# vImagePremultipliedAlphaBlendDarken_RGBA8888(_:_:_:_:) Performs alpha compositing of two 8-bit-per-channel, 4-channel BGRA buffers using the darken blend mode. ``` func vImagePremultipliedAlphaBlendDarken_RGBA8888(_ srcTop: UnsafePointer, _ srcBottom: UnsafePointer, _ dest: UnsafePointer, _ flags: vImage_Flags) -> vImage_Error ``` ## Parameters `srcTop` The vImage buffer that provides the source top image. `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: ```c // For color channels: uint8_t destColor = MIN( topColor + ((255 - srcTopAlpha) * srcBotomColor + 127) / 255, bottomColor + ((255 - srcBottomAlpha) * srcTopColor + 127) / 255); // For the alpha channel: uint8_t alpha = srcTopAlpha + ((255 - srcTopAlpha) * srcBottomAlpha + 127)/255; ``` ## 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)