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

compareimages.swift

/**
 @brief Returns true if images have same meta. Width, Height, bit depth.
 @discussion Assumes images are non null.
*/
func doImagesHaveSameMeta(#image1:CGImage, #image2:CGImage) -> Bool {
    if CGImageGetWidth(image1) != CGImageGetWidth(image2) {
        return false
    }
    
    if CGImageGetHeight(image1) != CGImageGetHeight(image2) {
        return false
    }
    
    if CGImageGetBitsPerComponent(image1) != CGImageGetBitsPerComponent(image2) {
        return false
    }
    
    if CGImageGetBytesPerRow(image1) != CGImageGetBytesPerRow(image2) {
        return false
    }
    
    if CGImageGetBitsPerPixel(image1) != CGImageGetBitsPerPixel(image2) {
        return false
    }
    
    return true
}

/**
 @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.valueForKey(kCIOutputImageKey),
        forKey: kCIInputImageKey)
    let compareRect = CGRectMake(0.0, 0.0, CGFloat(CGImageGetWidth(image1)),
        CGFloat(CGImageGetHeight(image1)))
    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>(count: 16, repeatedValue: 255)
    let context = CGBitmapContextCreate(&buf, 1, 1, 8, 16, colorSpace, bitmapInfo)
    
    // Now create the core image context CIContext from the bitmap context.
    let ciContextOpts = [
          kCIContextWorkingColorSpace : colorSpace,
        kCIContextUseSoftwareRenderer : false
    ]
    let ciContext = CIContext(CGContext: context, options: ciContextOpts)
    
    // Get the output CIImage and draw that to the Core Image context.
    let valueImage = areaMaxFilter.valueForKey(kCIOutputImageKey)! as CIImage
    ciContext.drawImage(valueImage, inRect: CGRectMake(0,0,1,1),
        fromRect: 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
}

Thanks for putting this together! I couldn't get it to compile on Xcode 13, so I created an updated version (works with Swift 5):

https://gist.github.com/nicolas-miari/519cb8fd31c16e5daac263412996d08a