A computer implemented method of processing image pixel data corresponding to an image pixel comprises determining if the image pixel is opaque or transparent. If the image pixel is determined to be opaque, a pixel color value from a first set of image pixel data is determined. If the image pixel is determined to be transparent, a transparency value from a second set of the image pixel data and a pixel color value from a third set of the image pixel data is determined. The second and third sets of the image pixel data are subsets of the first set of image pixel data.