# VTCompressionSession An object that compresses video data. ## Discussion A compression session supports the compression of a sequence of video frames. Here’s the workflow: 1. Create a compression session using [`VTCompressionSessionCreate(allocator:width:height:codecType:encoderSpecification:imageBufferAttributes:compressedDataAllocator:outputCallback:refcon:compressionSessionOut:)`](/documentation/VideoToolbox/VTCompressionSessionCreate(allocator:width:height:codecType:encoderSpecification:imageBufferAttributes:compressedDataAllocator:outputCallback:refcon:compressionSessionOut:)). 2. Optionally, configure the session with your desired [Compression Properties](/documentation/VideoToolbox/compression-properties) by calling [`VTSessionSetProperty(_:key:value:)`](/documentation/VideoToolbox/VTSessionSetProperty(_:key:value:)) or [`VTSessionSetProperties(_:propertyDictionary:)`](/documentation/VideoToolbox/VTSessionSetProperties(_:propertyDictionary:)). 3. Encode video frames using [`VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)) and receive the compressed video frames in the session’s [`VTCompressionOutputCallback`](/documentation/VideoToolbox/VTCompressionOutputCallback). 4. To force the completion of some or all pending frames, call [`VTCompressionSessionCompleteFrames(_:untilPresentationTimeStamp:)`](/documentation/VideoToolbox/VTCompressionSessionCompleteFrames(_:untilPresentationTimeStamp:)). 5. When you finish with the compression session, call [`VTCompressionSessionInvalidate(_:)`](/documentation/VideoToolbox/VTCompressionSessionInvalidate(_:)) to invalidate it and to free its memory. ## Topics ### Creating a Session [`func VTCompressionSessionCreate(allocator: CFAllocator?, width: Int32, height: Int32, codecType: CMVideoCodecType, encoderSpecification: CFDictionary?, imageBufferAttributes: CFDictionary?, compressedDataAllocator: CFAllocator?, outputCallback: VTCompressionOutputCallback?, refcon: UnsafeMutableRawPointer?, compressionSessionOut: UnsafeMutablePointer) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionCreate(allocator:width:height:codecType:encoderSpecification:imageBufferAttributes:compressedDataAllocator:outputCallback:refcon:compressionSessionOut:)) Creates an object that compresses video frames. ### Configuring a Session [Compression Properties](/documentation/VideoToolbox/compression-properties) Properties that you use to configure a compression session. ### Encoding Frames [`func VTCompressionSessionGetPixelBufferPool(VTCompressionSession) -> CVPixelBufferPool?`](/documentation/VideoToolbox/VTCompressionSessionGetPixelBufferPool(_:)) Returns a pool that provides ideal source pixel buffers for a compression session. [`func VTCompressionSessionPrepareToEncodeFrames(VTCompressionSession) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionPrepareToEncodeFrames(_:)) Enables the encoder to perform any necessary resource allocation before the encoder begins encoding frames (optional). [`func VTCompressionSessionEncodeFrame(VTCompressionSession, imageBuffer: CVImageBuffer, presentationTimeStamp: CMTime, duration: CMTime, frameProperties: CFDictionary?, sourceFrameRefcon: UnsafeMutableRawPointer?, infoFlagsOut: UnsafeMutablePointer?) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)) Presents frames to the compression session. [`func VTCompressionSessionEncodeFrame(VTCompressionSession, imageBuffer: CVImageBuffer, presentationTimeStamp: CMTime, duration: CMTime, frameProperties: CFDictionary?, infoFlagsOut: UnsafeMutablePointer?, outputHandler: VTCompressionOutputHandler) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:infoFlagsOut:outputHandler:)) Presents frames to the compression session and invokes the output callback when compression is complete. [`func VTCompressionSessionCompleteFrames(VTCompressionSession, untilPresentationTimeStamp: CMTime) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionCompleteFrames(_:untilPresentationTimeStamp:)) Forces the compression session to complete the encoding of frames. ### Encoding Multi-Image Frames [`func VTIsStereoMVHEVCEncodeSupported() -> Bool`](/documentation/VideoToolbox/VTIsStereoMVHEVCEncodeSupported()) Returns a Boolean value that indicates whether the system supports MV-HEVC encoding. [`func VTCompressionSessionEncodeMultiImageFrame(VTCompressionSession, taggedBuffers: [CMTaggedBuffer], presentationTimeStamp: CMTime, duration: CMTime, frameProperties: CFDictionary?, infoFlagsOut: UnsafeMutablePointer?, outputHandler: VTCompressionOutputHandler) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionEncodeMultiImageFrame(_:taggedBuffers:presentationTimeStamp:duration:frameProperties:infoFlagsOut:outputHandler:)) Passes a multi-image frame to a compression session for encoding and provides a callback to handle the output. [`extern OSStatus VTCompressionSessionEncodeMultiImageFrame(VTCompressionSessionRef session, CMTaggedBufferGroupRef taggedBufferGroup, CMTime presentationTimeStamp, CMTime duration, CFDictionaryRef frameProperties, void *sourceFrameRefcon, VTEncodeInfoFlags *infoFlagsOut);`](/documentation/VideoToolbox/VTCompressionSessionEncodeMultiImageFrame) Passes a multi-image frame to a compression session for encoding. [`extern OSStatus VTCompressionSessionEncodeMultiImageFrameWithOutputHandler(VTCompressionSessionRef session, CMTaggedBufferGroupRef taggedBufferGroup, CMTime presentationTimeStamp, CMTime duration, CFDictionaryRef frameProperties, VTEncodeInfoFlags *infoFlagsOut, VTCompressionOutputHandler outputHandler);`](/documentation/VideoToolbox/VTCompressionSessionEncodeMultiImageFrameWithOutputHandler) Passes a multi-image frame to a compression session for encoding and provides a callback to handle the output. ### Performing Multiple Passes [`func VTCompressionSessionBeginPass(VTCompressionSession, flags: VTCompressionSessionOptionFlags, UnsafeMutablePointer?) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionBeginPass(_:flags:_:)) Marks the start of a specific compression pass. [`func VTCompressionSessionEndPass(VTCompressionSession, furtherPassesRequestedOut: UnsafeMutablePointer?, UnsafeMutablePointer?) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionEndPass(_:furtherPassesRequestedOut:_:)) Marks the end of a compression pass. [`func VTCompressionSessionGetTimeRangesForNextPass(VTCompressionSession, timeRangeCountOut: UnsafeMutablePointer, timeRangeArrayOut: UnsafeMutablePointer?>) -> OSStatus`](/documentation/VideoToolbox/VTCompressionSessionGetTimeRangesForNextPass(_:timeRangeCountOut:timeRangeArrayOut:)) Retrieves the time ranges for the next pass. ### Invalidating a Session [`func VTCompressionSessionInvalidate(VTCompressionSession)`](/documentation/VideoToolbox/VTCompressionSessionInvalidate(_:)) Tears down a compression session. ### Accessing the Type Identifier [`func VTCompressionSessionGetTypeID() -> CFTypeID`](/documentation/VideoToolbox/VTCompressionSessionGetTypeID()) Retrieves the Core Foundation type identifier for the compression session. ### Data Types [`class VTCompressionSession`](/documentation/VideoToolbox/VTCompressionSession) A reference to a VideoToolbox compression session. [`struct VTEncodeInfoFlags`](/documentation/VideoToolbox/VTEncodeInfoFlags) Flags that indicate encoder state. --- Copyright © 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)