Image perspective correction

UIImage+CIPerspectiveCorrection.swift

extension UIImage {
    var flattened: UIImage? {
        let ciImage = CIImage(image: self)!

        guard let openGLContext = EAGLContext(api: .openGLES2) else { return nil }
        let ciContext =  CIContext(eaglContext: openGLContext)

        let detector = CIDetector(ofType: CIDetectorTypeRectangle,
                                  context: ciContext,
                                  options: [CIDetectorAccuracy: CIDetectorAccuracyHigh])!

        guard let rect = detector.features(in: ciImage).first as? CIRectangleFeature
            else { return nil }

        let perspectiveCorrection = CIFilter(name: "CIPerspectiveCorrection")!
        perspectiveCorrection.setValue(CIVector(cgPoint: rect.topLeft),
                                       forKey: "inputTopLeft")
        perspectiveCorrection.setValue(CIVector(cgPoint: rect.topRight),
                                       forKey: "inputTopRight")
        perspectiveCorrection.setValue(CIVector(cgPoint: rect.bottomRight),
                                       forKey: "inputBottomRight")
        perspectiveCorrection.setValue(CIVector(cgPoint :rect.bottomLeft),
                                       forKey: "inputBottomLeft")
        perspectiveCorrection.setValue(ciImage,
                                       forKey: kCIInputImageKey)


        if let output = perspectiveCorrection.outputImage,
            let cgImage = ciContext.createCGImage(output, from: output.extent) {
            
            return UIImage(cgImage: cgImage, scale: scale, orientation: imageOrientation)
        }

        return nil
    }
}