A context based arithmetic encoding CAE for binary shape coding of I (intra-frame) VOPs, P (unilaterally motion-compensated-predicted) VOPs and B (bidirectionally interpolated) VOPs (VOP=video object plane) is used in MPEG verification model version 8.0 (VM8.0), ISO/IEC JTC1/SC29/WG11, MPEG97/N1796.
FIG. 1 shows an example of a GOF (group of frames) structure in MPEG-1 standard for an image sequence composed of I-, P- and B-VOPs. VOPs coded exclusively by the intra-frame technique are labeled as I, unilaterally motion-compensated predicted VOPs are labeled as P, and the bidirectionally interpolated VOPs between them are labeled as B. The example according to FIG. 1 shows four P-VOPs per GOF and two B-VOPs between I- and/or P-VOPs. The length of the GOF is not fixed, but instead is determined by the number of different image types. One essential feature of the CAE algorithm is that a chronologically following VOP (backward reference VOP) is always used as the reference value in inter-CAE shape coding of a current B-VOP, namely for both the binary shape reference and the shape mode reference. However, since B-VOPs of a random shape sometimes access only image contents of a chronologically preceding VOP as the reference (forward reference VOP), the binary shape of this B-VOP must be coded in the same manner as an I-VOP when using the current CAE algorithm. This leads to a considerable degradation of coding efficiency with such B-VOPs.
According to the present invention, coding efficiency can be improved in particular for shape coding in bidirectionally interpolated VOPs (B-VOPs). The method according to the present invention provides time-adaptive shape coding for B-VOPs. During a context-based shape coding, a B-VOP can access either a chronologically preceding I-VOP or P-VOP (forward reference VOP) or a chronologically following I-VOP or P-VOP (backward reference VOP) as the reference value for shape coding.
In the method according to the present invention no changes in syntax are necessary and thus the method can easily be used with existing standards such as the MPEG-4 standard.