FullBox

FullBox is a designation used in the ISO Base Media File Format (ISOBMF), notably in MP4 and related container formats, to describe a subset of box types that include a version and flags field in their header. In a standard box, the header consists of a 4-byte size and a 4-byte type. FullBox adds additional 4 bytes of header data immediately after the standard header: 1 byte for version and 3 bytes for flags. The remaining bytes constitute the box payload, interpreted according to the specific box type and the value of the version and flags.

The version field enables backward- and forward-compatible evolution of a box’s structure. Different versions may change the size or interpretation of certain fields within the payload, while the flags field indicates which optional fields are present or how they should be processed. For example, in some well-known FullBox types, a version change can switch time representations from 32-bit to 64-bit, affecting fields such as creation_time, modification_time, and duration in certain header boxes.

Common FullBox types appear in MP4 and related formats, such as the Movie Header Box (mvhd) and Track Header Box (tkhd), among others. These boxes control essential metadata about media tracks, timing, and decoding parameters, with their version and flags guiding how decoders read the embedded data.

See also: Box (ISOBMF), MP4, 14496-12 (ISO Base Media File Format).