3. The method defined in claim 2 , wherein each of the reference data elements is associated with a respective one of the video frames in the reference video stream, and wherein said deriving the set of reference data elements comprises, for each particular video frame in the reference video stream: extracting at least one feature for each of a plurality of regions of the particular video frame in the reference video stream, wherein the reference data element associated with the particular video frame in the reference video stream comprises the at least one extracted feature.
3. The method defined in claim 2 , wherein each of the reference data elements is associated with a respective one of the video frames in the reference video stream, and wherein said deriving the set of reference data elements comprises, for each particular video frame in the reference video stream: extracting at least one feature for each of a plurality of regions of the particular video frame in the reference video stream, wherein the reference data element associated with the particular video frame in the reference video stream comprises the at least one extracted feature. 4. The method defined in claim 3 , wherein each of the query data elements is associated with a respective one of the video frames in the query video stream, and wherein said deriving the set of query data elements comprises, for each particular video frame in the query video stream: extracting at least one feature for each of a plurality of regions of the particular video frame in the query video stream, wherein the query data element associated with the particular video frame in the query video stream comprises the at least one extracted feature.