iOS MachineLearning 系列（7）—— 图片相似度分析

作者：珲少

2023-04-27
上海
本文字数：2550 字
阅读完需：约 8 分钟

图片相似度分析是 Vision 框架中提供的高级功能。其本质是计算图片的特征值，通过特征值的比较来计算出图片特征差距，从而可以获取到图片的相似程度。在实际应用中，图片的相似度分析有着广泛的应用。如人脸对比识别，相似物品的搜索和识别等。

进行图片相似度计算前，首先需要对图片的特征值进行分析。使用 VNGenerateImageFeaturePrintRequest 类创建图片特征分析请求。定义如下：

open class VNGenerateImageFeaturePrintRequest : VNImageBasedRequest {    // 图片的裁切和缩放配置    open var imageCropAndScaleOption: VNImageCropAndScaleOption    // 结果列表    open var results: [VNFeaturePrintObservation]? { get }}

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VNFeaturePrintObservatio 结果实例中封装了特征数据：

open class VNFeaturePrintObservation : VNObservation {    // 特征类型    open var elementType: VNElementType { get }    // 特征元素数量    open var elementCount: Int { get }    // 特征数据    open var data: Data { get }    // 进行差距比较    open func computeDistance(_ outDistance: UnsafeMutablePointer<Float>, to featurePrint: VNFeaturePrintObservation) throws}

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其中 computeDistance 方法即用来进行两个分析结果的特征差距计算。对于完全一样的图片，计算的差距为 0，差距越大，表明图片的相似度越小。

下面提供了完整的 Demo 代码：

import UIKitimport Vision
class ImageFeatureViewController: UIViewController {        let image1 = UIImage(named: "cat1")!    let image2 = UIImage(named: "cat2")!    let image3 = UIImage(named: "dog1")!    let image4 = UIImage(named: "dog2")!    lazy var imageView1 = UIImageView(image: image1)    lazy var imageView2 = UIImageView(image: image2)    lazy var imageView3 = UIImageView(image: image3)    lazy var imageView4 = UIImageView(image: image4)    
    override func viewDidLoad() {        super.viewDidLoad()        view.backgroundColor = .white                view.addSubview(imageView1)        view.addSubview(imageView2)        view.addSubview(imageView3)        view.addSubview(imageView4)                let width = (self.view.frame.width - 60) / 2        let h1 = width / (image1.size.width / image1.size.height)        let h2 = width / (image2.size.width / image2.size.height)        let h3 = width / (image3.size.width / image3.size.height)        let h4 = width / (image4.size.width / image4.size.height)                imageView1.frame = CGRect(x: 20, y: 100, width: width, height: h1)        imageView2.frame = CGRect(x: width + 40, y: 100, width: width, height: h2)        imageView3.frame = CGRect(x: 20, y: max(h1, h2) + 120, width: width, height: h3)        imageView4.frame = CGRect(x: width + 40, y: max(h1, h2) + 120, width: width, height: h4)        // 进行特征值分析        sendRequest(image: image1, number: 1)        sendRequest(image: image2, number: 2)        sendRequest(image: image3, number: 3)        sendRequest(image: image4, number: 4)               }        func sendRequest(image: UIImage, number: Int) {        let handler = VNImageRequestHandler(cgImage: image.cgImage!, orientation: .up)        let request = VNGenerateImageFeaturePrintRequest { result, error in            guard error == nil else {                print(error!)                return            }            let r = result as! VNGenerateImageFeaturePrintRequest            if number == 1 {                self.result1 = r.results?.first            }            if number == 2 {                self.result2 = r.results?.first            }            if number == 3 {                self.result3 = r.results?.first            }            if number == 4 {                self.result4 = r.results?.first            }            if let result1 = self.result1, let result2 = self.result2, let result3 = self.result3, let result4 = self.result4 {                // 进行相似性对比                var distance12 = Float(0)                try! result1.computeDistance(&distance12, to: result2)                var distance13 = Float(0)                try! result1.computeDistance(&distance13, to: result3)                var distance34 = Float(0)                try! result3.computeDistance(&distance34, to: result4)                print("图1与图2相似差距:", distance12)                print("图1与图3相似差距:", distance13)                print("图3与图4相似差距:", distance34)                DispatchQueue.main.async {                    let l = UILabel()                    l.text = "图1与图2相似差距:\(distance12)\n图1与图3相似差距:\(distance13)\n图3与图4相似差距:\(distance34)"                    l.font = .boldSystemFont(ofSize: 22)                    self.view.addSubview(l)                    l.frame = CGRect(x: 0, y: max(self.imageView3.frame.height, self.imageView4.frame.height) + self.imageView3.frame.origin.y, width: self.view.frame.width, height: 100)                    l.numberOfLines = 0                }            }        }        try? handler.perform([request])    }        var result1: VNFeaturePrintObservation?    var result2: VNFeaturePrintObservation?    var result3: VNFeaturePrintObservation?    var result4: VNFeaturePrintObservation?
}