A method of processing multi-layer video data, the method comprising: generating a file that comprises a media data box for each track of media data, the media data box containing samples, each of the samples being a video access unit of the multi-layer video data, and a movie box, the movie box including a track box for each track of the media data, wherein each track box includes a media box and contains the metadata for a track of media data, characterised in that generating the file comprises: generating each media box to include a media information box, each media information box includes a sample table box, wherein the sample table box includes a sample to group box; defining, in the file, a sample group that documents all samples containing at least one Intra Random Access Point (IRAP) picture, wherein: the sample group is defined to include a sample group entry that includes: a value indicating whether all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, a value indicating the number of IRAP pictures in the particular sample documented by the sample group, and when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, values indicating layer identifiers of the IRAP pictures in the particular sample documented by the sample group, and wherein at least one of the samples in the generated file contains an IRAP picture in a base layer and also contains one or more non-IRAP pictures in other layers. generating each media box to include a media information box, each media information box includes a sample table box, wherein the sample table box includes a sample to group box; defining, in the file, a sample group that documents all samples containing at least one Intra Random Access Point (IRAP) picture, wherein: the sample group is defined to include a sample group entry that includes: a value indicating whether all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, a value indicating the number of IRAP pictures in the particular sample documented by the sample group, and when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, values indicating layer identifiers of the IRAP pictures in the particular sample documented by the sample group, and a value indicating whether all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, a value indicating the number of IRAP pictures in the particular sample documented by the sample group, and when not all coded pictures in a particular sample in the sequence of samples documented by the sample group are IRAP pictures, values indicating layer identifiers of the IRAP pictures in the particular sample documented by the sample group, and wherein at least one of the samples in the generated file contains an IRAP picture in a base layer and also contains one or more non-IRAP pictures in other layers.