1. A method of determining a blocking artefact measure relating to blocking artefacts in a digital image comprising a plurality of pixels each pixel having a value, the method comprising the steps of: for each of a plurality of pixels determining a vertical gradient measure in dependence upon the values of said pixel and neighbouring pixels; comparing said vertical gradient measure with a vertical gradient threshold and defining said pixel as a potential horizontal boundary in dependence thereon determining a horizontal gradient measure in dependence upon the values of said pixel and neighbouring pixels; comparing said horizontal gradient measure with a horizontal gradient threshold and defining said pixel as a potential vertical boundary in dependence thereon; and determining said blocking artefact measure in dependence upon the vertical gradient measure of pixels defined as a potential horizontal boundary and upon the horizontal gradient measure of pixels defined as a potential vertical boundary.