ftyp

ftyp, or the File Type Box, is a metadata box used by the ISO Base Media File Format (ISOBMFF) and its derivatives, such as MP4, M4A, and 3GP. It is typically the first box in a file and provides a quick means for software to identify the file type and determine compatibility with various features and codecs.

The box contains a size field and the ASCII code 'ftyp' that marks the box type, followed by a major_brand, a minor_version, and an array of compatible_brands. The major_brand is a four-character code that indicates the primary file type or specification the file adheres to. The minor_version is a 32-bit unsigned integer describing the version of the major_brand. The compatible_brands is a list of additional four-character codes that signal other formats that can interpret the file's contents.

Commonly seen major brands include isom and mp41/mp42, with compatible_brands often listing isom, iso2, avc1, mp41, and mp42 among others. This structure enables media players and editors to determine whether they can decode the video and audio tracks without fully parsing the entire file.

In practice, the ftyp box helps software decide how to handle the file, select appropriate decoders, and ensure interoperability across devices and applications. It is typically present at the start of ISOBMFF-based files; files that are not ISOBMFF-based may lack an ftyp box.