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# Track header atom ('tkhd')

An atom that specifies the characteristics of a single track within a movie.

## Overview

The track header atom specifies the characteristics of a single track within a movie. A track header atom contains a size field that specifies the number of bytes and a type field that indicates the format of the data (defined by the atom type `'tkhd'`).

The layout of a track header atom is as follows.

|Data field                                                                                      |Bytes|
|------------------------------------------------------------------------------------------------|-----|
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Size``             |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Type``             |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Version``          |1    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Flags``            |3    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Creation_time``    |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Modification_time``|4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Track_ID``         |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Reserved``         |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Duration``         |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Reserved_2``       |8    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Layer``            |2    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Alternate_group``  |2    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Volume``           |2    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Reserved_3``       |2    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Matrix_structure`` |36   |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Track_width``      |4    |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Track_header_atom/Track_height``     |4    |

The track header atom contains the track characteristics for the track, including temporal, spatial, and volume information.

## Topics

### Data fields

[`Size`](/documentation/quicktime-file-format/Track_header_atom/Size)

A 32-bit integer that specifies the number of bytes in this track header atom.

[`Type`](/documentation/quicktime-file-format/Track_header_atom/Type)

A 32-bit integer that identifies the atom type.

[`Version`](/documentation/quicktime-file-format/Track_header_atom/Version)

A 1-byte specification of the version of this track header.

[`Flags`](/documentation/quicktime-file-format/Track_header_atom/Flags)

Three bytes that are reserved for the track header flags.

[`Creation time`](/documentation/quicktime-file-format/Track_header_atom/Creation_time)

A 32-bit integer that indicates the creation calendar date and time for the track header.

[`Modification time`](/documentation/quicktime-file-format/Track_header_atom/Modification_time)

A 32-bit integer that indicates the last change date for the track header.

[`Track ID`](/documentation/quicktime-file-format/Track_header_atom/Track_ID)

A 32-bit integer that uniquely identifies the track.

[`Reserved`](/documentation/quicktime-file-format/Track_header_atom/Reserved)

A 32-bit integer that is reserved for use by Apple.

[`Duration`](/documentation/quicktime-file-format/Track_header_atom/Duration)

A time value that indicates the duration of this track, in the movie’s time coordinate system.

[`Reserved`](/documentation/quicktime-file-format/Track_header_atom/Reserved_2)

An 8-byte value that is reserved for use by Apple.

[`Layer`](/documentation/quicktime-file-format/Track_header_atom/Layer)

A 16-bit integer that indicates this track’s spatial priority in its movie.

[`Alternate group`](/documentation/quicktime-file-format/Track_header_atom/Alternate_group)

A 16-bit integer that identifies a collection of movie tracks that contain alternate data for one another.

[`Volume`](/documentation/quicktime-file-format/Track_header_atom/Volume)

A 16-bit fixed-point value that indicates how loudly to play this track’s sound.

[`Reserved`](/documentation/quicktime-file-format/Track_header_atom/Reserved_3)

A 16-bit integer that is reserved for use by Apple.

[`Matrix structure`](/documentation/quicktime-file-format/Track_header_atom/Matrix_structure)

The matrix structure associated with this track.

[`Track width`](/documentation/quicktime-file-format/Track_header_atom/Track_width)

A 32-bit fixed-point number that specifies the width of this track in pixels.

[`Track height`](/documentation/quicktime-file-format/Track_header_atom/Track_height)

A 32-bit fixed-point number that indicates the height of this track in pixels.


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