# vImagePremultipliedConstAlphaBlend_PlanarF(_:_:_:_:_:_:) Performs premultiplied alpha compositing of two 32-bit planar buffers and applies an extra alpha value to the top buffer. ``` func vImagePremultipliedConstAlphaBlend_PlanarF(_ srcTop: UnsafePointer, _ constAlpha: Pixel_F, _ srcTopAlpha: UnsafePointer, _ srcBottom: UnsafePointer, _ dest: UnsafePointer, _ flags: vImage_Flags) -> vImage_Error ``` ## Parameters `srcTop` The vImage buffer that provides the source top image. `constAlpha` The constant alpha value that the function applies to the top image. `srcTopAlpha` The vImage buffer that provides the source top alpha. `srcBottom` The vImage buffer that provides the source bottom image. `dest` A pointer to the destination vImage buffer structure. You’re responsible for filling out the [`height`](/documentation/Accelerate/vImage_Buffer/height), [`width`](/documentation/Accelerate/vImage_Buffer/width), and [`rowBytes`](/documentation/Accelerate/vImage_Buffer/rowBytes) fields of this structure, and for allocating a data buffer of the appropriate size. On return, the data buffer this structure points to contains the destination image data. When you no longer need the data buffer, deallocate the memory to prevent memory leaks. `flags` The options to use when performing the operation. If your code implements its own tiling or its own multithreading, pass [`kvImageDoNotTile`](/documentation/Accelerate/kvImageDoNotTile); otherwise, pass [`kvImageNoFlags`](/documentation/Accelerate/kvImageNoFlags). ## Return Value [`kvImageNoError`](/documentation/Accelerate/kvImageNoError); otherwise, one of the error codes in . ## Discussion On return of this function, the value of each pixel in the destination buffer is: ```objc float destColor = srcTopColor * constAlpha + (1.0 - srcTopAlpha * constAlpha) * srcBottomColor; ``` ## 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)