Image decoding

In order to prevent accumulation of rounding errors caused by the integer-based bilinear interpolation used in the motion compensation process of image coding and decoding methods, two types of P frames are used for unidirectional motion compensation prediction, namely:P+ frames, which round the results of bilinear interpolation with real number operations to the nearest integer and rounds half integer values (0.5 added to an integer) away from zero, andP- frames, which differs from P+ frames in that the above mentioned half integer values are rounded towards zero. Utilising both of these P+ and P- frames enables cancelling of rounding errors and prevents the accumulation of rounding errors.