Methods and apparatus for compressing image data are described along with corresponding methods and apparatus for decompressing the compressed image data. A decoder unit identifies neighbouring pixels based on coordinates of a sample position and fetches encoded data from the compressed image data for each of the neighbouring pixels. A first decoder decodes fetched encoded blocks of a first image and a difference decoder decodes fetched encoded sub-blocks of differences between the first image and a second image and outputs a difference quad and a prediction value for each of the four pixels, and a reconstruction of the image is generated at the sample position using the decoded blocks of the first image, difference quads and prediction values.