Video data processing system

A method of encoding sequential frames of video data where, when it is desired to encode 106 a new source frame which has a different resolution to another frame which is to be used as a reference frame, a scaled 104 copy of the another frame is generated which is scaled to the resolution of the new source frame. The scaled copy of the another frame is encoded, preferably using the original another frame. The new source frame is encoded using the scaled copy of the another frame. This encoded video data is then sent to a decoder which uses the scaled copy of the frame as the reference frame to decode the video image data. Preferably the scaled copy of the frame is not displayed. There are also related apparatus and system claims.