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# VTCompressionSessionCreate(allocator:width:height:codecType:encoderSpecification:imageBufferAttributes:compressedDataAllocator:outputCallback:refcon:compressionSessionOut:)

Creates an object that compresses video frames.

```
func VTCompressionSessionCreate(allocator: CFAllocator?, width: Int32, height: Int32, codecType: CMVideoCodecType, encoderSpecification: CFDictionary?, imageBufferAttributes sourceImageBufferAttributes: CFDictionary?, compressedDataAllocator: CFAllocator?, outputCallback: VTCompressionOutputCallback?, refcon outputCallbackRefCon: UnsafeMutableRawPointer?, compressionSessionOut: UnsafeMutablePointer<VTCompressionSession?>) -> OSStatus
```

## Parameters

`allocator`

An allocator for the session. Pass `NULL` to use the default allocator.

`width`

The pixel width of video frames.

`height`

The pixel height of video frames.

`codecType`

The codec type.

`encoderSpecification`

A video encoder to use. Pass `NULL` to let VideoToolbox choose an encoder.

`sourceImageBufferAttributes`

Required attributes for source pixel buffers, used when creating a pixel buffer pool for source frames. If you don’t want VideoToolbox to create one for you, pass `NULL`.

    Using pixel buffers not allocated by VideoToolbox increases the chance that you’ll have to copy image data.

`compressedDataAllocator`

An allocator for the compressed data. Pass `NULL` to use the default allocator.

    In MacOS 10.12 and later, using a     `compressedDataAllocator`     may trigger an extra buffer copy.

`outputCallback`

The callback to invoke with compressed frames. The system may call this function asynchronously, on a different thread from the one that calls [`VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)).

    Pass     `NULL`     only if you’ll be calling [`VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:infoFlagsOut:outputHandler:)`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:infoFlagsOut:outputHandler:))     for encoding frames.

`outputCallbackRefCon`

Client-defined reference value for the output callback.

`compressionSessionOut`

A pointer to a variable to receive the new compression session.

## Discussion

The session outputs compressed frames through the output callback.

## Topics

### Callbacks

[`VTCompressionOutputCallback`](/documentation/VideoToolbox/VTCompressionOutputCallback)

A callback for the system to invoke when it’s finished compressing a frame.

[`VTCompressionOutputHandler`](/documentation/VideoToolbox/VTCompressionOutputHandler)

A callback for the system to invoke when it’s finished compressing a frame.


---

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