is there any way to detect shape contours in Swift without having to branch out to OpenCV?

Question

Obviously, we can use CoreImage to convert to gray, denoise and blur an image but then when it comes to Canny Edge Detection, contour detection and to draw a contour of a shape it seems that this is not possible just using swift features. Am I right or is there something that I am missing here?

Resa

Answer 1

In June 2020 Apple added a new Contour Detection feature to their Vision Framework in iOS 14. (currently in beta), so you can now use Swift (w/o OpenCV) in order to detect contours.

You can find more details in this documentation from Apple: https://developer.apple.com/documentation/vision/vndetectcontoursrequest

Also relevant is this WWDC2020 video: https://developer.apple.com/videos/play/wwdc2020/10673/

Joacimah

Answer 2

Now you can use VNDetectContoursRequest to make detection with SwiftUI for iOS 14:

func detectVisionContours(){
    
    let context = CIContext()
    if let sourceImage = UIImage.init(named: "coin")
    {
        var inputImage = CIImage.init(cgImage: sourceImage.cgImage!)
        
        let contourRequest = VNDetectContoursRequest.init()
        contourRequest.revision = VNDetectContourRequestRevision1
        contourRequest.contrastAdjustment = 1.0
        contourRequest.detectDarkOnLight = true            
        contourRequest.maximumImageDimension = 512


        let requestHandler = VNImageRequestHandler.init(ciImage: inputImage, options: [:])

        try! requestHandler.perform([contourRequest])
        let contoursObservation = contourRequest.results?.first as! VNContoursObservation
        
        self.points  = String(contoursObservation.contourCount)
        self.contouredImage = drawContours(contoursObservation: contoursObservation, sourceImage: sourceImage.cgImage!)

    } else {
        self.points = "Could not load image"
    }
}

and for drawing contours do this:

public func drawContours(contoursObservation: VNContoursObservation, sourceImage: CGImage) -> UIImage {
    let size = CGSize(width: sourceImage.width, height: sourceImage.height)
    let renderer = UIGraphicsImageRenderer(size: size)
    
    let renderedImage = renderer.image { (context) in
        let renderingContext = context.cgContext

        let flipVertical = CGAffineTransform(a: 1, b: 0, c: 0, d: -1, tx: 0, ty: size.height)
        renderingContext.concatenate(flipVertical)

        renderingContext.draw(sourceImage, in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
        
        renderingContext.scaleBy(x: size.width, y: size.height)
        renderingContext.setLineWidth(5.0 / CGFloat(size.width))
        let redUIColor = UIColor.red
        renderingContext.setStrokeColor(redUIColor.cgColor)
        renderingContext.addPath(contoursObservation.normalizedPath)
        renderingContext.strokePath()
    }
    
    return renderedImage
}

Reference

Cruiser