A sequence of picture slices is encoded as reference slices and non-reference slices, wherein the reference slices include B slices, by forming, for each B slice, at least one data packet containing data values derived from brightness and color information pertaining to the slice. The data packet for each B slice includes a header element indicating whether the B slice is a reference slice. The header element of each reference B slice has a value that depends on depth of the reference B slice in a hierarchy of discardability.