Similarity of images in Swift

main.swift

import Accelerate

/// A function for checking two images for similarity.
/// The returned result is the percentage of similarity, where 1 is the result of comparing identical images.
/// The implementation uses Accelerate to speed up similar operations on vectors.

func compareImages(_ image1: UIImage, _ image2: UIImage) -> Double? {
    guard image1.size == image2.size else {
        return nil
    }
    
    guard
        let imageData1 = getGrayscaleData(from: image1),
        let imageData2 = getGrayscaleData(from: image2)
    else {
        return nil
    }

    let width = Int(image1.size.width)
    let height = Int(image1.size.height)
    
    var floatData1 = [Float](repeating: 0, count: width * height)
    var floatData2 = [Float](repeating: 0, count: width * height)
    var resultData = [Float](repeating: 0, count: width * height)
    
    // Convert [UInt8] to [Float]
    vDSP.convertElements(of: imageData1, to: &floatData1)
    vDSP.convertElements(of: imageData2, to: &floatData2)
    
    // Subtract images
    vDSP.subtract(floatData2, floatData1, result: &resultData)
    
    // Calculate the absolute values of the differences
    vDSP.absolute(resultData, result: &resultData)
    
    // Clip values to the range from 0 to 1
    vDSP.clip(resultData, to: 0...1, result: &resultData)
    
    let sum: Float = vDSP.sum(resultData)
    let ratio = Double(sum) / Double(width * height)
    
    // Calculate the similarity percentage
    let similarity = 1 - ratio
    return similarity
}

func getGrayscaleData(from image: UIImage) -> [UInt8]? {
    guard let cgImage = image.cgImage else {
        return nil
    }
    
    let width = Int(image.size.width)
    let height = Int(image.size.height)
    let bitsPerComponent = 8
    let bytesPerPixel = 1
    let bytesPerRow = width * bytesPerPixel
    
    let colorSpace = CGColorSpaceCreateDeviceGray()
    let bitmapInfo: UInt32 = CGImageAlphaInfo.none.rawValue
    
    var imageData = [UInt8](repeating: 0, count: width * height)
    
    guard let context = CGContext(
        data: &imageData,
        width: width,
        height: height,
        bitsPerComponent: bitsPerComponent,
        bytesPerRow: bytesPerRow,
        space: colorSpace,
        bitmapInfo: bitmapInfo
    ) else {
        return nil
    }
    
    let rect = CGRect(x: 0, y: 0, width: width, height: height)
    context.draw(cgImage, in: rect)
    
    return imageData
}

compareImages(firstImage, secondImage)