---
title: "withUnsafeMutableBufferPointer(_:)"
framework: accelerate
role: symbol
role_heading: Instance Method
path: "accelerate/vimage/pixelbuffer/withunsafemutablebufferpointer(_:)"
---

# withUnsafeMutableBufferPointer(_:)

Calls the given closure with a pointer to the buffer’s mutable contiguous storage.

## Declaration

```swift
func withUnsafeMutableBufferPointer<R>(_ body: (inout UnsafeMutableBufferPointer<Format.ComponentType>) throws -> R) rethrows -> R
```

## Parameters

- `body`: A closure with an doc://com.apple.documentation/documentation/Swift/UnsafeMutableBufferPointer parameter that points to the contiguous storage for the pixel buffer.

## Return Value

Return Value The return value, if any, of the body closure parameter.

## Discussion

Discussion You can use this function to simplify interoperation with other libraries and frameworks. note: The contiguous storage may include space outside of the buffer’s width that doesn’t contain image information. The storage contains rowStride * channelCount * height elements. Add Text to a Pixel Buffer The following code creates a CGContext instance from the pointer to the buffer’s storage. The code renders a gray rectangle over the image and draws text inside the rectangle by calling CTLineDraw(_:_:). let srcImage =  imageLiteral(resourceName: " ... ").cgImage(     forProposedRect: nil,     context: nil,     hints: nil)!

var cgImageFormat = vImage_CGImageFormat()

let buffer = try vImage.PixelBuffer(     cgImage: srcImage,     cgImageFormat: &cgImageFormat,     pixelFormat: vImage.Interleaved8x4.self)

buffer.withUnsafeMutableBufferPointer { ptr in          // Define font.     let fontAttributes = [         kCTFontFamilyNameAttribute : "Futura",         kCTFontStyleNameAttribute : "Medium Italic"     ] as NSDictionary     let fontDescriptor = CTFontDescriptorCreateWithAttributes(fontAttributes)     let font = CTFontCreateWithFontDescriptor(fontDescriptor, 48, nil)     let attributes = [kCTFontAttributeName : font] as CFDictionary          // Create `CGContext` and attributed string.     guard         let context = CGContext(data: ptr.baseAddress!,                                 width: buffer.width,                                 height: buffer.height,                                 bitsPerComponent: Int(cgImageFormat.bitsPerComponent),                                 bytesPerRow: buffer.rowStride * buffer.byteCountPerPixel,                                 space: cgImageFormat.colorSpace.takeRetainedValue(),                                 bitmapInfo: cgImageFormat.bitmapInfo.rawValue),         let text = CFAttributedStringCreate(nil,                                             "vImage Pixel Buffer" as CFString,                                             attributes) else {         return     }          let line = CTLineCreateWithAttributedString(text)          context.textPosition = CGPoint(x: 25, y: 25)          // Draw text background gray rectangle.     let boundingRect = CTLineGetImageBounds(line, context).insetBy(dx: -5, dy: -5)     context.setFillColor(.init(gray: 0.75, alpha: 0.75))     context.addRect(boundingRect)     context.drawPath(using: .fill)          // Draw text to `CGContext`.     CTLineDraw(line, context) }

let result = buffer.makeCGImage(cgImageFormat: cgImageFormat)! On return, the buffer contains the original image with a text overlay.

Fill a Pixel Buffer with Random Values The following code uses Accelerate’s BNNS library to fill a pixel buffer with random values: let buffer = vImage.PixelBuffer(     size: vImage.Size(width: 512,                       height: 512),     pixelFormat: vImage.Interleaved8x3.self)

buffer.withUnsafeMutableBufferPointer { bufferPtr in

guard         var descriptor = BNNSNDArrayDescriptor(             data: bufferPtr,             shape: .vector(bufferPtr.count)),         let randomNumberGenerator = BNNSCreateRandomGenerator(             BNNSRandomGeneratorMethodAES_CTR,             nil) else {                 fatalError()             }

BNNSRandomFillUniformInt(randomNumberGenerator,                              &descriptor,                              0, 255)

BNNSDestroyRandomGenerator(randomNumberGenerator) } On return, the buffer contains random color values.