Building a basic image conversion workflow

Learn the fundamentals of the convert-any-to-any function by converting a CMYK image to an RGB image.

Overview

The functions in the vImage library that perform image-processing operations are specific to properties such as bit-depth, the number of channels, and channel ordering. For example, the vImageAlphaBlend_ARGB8888(_:_:_:_:) function works with 8-bit-per-channel, ARGB image data. The convert-any-to-any functionality allows you to convert images with formats you know at runtime to a format you define at compile time.

You can use the vImage convert-any-to-any functionality to convert image data between different bit depths, different channel counts, and different color spaces. In this example, the code converts a 16-bit-per-channel CMYK source image to an 8-bit-per-channel RGB destination image. In some cases, working in a non-RGB color space simplifies image-processing tasks. For an example of using convert-any-to-any to work in L*a*b* color space, see Adjusting the hue of an image.

Create the source and destination image formats

A vImageConverter instance – that contains the information to perform image conversion – requires two vImage_CGImageFormat structures that describe the source and destination formats. The code below defines the CMYK source format and the RGB destination format:

let cmykSourceImageFormat = vImage_CGImageFormat(
    bitsPerComponent: 16,
    bitsPerPixel: 16 * 4,
    colorSpace: CGColorSpaceCreateDeviceCMYK(),
    bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue))!

let rgbDestinationImageFormat = vImage_CGImageFormat(
    bitsPerComponent: 8,
    bitsPerPixel: 8 * 3,
    colorSpace: CGColorSpaceCreateDeviceRGB(),
    bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue))!

Create the converter

The Swift make(sourceFormat:destinationFormat:flags:) method calls the underlying vImageConverter_CreateWithCGImageFormat(_:_:_:_:_:) function and returns a new any-to-any converter instance.

let cmykToRgbConverter = try vImageConverter.make(
    sourceFormat: cmykSourceImageFormat,
    destinationFormat: rgbDestinationImageFormat)

Perform the conversion using pixel buffers

The code below converts the CMYK image data that cmykSourceBuffer contains and writes the RGB result to rgbDestinationBuffer. In this example, cmykSourceBuffer is a vImage.PixelBuffer structure with a vImage.Interleaved16Ux4 format.

The Swift convert(from:to:) method calls the underlying vImageConvert_AnyToAny(_:_:_:_:_:) function.

let rgbDestinationBuffer = vImage.PixelBuffer<vImage.Interleaved8x3>(
    size: cmykSourceBuffer.size)

try cmykToRgbConverter.convert(from: cmykSourceBuffer,
                               to: rgbDestinationBuffer)

On return, rgbDestinationBuffer contains the RGB representation of the CMYK source image.

Perform the conversion using vImage buffers

If you’re creating apps for older operating systems that don’t support the vImage.PixelBuffer API, the following code performs the same conversion using vImage_Buffer structures.

The Swift convert(source:destination:flags:) method calls the underlying vImageConvert_AnyToAny(_:_:_:_:_:) function

var rgbDestinationBuffer = try vImage_Buffer(
    size: cmykSourceBuffer.size,
    bitsPerPixel: rgbDestinationImageFormat.bitsPerPixel)

try cmykToRgbConverter.convert(source: cmykSourceBuffer,
                               destination: &rgbDestinationBuffer)