A device generates, in a container box of a file for storage of video contents, a SampleGroupDescription box that provides a sample group description for a sample grouping. The SampleGroupDescription box includes a grouping type syntax element having a particular value. Additionally, the SampleGroupDescription box includes one or more sample group entries. The device generates, in the same container box, a plurality of SampleToGroup boxes. Each respective SampleToGroup box of the plurality of SampleToGroup boxes includes a respective grouping type syntax element having the particular value, includes a respective sample count syntax element indicating a number of samples in a respective sample grouping, and includes a respective group description index syntax element indicating an index of an entry in the SampleGroupDescription box which describes samples of the respective sample grouping. Each sample of the respective sample grouping comprises a respective picture of the video contents.