iOS：在相机预览期间捕获图像而不采取行动

Question

我正在尝试从相机预览中捕获图像，但无法从预览层获取图像。我想做的有点类似于照片应用程序中的 iOS 15 OCR 模式，它在相机预览期间处理图像，不需要用户拍摄或开始录制视频，只需在预览中处理图像。我查看了文档并在网上搜索，但找不到任何有用的信息。

我尝试的是，保存 previewLayer 并定期调用 previewLayer.draw(in: context) 。但是在上下文中绘制的图像是空白的。现在我想知道这是否可能首先。 可能存在一些安全问题，以限制在相机预览中处理图像，我猜只有正版应用程序才能访问，所以我可能需要找到其他方法。

如果有任何解决方法，请赐教。

谢谢！

Answer 1

好的。在 MadProgrammer 的帮助下，我得到了正常工作。 Anurag Ajwani's site 很有帮助。

这是我用来捕捉视频帧的简单 sn-p。您需要在 CameraView 实例化之前确保权限。

class VideoCapture: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {
    //private var previewLayer: AVCaptureVideoPreviewLayer? = nil
    private var session: AVCaptureSession? = nil
    private var videoOutput: AVCaptureVideoDataOutput? = nil

    private var videoHandler: ((UIImage) -> Void)?

    override init() {
        super.init()
        
        let deviceSession = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInDualWideCamera, .builtInWideAngleCamera], mediaType: .video, position: .back)
        guard deviceSession.devices.count > 0 else { return }

        if let input = try? AVCaptureDeviceInput(device: deviceSession.devices.first!) {
            let session = AVCaptureSession()
            session.addInput(input)

            let videoOutput = AVCaptureVideoDataOutput()
            videoOutput.videoSettings = [(kCVPixelBufferPixelFormatTypeKey as NSString): NSNumber(value: kCVPixelFormatType_32BGRA)] as [String:Any]
            videoOutput.setSampleBufferDelegate(self, queue: DispatchQueue(label: "my.image.handling.queue"))
            videoOutput.alwaysDiscardsLateVideoFrames = true
            if session.canAddOutput(videoOutput) {
                session.sessionPreset = .high
                session.addOutput(videoOutput)
                self.videoOutput = videoOutput
            }

            for connection in videoOutput.connections {
                if connection.isVideoOrientationSupported {
                    connection.videoOrientation = .portrait
                }
            }

            session.commitConfiguration()

            self.session = session

            /*
            self.previewLayer = AVCaptureVideoPreviewLayer(session: session)
            if let previewLayer = self.previewLayer {
                previewLayer.videoGravity = .resizeAspectFill
                layer.insertSublayer(previewLayer, at: 0)
                CameraPreviewView.initialized = true
            }
             */
       }
    }

    func startCapturing(_ videoHandler: @escaping (UIImage) -> Void) -> Void {
        if let session = session {
            session.startRunning()
        }
        self.videoHandler = videoHandler
    }

    // AVCaptureVideoDataOutputSampleBufferDelegate
    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        guard let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else {
            debugPrint("unable to get video frame")
            return
        }
        //print("got video frame")
        if let videoHandler = self.videoHandler {
            let rect = CGRect(x: 0, y: 0, width: CVPixelBufferGetWidth(imageBuffer), height: CVPixelBufferGetHeight(imageBuffer))
            let ciImage = CIImage.init(cvImageBuffer: imageBuffer)
            let ciContext = CIContext()
            let cgImage = ciContext.createCGImage(ciImage, from: rect)
            guard cgImage != nil else {return }
            let uiImage = UIImage(cgImage: cgImage!)
            videoHandler(uiImage)
        }
    }
}

struct CameraView: View {
    @State var capturedVideo: UIImage? = nil

    let videoCapture = VideoCapture()

   var body: some View {
       VStack {
           ZStack(alignment: .center) {
               if let capturedVideo = self.capturedVideo {
                   Image(uiImage: capturedVideo)
                       .resizable()
                       .scaledToFill()
               }
           }
       }
       .background(Color.black)
       .onAppear {
           self.videoCapture.startCapturing { uiImage in
               self.capturedVideo = uiImage
           }
       }
   }