# withUnsafeRegionOfInterest(_:_:) Calls the given closure with a pixel buffer that references the image data within the specified region of interest. ``` func withUnsafeRegionOfInterest(_ regionOfInterest: CGRect, _ body: (vImage.PixelBuffer) throws -> R) rethrows -> R ``` ## Parameters `regionOfInterest` The region of interest. `body` A closure with a [`vImage.PixelBuffer`](/documentation/Accelerate/vImage/PixelBuffer) parameter that references the region of interest. ## Discussion Use this function to apply a vImage operation to a region of interest in a pixel buffer. This region of interest pixel buffer that this function passes to the closure as an argument references the same underlying memory as `self`. Therefore, changes the pixel buffer in the closure are changes in `self`. For example, the following code generates a 5 x 5 pixel buffer filled with `1`s. The code applies a multiplication operation to a region of interest with the pixel buffer that mutplies each pixel by 5: ```swift let src = vImage.PixelBuffer ( pixelValues: [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ], size: vImage.Size(width: 5, height: 5)) let roi = CGRect(x: 1, y: 1, width: 3, height: 3) src.withUnsafeRegionOfInterest(roi) { roiPixelBuffer in roiPixelBuffer.multiply(by: 5, divisor: 1, preBias: 0, postBias: 0, destination: roiPixelBuffer) } // Prints: // "[ 1, 1, 1, 1, 1, // 1, 5, 5, 5, 1, // 1, 5, 5, 5, 1, // 1, 5, 5, 5, 1, // 1, 1, 1, 1, 1 ]" print(src.array) ``` To apply an operation that doesn’t work in place to a region of interest, create a temporary buffer to receive the result and call `[`copy(to:)`](/documentation/Accelerate/vImage/PixelBuffer/copy(to:))` to write the result back to the region of interest buffer. For example, the following code applies a box blur to a centered rectangle within the source buffer: ```swift let srcImage = imageLiteral(resourceName: " ... ").cgImage( forProposedRect: nil, context: nil, hints: nil)! var cgImageFormat = vImage_CGImageFormat( bitsPerComponent: 8, bitsPerPixel: 8 * 4, colorSpace: CGColorSpaceCreateDeviceRGB(), bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.noneSkipFirst.rawValue))! let buffer = try vImage.PixelBuffer( cgImage: srcImage, cgImageFormat: &cgImageFormat, pixelFormat: vImage.Interleaved8x4.self) let roi = CGRect(origin: .zero, size: CGSize(width: buffer.width, height: buffer.height)).insetBy(dx: 150, dy: 150) buffer.withUnsafeRegionOfInterest(roi) { buf in let tmp = vImage.PixelBuffer( size: vImage.Size(exactly: roi.size)!) buf.boxConvolve(kernelSize: vImage.Size(width: 17, height: 17), edgeMode: .truncateKernel, destination: tmp) tmp.copy(to: buf) } ``` --- 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)