Methods of encapsulating a bitstream include generating reference information to enable image items identifying image portions of a bit stream to be referenced to corresponding description information. The resulting encapsulated file may also include information identifying a timed portion of the data stream corresponding to a video sequence. Description information may include spatial parameters, such as position and width and height of sub-parts of an overall image, and/or display parameters such as colour display, pixel aspect ratio, and clean aperture parameters.