorted image and a normalized reference image; and a convolution layer configured to convolve each of the normalized distorted image and the normalized reference image with N1 filters to provide N1 pairs of feature maps, each pair containing one filtered normalized distorted image and one correspondingly filtered and normalized reference image, where N1 is an integer greater than unity.