1. A method of compression coding, the method comprising the steps of: forming a difference between input picture values and picture prediction values; and transforming the difference in a transform; wherein a picture prediction P is formed by the combination of a predictor P C which is a temporal predictor or a spatial predictor and which is restricted to prediction values capable of exact reconstruction in a downstream decoder and a predictor P O which is not restricted to prediction values capable of exact reconstruction in a downstream decoder, the predictor P O being a spatial predictor with the transform being a spatial transform or a temporal transform or the predictor P O being a temporal predictor with the transform being a temporal transform.