Using AVKit to detect luminosity - ios

Question

I am working on an app using SwiftUi that leverages the device camera to detect luminosity as described in top answer of this post. The captureOutput(_:didOutput:from:) function in the top answer was used to calculate luminosity. According to Apple Docs this function is intended to notify a delegate that a new video frame was written, and so I have placed this function in a VideoDelegate class. This delegate is then set in a VideoStream class that handles the logic of asking permissions and setting up an AVCaptureSession. My question is how to access the luminosity value calculated within the delegate inside my SwiftUI view?
struct ContentView: View {
#StateObject var videoStream = VideoStream()
var body: some View {
Text("\(videoStream.luminosityReading) ?? Detecting...")
.padding()
}
}
class VideoStream: ObservableObject {
#Published var luminosityReading : Double = 0.0 // TODO get luminosity from VideoDelegate
var session : AVCaptureSession!
init() {
authorizeCapture()
}
func authorizeCapture() {
// permission logic and call to beginCapture()
}
func beginCapture() {
session = AVCaptureSession()
session.beginConfiguration()
let videoDevice = bestDevice() // func definition omitted for readability
guard
let videoDeviceInput = try? AVCaptureDeviceInput(device: videoDevice),
session.canAddInput(videoDeviceInput)
else {
print("Camera selection failed")
return
}
let videoOutput = AVCaptureVideoDataOutput()
guard
session.canAddOutput(videoOutput)
else {
print("Error creating video output")
return
}
session.sessionPreset = .high
session.addOutput(videoOutput)
let queue = DispatchQueue(label: "VideoFrameQueue")
let delegate = VideoDelegate()
videoOutput.setSampleBufferDelegate(delegate, queue: queue)
session.commitConfiguration()
session.startRunning()
}
}
class VideoDelegate: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {
func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
//Retrieving EXIF data of camara frame buffer
let rawMetadata = CMCopyDictionaryOfAttachments(allocator: nil, target: sampleBuffer, attachmentMode: CMAttachmentMode(kCMAttachmentMode_ShouldPropagate))
let metadata = CFDictionaryCreateMutableCopy(nil, 0, rawMetadata) as NSMutableDictionary
let exifData = metadata.value(forKey: "{Exif}") as? NSMutableDictionary
let FNumber : Double = exifData?["FNumber"] as! Double
let ExposureTime : Double = exifData?["ExposureTime"] as! Double
let ISOSpeedRatingsArray = exifData!["ISOSpeedRatings"] as? NSArray
let ISOSpeedRatings : Double = ISOSpeedRatingsArray![0] as! Double
let CalibrationConstant : Double = 50
//Calculating the luminosity
let luminosity : Double = (CalibrationConstant * FNumber * FNumber ) / ( ExposureTime * ISOSpeedRatings )
// how to pass value of luminosity to `VideoStream`?
}
}

Answer 1

As discussed in the comments, the lowest friction option would be to have VideoStream conform to AVCaptureVideoDataOutputSampleBufferDelegate and implement the delegate method there.