The invention relates to a method of generating a media file, the method comprising: generating a first data structure assigning a subset of samples or subsamples of a track to one or more sample groups; generating a second data structure comprising data for describing each of the one or more sample groups, the first data structure comprising a first grouping type and the second data structure comprising a second grouping type, wherein the second data structure comprises data for indicating whether the data for describing each of the one or more sample groups is invariant along time or not; and generating a media file including the samples, and the first and second data structures