1. A computer implemented method of hashing an image, comprising: receiving a reference image frame; deriving an array of reference hash values to represent the reference image frame, wherein the array of reference hash values includes a first reference hash value, a second reference hash value, and a third reference hash value; receiving an test image frame; deriving an array of hash values to represent the test image frame, wherein the array of hash values includes a first test hash value, a second test hash value, and a third test hash value; subtracting the array of test hash values representing the test image frame from the array of reference hash values representing the reference image frame to produce a delta array, wherein the delta array is indicative of an area where the test image frame differs from the reference image frame.