# VTCompressionSessionBeginPass(_:flags:_:) Marks the start of a specific compression pass. ``` func VTCompressionSessionBeginPass(_ session: VTCompressionSession, flags beginPassFlags: VTCompressionSessionOptionFlags, _ reserved: UnsafeMutablePointer?) -> OSStatus ``` ## Parameters `session` A compression session. `beginPassFlags` Pass [`beginFinalPass`](/documentation/VideoToolbox/VTCompressionSessionOptionFlags/beginFinalPass) to inform the encoder that the pass must be the final pass. `reserved` A reserved value. ## Discussion During multipass encoding, this function must be called before [`VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)`](/documentation/VideoToolbox/VTCompressionSessionEncodeFrame(_:imageBuffer:presentationTimeStamp:duration:frameProperties:sourceFrameRefcon:infoFlagsOut:)). It’s an error to call this function when multipass encoding is not enabled by setting [`kVTCompressionPropertyKey_MultiPassStorage`](/documentation/VideoToolbox/kVTCompressionPropertyKey_MultiPassStorage). ## Topics ### Option Flags [`VTCompressionSessionOptionFlags`](/documentation/VideoToolbox/VTCompressionSessionOptionFlags) Flags to pass to a compression session. --- 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)