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

# withBNNSTensor(_:)

Calls the given closure with a pointer to a BNNS tensor that shares memory with the pixel buffer.

## Declaration

```swift
func withBNNSTensor<R>(_ body: (BNNSTensor) throws -> R) rethrows -> R
```

## Parameters

- `body`: A closure with a BNNSTensor parameter

## Discussion

Discussion Use this function to pass image data to a BNNSGraph. For example, the following code creates a graph that calculates the mean value of an image and apply a threshold based on the mean. let context = try BNNSGraph.makeContext {    builder in

let src = builder.argument(name: "source",                               dataType: Float16.self,                               shape: [<height, width, channels count>])

let mean = src.mean(axes: [0, 1], keepDimensions: false)

let thresholded = src .> mean

let result = thresholded.cast(to: Float16.self)

return [result] } Use withBNNSTensor() to pass source and destination pixel buffers to the context: let source = vImage.PixelBuffer<vImage.Planar16F>(...) let destination = vImage.PixelBuffer<vImage.Planar16F>(...) source.withBNNSTensor { src in     destination.withBNNSTensor { dst in

var args = [dst, src]

try! context.executeFunction(arguments: &args)     } } On return, destination contains the thresholded image data. Note that the tensor that this function creates has the shape height, width, channels count.  If you appy a slice operation to a pixel buffer back tensor, the first dimension is the height and the second dimension is the width. For example, given the following code:    let sourcePixels: [Float] = [        001, 002, 003, 004,     005, 006, 007, 008,     009, 010, 011, 012,        013, 014, 015, 016,     017, 018, 019, 020,     021, 022, 023, 024,        025, 026, 027, 028,     029, 030, 031, 032,     033, 034, 035, 036,        037, 038, 039, 040,     041, 042, 043, 044,     045, 046, 047, 048,        049, 050, 051, 052,     053, 054, 055, 056,     057, 058, 059, 060    ]

let source = vImage.PixelBuffer(pixelValues: sourcePixels,                                    size: .init(width: 3, height: 5),                                    pixelFormat: vImage.InterleavedFx4.self)

let destination = vImage.PixelBuffer(size: .init(width: 2, height: 3),                                         pixelFormat: vImage.InterleavedFx4.self)

let context = try BNNSGraph.makeContext {        builder in

let src = builder.argument(name: "source",                                   dataType: Float.self,                                   shape: [ -1, -1, -1])

let result = src[ 0...2,    // rows 0, 1, 2                          1...2,    // cols 1, 2                          0...3 ]   // channels 0, 1, 2, 3

return [result]    }

source.withBNNSTensor { src in        destination.withBNNSTensor { dst in

var args = [dst, src]            try! context.executeFunction(arguments: &args)        }    } The slice selects rows 0, 1, and 2 and columns 1 and 2. On return, destination contains the following values:        [            005, 006, 007, 008,     009, 010, 011, 012,            017, 018, 019, 020,     021, 022, 023, 024,            029, 030, 031, 032,     033, 034, 035, 036,        ] Returns The return value, if any, of the body closure parameter.