The invention relates to an encoder for forming an encoded picture stream. The pictures are defined as reference pictures or non-reference pictures, and information relating to decoding order and output order of a picture is defined for pictures of the picture stream. The encoder is arranged to form to the bitstream an indication of such a number of pictures arranged to be buffered in decoding order that is sufficient to recover the output order of pictures. The invention also relates to a signal, hypothetical encoder, decoder, hypothetical decoder, electronic device, system, software program and a storage medium.