Method and apparatus for describing subsamples in a media file

Encapsulating media data in a file, or reading encoded media data from a file, the media data comprising samples (e.g. video frames, images or point cloud frames), at least some of the samples being organized into subsamples (e.g. spatial divisions such as tiles). Subsample descriptive metadata (e.g. in a SubSampleInformationBox 790) describes the subsample organization, the metadata comprising a subsample description for each subsample, where at least one subsample description comprises a reference to a set of property values. In embodiments the reference is a reference_id parameter 705 pointing to a reference table such as a SubsampleRef array containing the common properties. Flags 715 may be used on the description box (SubSampleInformationBox) to indicate other properties. One "applies_to_all_samples" flag may indicate all samples are divided into subsamples, allowing an entry_count parameter 720 to be omitted. A "regular_sample_pattern" flag may indicate that samples containing subsamples are regularly spaced, so a sample_delta parameter 730, 735 need only be included once. A "fixed_nb_subsamples_per_sample" 725, 745 may indicate that all samples are organised into the same number of subsamples, avoiding repetition of the subsample_count 740 parameter and the "merged property" flag may mean certain parameters are merged into a single combined parameter for efficiency.