A couple of swift functions for comparing two CGImage using CIImage in OS X

CompareImages.swift

import CoreGraphics
import CoreImage

func imageMetadataString(image: CGImage) -> String {
    return "\(image.width)x\(image.height) bitsPerComponent:\(image.bitsPerComponent) bytesPerRow:\(image.bytesPerRow) bitsPerPixel:\(image.bitsPerPixel)"
}

/**
 @brief Returns the maximum difference of pixel values in the image.
 @discussion Assumes doImagesHaveSameMeta has already returned true on
 the images passed into this function. OSX only as iOS doesn't have the
 CIAreaMaximum filter.
 */
func compareImages(_ image1: CGImage, _ image2: CGImage) -> Int {
    var diff = 0

    // First create the CIImage representations of the CGImage.
    let ciImage1 = CIImage(cgImage: image1)
    let ciImage2 = CIImage(cgImage: image2)

    // Create the difference blend mode filter and set its properties.
    let diffFilter = CIFilter(name: "CIDifferenceBlendMode")!
    diffFilter.setDefaults()
    diffFilter.setValue(ciImage1, forKey: kCIInputImageKey)
    diffFilter.setValue(ciImage2, forKey: kCIInputBackgroundImageKey)

    // Create the area max filter and set its properties.
    let areaMaxFilter = CIFilter(name: "CIAreaMaximum")!
    areaMaxFilter.setDefaults()
    areaMaxFilter.setValue(diffFilter.value(forKey: kCIOutputImageKey),
                           forKey: kCIInputImageKey)
    let compareRect = CGRect(x: 0.0, y: 0.0, width: CGFloat(image1.width),
                             height: CGFloat(image1.height))
    let extents = CIVector(cgRect: compareRect)
    areaMaxFilter.setValue(extents, forKey: kCIInputExtentKey)

    // The filters have been setup, now set up the CGContext bitmap context the
    // output is drawn to. Setup the context with our supplied buffer.
    let alphaInfo = CGImageAlphaInfo.premultipliedLast
    let bitmapInfo = CGBitmapInfo(rawValue: alphaInfo.rawValue)
    let colorSpace = CGColorSpaceCreateDeviceRGB()
    var buf: [CUnsignedChar] = Array<CUnsignedChar>(repeating: 255, count: 16)
    let context = CGContext(data: &buf, width: 1, height: 1, bitsPerComponent: 8, bytesPerRow: 16, space: colorSpace, bitmapInfo: bitmapInfo.rawValue)!

    // Now create the core image context CIContext from the bitmap context.
    let ciContext = CIContext(cgContext: context, options: [
        kCIContextWorkingColorSpace: colorSpace,
        kCIContextUseSoftwareRenderer: false,
    ])

    // Get the output CIImage and draw that to the Core Image context.
    let valueImage = areaMaxFilter.value(forKey: kCIOutputImageKey) as! CIImage
    ciContext.draw(valueImage, in: CGRect(x: 0, y: 0, width: 1, height: 1),
                   from: valueImage.extent)

    // This will have modified the contents of the buffer used for the CGContext.
    // Find the maximum value of the different color components. Remember that
    // the CGContext was created with a Premultiplied last meaning that alpha
    // is the fourth component with red, green and blue in the first three.
    let maxVal = max(buf[0], max(buf[1], buf[2]))
    diff = Int(maxVal)
    return diff
}