# 234-Text Recognition in Vision Framework

以前 Vision 只能识别 text rectangles ( VNDetectTextRectanglesRequest )。iOS 13 新增 VNRecognizeTextRequest 类，用于识别 text。

现在只支持 en_US。

iOS 14 的 VNRecognizeTextRequestRevision2 现在已经支持 en-US, fr-FR, it-IT, de-DE, es-ES, pt-BR, zh-Hans, zh-Hant。

let array = try? VNRecognizeTextRequest.supportedRecognitionLanguages(for: .fast, revision: VNRecognizeTextRequestRevision1)
// 输出：["en_US"]
// .accurate 也是一样的
// 说明在 VNRecognizeTextRequestRevision1 下只支持英语

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# How Text Recognition works

Text recognition 有两种方式，Fast 和 Accurate

Fast：一个字符一个字符识别，组成 text

Accurate：使用先进的神经网络 (neural network) 识别 text

# 结构

# 对比

# Example applications

# 样例代码

// 创建 request handler
let myRequestHandler = VNImageRequestHandler(url: fileURL, options: [:])
// 创建 request
let myTextRecognitionRequest = VNRecognizeTextRequest()
// 设置 request completion handler
myTextRecognitionRequest.completionHandler = { (request, error) in
  // 确定 VNRecognizedTextObservation
  guard let results = request.results as? [VNRecognizedTextObservation] else {
    return
  }
  // 多行
  for visionResult in results { 
    // 候选数量
    let maximumCandidates = 1
    guard let candidate = visionResult.topCandidates(maximumCandidates).first else {
      continue
    }
    
    // 识别出的 text
    print(candidate.string)
 
    // bounding box
    let boundingBox = visionResult.boundingBox
     
    if let range = candidate.string.range(of: "stress"),
      let boxObservation = try? candidate.boundingBox(for: range) {
      // stress 单词的 bounding box
      let foundTextBox = boxObservation.boundingBox
    }
  }
}
 
// Select the recognition level
myTextRecognitionRequest.recognitionLevel = VNRequestTextRecognitionLevel.accurate
// Set the revision
myTextRecognitionRequest.revision = VNRecognizeTextRequestRevision1
// 是否自动更正
myTextRecognitionRequest.usesLanguageCorrection = true
  
// Send the request to the request handler
try myRequestHandler.perform([myTextRecognitionRequest])

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# Phone Number Reader

Reading Phone Numbers in Real Time (opens new window)

# Document Camera

import VisionKit

func present() {
  let documentCameraViewController = VNDocumentCameraViewController()
  documentCameraViewController.delegate = self
  present(documentCameraViewController, animated: true)
}

func documentCameraViewController(_ controller: VNDocumentCameraViewController, didFinishWith scan: VNDocumentCameraScan) {
  controller.dismiss(animated: true) {
    textRecognitionWorkQueue.async {
      for pageIndex in 0 ..< scan.pageCount {
        let image = scan.imageOfPage(at: pageIndex) 
        if let cgImage = image.cgImage {
          let requestHandler = VNImageRequestHandler(cgImage: cgImage, options: [:])
          do {
            try requestHandler.perform(self.requests)
          } catch {
            print(error)
          } // do
        } // if
      }  // for
    } // queue
  } // dismiss
}

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Simple Text Recognition (opens new window)

# Best practices

My First Image Reader (opens new window)

Locating and Displaying Recognized Text on a Document (opens new window)

# Processing Results

# Use Geometry to Map Results

Compare spatial information

Position and scale
Rotation

# Use Parsers to Label Results

Data Detectors

NSDataDetector for types of interest
Addresses, URLs, dates, and phone numbers

Domain-specific filters

Your own lexicon
Regular expressions

# VNRecognizeTextRequest 其它属性

// 识别的语言，目前只支持 en_US
var recognitionLanguages: [String]

// 自定义单词，某些特殊单词
var customWords: [String]

// 识别单词最小高度，单位是图片高度
var minimumTextHeight: Float

// 识别单词最小高度，单位是图片高度
var minimumTextHeight: Float

// 识别区域 0~1.0
var regionOfInterest: CGRect

// 只使用CPU，把 GPU 和 Neural Engine 用于优先级高的应用
var usesCPUOnly: Bool

// 识别进度报告
var progressHandler: VNRequestProgressHandler

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