Video codec consists of an encoder that transforms the images of the input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. Image sequences can be compressed either as sequences of still pictures coded with spatial prediction means or inter pictures coded with spatial and temporal prediction means. Image sequences with random access and support for editing individual pictures have been traditionally enabled by representing the sequence as a series of independently coded intra pictures. Such formats include, for example, Motion JPEG, animated GIF and the Intra profiles of H.264.
If a sequence of images is represented as a series of still pictures, the coding efficiency is typically poor and the file size requirement for a high resolution sequence can become massive. In order to access a predicted picture, the decoder needs to conclude which reference pictures are required to be decoded first. The delay may be significantly reduced, if only those pictures that are used as reference are decoded rather than decoding all pictures starting from the previous IDR picture or alike. However, in the known methods, random access to any sample may require decoding all reference pictures starting from the previous IDR picture or alike, as concluding which subset of reference pictures are required to be decoded may not be easily possible.
Therefore there is a need for an improved method for identifying pictures that are required to be decoded in order to decode a desired random-accessed picture correctly.