1. A method for encoding a video sequence in which a keyframe is used to bi-directionally predict frames in the sequence, the method comprising: coding the keyframe independently of other frames in the sequence by a video encoder apparatus; and predicting by the video encoder apparatus a prior unidirectional predicted frame occurring before the keyframe using data from the keyframe and not from any other keyframe, directly or indirectly; and bi-directionally predicting by the video encoder apparatus a prior intervening frame using the data from the keyframe and data from the prior unidirectional predicted frame, without using data derived from any other keyframe, wherein the prior intervening frame occurs between the keyframe and the prior unidirectional predicted frame.