# vDSP_DCT_CreateSetup Builds a data structure that contains precalculated data to perform a discrete cosine transform. ``` vDSP_DFT_SetupvDSP_DCT_CreateSetup(vDSP_DFT_Setup __Previous, vDSP_Length __Length, vDSP_DCT_Type __Type); ``` ## Parameters `__Previous` An existing [`vDSP_DFT_Setup`](/documentation/Accelerate/vDSP_DFT_Setup) structure that shares memory with the returned setup structure. Pass `nil` to create an object with newly initialized and allocated memory. `__Length` The number of real elements to transform. The supported values are `Length` = *f* * 2***n*, where *f* is 1, 3, 5, or 15 and *n* is at least 4. `__Type` A constant that defines which discrete cosine transform (DCT) variant to perform. ## Return Value When successful, this function returns a [`vDSP_DFT_Setup`](/documentation/Accelerate/vDSP_DFT_Setup) structure. When unsuccessful, this function returns `nil`, either because of insufficient memory or because `Length` doesn’t satisfy the requirements given above. ## Discussion> Important: > To prevent potential memory leaks, if the `Previous` parameter is not `nil`, the return value and the `Previous` value must be different variables. ## Discussion Use this function to create a new setup structure. Don’t use the return value to reconfigure an existing setup structure passed to this function as the `Previous` parameter. The following code creates two setup structures that share the same memory. Note that you must call [`vDSP_DFT_DestroySetup`](/documentation/Accelerate/vDSP_DFT_DestroySetup) for both setup structures. Destroying a setup with shared data is safe; the destroy function only releases memory that’s not needed by other undestroyed setups. ```swift var signal = [ ... ] guard // Create the first setup object. let setupForward = vDSP_DCT_CreateSetup(nil, vDSP_Length(signal.count), .II), // Create the second setup object that shares resources with `setupForward`. let setupInverse = vDSP_DCT_CreateSetup(setupForward, vDSP_Length(signal.count), .III) else { NSLog("Failed to create `vDSP_DCT_CreateSetup` setup structures.") return } // Perform DCT-II transform. vDSP_DCT_Execute(setupForward, signal, &signal) // `signal` contains frequency-domain representation of the original signal. // Perform DCT-III transform. vDSP_DCT_Execute(setupInverse, signal, &signal) // `signal` contains time-domain representation of the original signal. // Call `vDSP_DFT_DestroySetup` on both setup structures. vDSP_DFT_DestroySetup(setupForward) vDSP_DFT_DestroySetup(setupInverse) ``` You should only use the returned setup structure with [`vDSP_DCT_Execute`](/documentation/Accelerate/vDSP_DCT_Execute) using an input vector that contains `Length` elements. ## See Also --- 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)