VNFaceObservation: Computes the landmarks normalized on the smallest bounding box which includes all of the landmarks.

LandmarksBBox.swift

extension VNFaceObservation {
    
    func landmarksBBox(_ imageSize: CGSize) -> (bbox: CGRect, landmarks: [CGPoint]) {
        guard let allPoints = self.landmarks?.allPoints?.pointsInImage(imageSize: imageSize),
              allPoints.count > 0 else { return (CGRect.zero, []) }
            
        var maxX : CGFloat = -CGFloat.greatestFiniteMagnitude
        var minX : CGFloat = CGFloat.greatestFiniteMagnitude
        var maxY = maxX
        var minY = minX

        allPoints.forEach { point in
            if point.x > maxX {
                maxX = point.x
            }
            if point.y > maxY {
                maxY = point.y
            }
            if point.x < minX {
                minX = point.x
            }
            if point.y < minY {
                minY = point.y
            }
        }
      
        let bboxWidth = (maxX - minX)
        let bboxHeight = (maxY - minY)
        let normalizedBBoxOrigin = CGPoint(x: minX/imageSize.width, y: minY/imageSize.height)
        let normalizedBBoxSize = CGSize(width: bboxWidth / imageSize.width, height: bboxHeight / imageSize.height)
        
        let normalizedBBox = CGRect(origin: normalizedBBoxOrigin, size: normalizedBBoxSize)
        //compute the normalized points with respect to the new bounding box
        let normalizedPoints = allPoints.map { point -> CGPoint in
            let normalizedPoint = CGPoint(x: point.x/imageSize.width, y: point.y/imageSize.height)
            let normalizationFactor = CGSize(width: imageSize.width / bboxWidth, height: imageSize.height / bboxHeight)
            return CGPoint(x: (normalizedPoint.x - normalizedBBoxOrigin.x) * normalizationFactor.width, y: (normalizedPoint.y - normalizedBBoxOrigin.y) * normalizationFactor.height)
        }
        
        return (normalizedBBox, normalizedPoints)
    }
}

The usual VNFaceObservation bounding box does not always contain all the landmarks. This extension computes the smallest bounding box containing all the landmarks. The bounding box is expressed as normalized in the image dimension, while the landmarks are expressed as normalized in the bounding box dimension.