AVCapture 似乎滞后，文本识别不会立即开始

Question

我是 Swift 新手，目前正在开发一个包含文本识别的功能。我正在使用 Firebase 的 MLKit 并拥有代码，虽然它不是很复杂（我愿意接受每一个改进我的编码的建议），但几乎已经设置好了。

总之有两件事困扰着我：

1. 自从我添加了文本识别后，实时提要似乎滞后（大约每秒 1 帧） - 我认为这是由文本识别引起的，以防止过载？如果是这样，如何断开实时取景和处理的帧？
2. 文本识别似乎在 10 秒后开始。有没有让它立即开始？

CameraViewController：

import UIKit
import AVKit
import Vision
import FirebaseMLVision


class CameraViewController: UIViewController, AVCaptureVideoDataOutputSampleBufferDelegate {    

    private lazy var vision = Vision.vision()
    private lazy var textRecognizer = vision.onDeviceTextRecognizer()

    override func viewDidLoad() {
        super.viewDidLoad()
        captureSession()
    }

    func captureSession () {
        let captureSession = AVCaptureSession()

        guard let captureDevice = AVCaptureDevice.default(for: .video) else { return }
        guard let input = try? AVCaptureDeviceInput(device: captureDevice) else{ return }
        captureSession.addInput(input)

        captureSession.startRunning()

        let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        view.layer.addSublayer(previewLayer)
        previewLayer.frame = view.frame

        let dataOutput = AVCaptureVideoDataOutput()
        dataOutput.setSampleBufferDelegate(self, queue: DispatchQueue(label: "videoQueue"))
        captureSession.addOutput(dataOutput)
    }

    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {


        let metadata = VisionImageMetadata()

        let devicePosition: AVCaptureDevice.Position = .back

        let deviceOrientation = UIDevice.current.orientation

        switch deviceOrientation {
        case .portrait:
            metadata.orientation = devicePosition == .front ? .leftTop : .rightTop
        case .landscapeLeft:
            metadata.orientation = devicePosition == .front ? .bottomLeft : .topLeft
        case .portraitUpsideDown:
            metadata.orientation = devicePosition == .front ? .rightBottom : .leftBottom
        case .landscapeRight:
            metadata.orientation = devicePosition == .front ? .topRight : .bottomRight
        case .faceDown, .faceUp, .unknown:
            metadata.orientation = .leftTop
        }

        let image = VisionImage(buffer: sampleBuffer)
        image.metadata = metadata

        textRecognizer.process(image) { result, error in
            guard error == nil, let result = result else {
                return
            }

            for block in result.blocks {
                for line in block.lines {
                    for element in line.elements {
                        let elementText = element.text
                        print(element.text)
                    }
                }
            }
        }
    }

}

Answer 1

您需要更新您的 AVCaptureVideoDataOutput

output.alwaysDiscardsLateVideoFrames = true

如https://github.com/googlecodelabs/mlkit-ios/blob/master/translate/TranslateDemo/CameraViewController.swift#L307