<!--
{
  "availability" : [
    "iOS: 4.0.0 -",
    "iPadOS: 4.0.0 -",
    "macCatalyst: 13.1.0 -",
    "macOS: 10.7.0 -",
    "tvOS: 9.0.0 -",
    "visionOS: 1.0.0 -",
    "watchOS: 6.0.0 -"
  ],
  "documentType" : "symbol",
  "framework" : "CoreMedia",
  "identifier" : "/documentation/CoreMedia/CMFormatDescription",
  "metadataVersion" : "0.1.0",
  "role" : "Class",
  "symbol" : {
    "kind" : "Class",
    "modules" : [
      "Core Media"
    ],
    "preciseIdentifier" : "c:@T@CMFormatDescriptionRef"
  },
  "title" : "CMFormatDescription"
}
-->

# CMFormatDescription

An object that describes a media format descriptor.

```
class CMFormatDescription
```

## Overview

A `CMFormatDescription` object is an object that describes media types (audio, video, muxed, and so on).

## Topics

### Inspecting Format Descriptions

[`audioFormatList`](/documentation/CoreMedia/CMFormatDescription/audioFormatList)

The audio format list items that describe the audio formats.

[`audioStreamBasicDescription`](/documentation/CoreMedia/CMFormatDescription/audioStreamBasicDescription)

The audio stream description.

[`audioChannelLayout`](/documentation/CoreMedia/CMFormatDescription/audioChannelLayout)

The audio channel layout.

[`dimensions`](/documentation/CoreMedia/CMFormatDescription/dimensions)

The encoded pixels not including the pixel aspect ratio or clean aperture tags.

[`extensions`](/documentation/CoreMedia/CMFormatDescription/extensions-swift.property)

A dictionary that contains all of the extensions.

[`frameDuration`](/documentation/CoreMedia/CMFormatDescription/frameDuration)

The duration of each frame.

[`frameQuanta`](/documentation/CoreMedia/CMFormatDescription/frameQuanta)

The frames per second for the time code, or the frame per tick for counter mode.

[`identifiers`](/documentation/CoreMedia/CMFormatDescription/identifiers)

An array of metadata identifiers.

[`magicCookie`](/documentation/CoreMedia/CMFormatDescription/magicCookie)

A copy of the magic cookie, if any.

[`mediaSubType`](/documentation/CoreMedia/CMFormatDescription/mediaSubType-swift.property)

The media subtype.

[`mediaType`](/documentation/CoreMedia/CMFormatDescription/mediaType-swift.property)

The media type.

[`mostCompatibleFormat`](/documentation/CoreMedia/CMFormatDescription/mostCompatibleFormat)

The most compatible audio format list item.

[`nalUnitHeaderLength`](/documentation/CoreMedia/CMFormatDescription/nalUnitHeaderLength)

The size, in bytes, of the unit length field in an AVC or HEVC video sample or parameter set sample.

[`parameterSets`](/documentation/CoreMedia/CMFormatDescription/parameterSets)

The parameter sets that an H.264 or HEVC format contains.

[`richestDecodableFormat`](/documentation/CoreMedia/CMFormatDescription/richestDecodableFormat)

The audio format list item the system validates.

[`timeCodeFlags`](/documentation/CoreMedia/CMFormatDescription/timeCodeFlags)

The flags for the available time codes.

[`tagCollections`](/documentation/CoreMedia/CMFormatDescription/tagCollections)

The tag collections associated with this media.

[`matchesTaggedBufferGroup(_:)`](/documentation/CoreMedia/CMFormatDescription/matchesTaggedBufferGroup(_:))

Whether the format description matches a set of tagged buffers.

### Working with Audio Descriptions

[`withMagicCookie(_:)`](/documentation/CoreMedia/CMFormatDescription/withMagicCookie(_:))

Returns the magic cookie.

### Working with Video Descriptions

[`cleanAperture(originIsAtTopLeft:)`](/documentation/CoreMedia/CMFormatDescription/cleanAperture(originIsAtTopLeft:))

Returns the clean aperture.

[`matchesImageBuffer(_:)`](/documentation/CoreMedia/CMFormatDescription/matchesImageBuffer(_:))

Returns a Boolean value that indicates whether the format description matches an image buffer.

[`presentationDimensions(usePixelAspectRatio:useCleanAperture:)`](/documentation/CoreMedia/CMFormatDescription/presentationDimensions(usePixelAspectRatio:useCleanAperture:))

Returns the dimensions to take the pixel aspect ratio or clean aperture into account.

### Working with Metadata Descriptions

[`keyWithLocalID(_:)`](/documentation/CoreMedia/CMFormatDescription/keyWithLocalID(_:))

Returns the metadata for the local identifier you specify.

### Working with Text Descriptions

[`defaultStyle()`](/documentation/CoreMedia/CMFormatDescription/defaultStyle())

Returns the default text style.

[`defaultTextBox(originIsAtTopLeft:heightOfTextTrack:)`](/documentation/CoreMedia/CMFormatDescription/defaultTextBox(originIsAtTopLeft:heightOfTextTrack:))

Returns the default text box.

[`displayFlags()`](/documentation/CoreMedia/CMFormatDescription/displayFlags())

Returns the display mode flags for the text media.

[`fontName(localFontID:)`](/documentation/CoreMedia/CMFormatDescription/fontName(localFontID:))

Returns the font name for the local font identifier.

[`justification()`](/documentation/CoreMedia/CMFormatDescription/justification())

Returns the horizontal and vertical justification.

### Comparing Format Descriptions

[`==(_:_:)`](/documentation/CoreMedia/CMFormatDescription/==(_:_:))

Equality is derived from

[`equalTo(_:equalityMask:)`](/documentation/CoreMedia/CMFormatDescription/equalTo(_:equalityMask:))

Evaluates equality for the parts of two audio format descriptions.

[`equalTo(_:extensionKeysToIgnore:sampleDescriptionExtensionAtomKeysToIgnore:)`](/documentation/CoreMedia/CMFormatDescription/equalTo(_:extensionKeysToIgnore:sampleDescriptionExtensionAtomKeysToIgnore:))

Evaluates equality for the parts of two audio format descriptions, ignoring the extensions you specify.

### Errors

[`CMFormatDescription.Error`](/documentation/CoreMedia/CMFormatDescription/Error)

A type that describes format description errors.

### Errors

[`kCMFormatDescriptionError_InvalidParameter`](/documentation/CoreMedia/kCMFormatDescriptionError_InvalidParameter)

An error that indicates that the function recieves an empty value for a parameter it requires.

[`kCMFormatDescriptionError_AllocationFailed`](/documentation/CoreMedia/kCMFormatDescriptionError_AllocationFailed)

An error that indicates when an allocation fails.

[`kCMFormatDescriptionError_ValueNotAvailable`](/documentation/CoreMedia/kCMFormatDescriptionError_ValueNotAvailable)

An error that indicates the format description doesn’t contain the value you request.

### Constants

[`extensionKeysCommonWithImageBuffers`](/documentation/CoreMedia/CMFormatDescription/extensionKeysCommonWithImageBuffers)

A constant that represents keys you use with video format description extensions and image buffers.

[`typeID`](/documentation/CoreMedia/CMFormatDescription/typeID)

A type identifier that corresponds to a description object.

[`CMFormatDescription.MediaType`](/documentation/CoreMedia/CMFormatDescription/MediaType-swift.struct)

A type that describes format description media.

[`CMFormatDescription.MediaSubType`](/documentation/CoreMedia/CMFormatDescription/MediaSubType-swift.struct)

A type that describes format description media subtypes.

[`CMFormatDescription.TimeCode`](/documentation/CoreMedia/CMFormatDescription/TimeCode)

A type that describes format description time codes.

[`CMFormatDescription.EqualityMask`](/documentation/CoreMedia/CMFormatDescription/EqualityMask)

A type that describes format description equality masks.

[`CMFormatDescription.Extensions`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct)

A type that describes format description extensions.

[`CMFormatDescription.ParameterSetCollection`](/documentation/CoreMedia/CMFormatDescription/ParameterSetCollection)

Collection of parameter sets in a video format description.


---

Copyright &copy; 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)