4.3 File Type Box

4.3.1 Definition

Box Type: `ftyp’
Container: File
Mandatory: Yes   [ed note: but see emphasized text below]
Quantity: Exactly one

A media-file structured to this part of this specification may be compatible with more than one detailed specification, and it is therefore not always possible to speak of a single ‘type’ or ‘brand’ for the file. This means that the utility of the file name extension and [mime] Multipurpose Internet Mail Extension (MIME) type are somewhat reduced.

This box must be placed as early as possible in the file (e.g. after any obligatory signature, but before any significant variable-size boxes such as a Movie Box, Media Data Box, or Free Space). It identifies which specification is the ‘best use’ of the file, and a minor version of that specification; and also a set of other specifications to which the file complies. Readers implementing this format should attempt to read files that are marked as compatible with any of the specifications that the reader implements. Any incompatible change in a specification should therefore register a new ‘brand’ identifier to identify files conformant to the new specification.

The minor version is informative only. It does not appear for compatible-brands, and must not be used to determine the conformance of a file to a standard. It may allow more precise identification of the major specification, for inspection, debugging, or improved decoding.

The type ‘isom’ (ISO Base Media file) is defined in this section of this specification, as identifying files that conform to [the first version of] ISO Base Media File Format.

More specific identifiers can be used to identify precise versions of specifications providing more detail. This brand should not be used as the major brand; this base file format should be derived into another specification to be used. There is therefore no defined normal file extension, or [mime] MIME type assigned to this brand, nor definition of the minor version when ‘isom’ is the major brand.

Files would normally be externally identified (e.g. with a file extension or [mime] MIME type) that identifies the ‘best use’ (major brand), or the brand that the author believes will provide the greatest compatibility.

The brand ‘iso2’ shall be used to indicate compatibility with this amended version of the ISO Base Media File Format; it may be used in addition to or instead of the ‘isom’ brand and the same usage rules apply. If used without the brand 'isom' identifying the first version of this specification, it indicates that support for some or all of the technology introduced by this amendment is required, such as the functionality in sub-clauses [8.40] through [8.45], or the SRTP support in sub-clause [10], is required.

The brand ‘avc1’ shall be used to indicate that the file is conformant with the ‘AVC Extensions’ in sub-clause [8.40]. If used without other brands, this implies that support for those extensions is required. The use of ‘avc1’ as a major-brand may be permitted by specifications; in that case, that specification defines the file extension and required behavior.

If a Meta-box with an MPEG-7 handler type is used at the file level, then the brand ‘mp71’ should be a member of the compatible-brands list in the file-type box.

Files would normally be externally identified (e.g. with a file extension or mime type) that identifies the ‘best use’ (major brand), or the brand that the author believes will provide the greatest compatibility.

This section of this specification does not define any brands. However, see subclause 6.3 below for brands for files conformant to whole specification and not just this section.

4.3.2 Syntax

aligned(8) class FileTypeBox

extends Box(‘ftyp’)
{
    unsigned int(32) major_brand;
    unsigned int(32) minor_version;
    unsigned int(32) compatible_brands[];
    // to end of the box
}

4.3.3 Semantics

This box identifies the specifications to which this file complies.

Each brand is a printable four-character code, registered with ISO, that identifies a precise specification. Only one brand is defined here: ‘isom’ (ISO Base Media File), identifies files structurally conformant to this media independent part of this specification.

major_brand – is a brand identifier
minor_version – is an informative integer for the minor version of the major brand
compatible_brands – is a list, to the end of the box, of brands

6.3 Brand Identification

The type ‘isom’ (ISO Base Media file) is defined in this section of this specification, as identifying files that conform to the first version of ISO Base Media File Format.

More specific identifiers can be used to identify precise versions of specifications providing more detail. This brand should not be used as the major brand; this base file format should be derived into another specification to be used. There is therefore no defined normal file extension, or mime type assigned to this brand, nor definition of the minor version when ‘isom’ is the major brand.

The brand ‘iso2’ shall be used to indicate compatibility with this amended version of the ISO Base Media File Format; it may be used in addition to or instead of the ‘isom’ brand and the same usage rules apply. If used without the brand 'isom' identifying the first version of this specification, it indicates that support for some or all of the technology introduced by this amendment is required, such as the functionality in subclauses 8.40 through 8.45, or the SRTP support in clause 10, is required.

The brand ‘avc1’ shall be used to indicate that the file is conformant with the ‘AVC Extensions’ in subclause [8.40]. If used without other brands, this implies that support for those extensions is required. The use of ‘avc1’ as a major-brand may be permitted by specifications; in that case, that specification defines the file extension and required behavior.

If a Meta-box with an MPEG-7 handler type is used at the file level, then the brand ‘mp71’ should be a member of the compatible-brands list in the file-type box.

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