# vImageFlatten_RGBA16U(_:_:_:_:_:) Performs an alpha composite of an unsigned 16-bit-per-channel, 4-channel RGBA buffer over a solid background color. ``` func vImageFlatten_RGBA16U(_ rgbaSrc: UnsafePointer, _ rgbaDst: UnsafePointer, _ rgbaBackgroundColorPtr: UnsafePointer, _ isImagePremultiplied: Bool, _ flags: vImage_Flags) -> vImage_Error ``` ## Parameters `rgbaSrc` The source vImage buffer. `rgbaDst` 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. `rgbaBackgroundColorPtr` A pixel value that defines the solid background color. `isImagePremultiplied` A Boolean value that specifes whether the source image has premultiplied alpha. `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 The function uses the following calculation to flatten the source image: ```c resultAlpha = (pixelAlpha * 65535 + (65535 - pixelAlpha) * backgroundAlpha + 32767) / 65535 if(isImagePremultiplied) resultColor = (pixelColor * 65535 + (65535 - pixelAlpha) * backgroundColor + 32767) / 65535 else resultColor = (pixelColor * pixelAlpha + (65535 - pixelAlpha) * backgroundColor + 32767) / 65535 ``` --- 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)