How to create border in ciimage in swift

Question

I'm writing a feature for correcting perspective of documents scanned from ios camera.

I want to create a border effect around the document whose edge has been determined. CiImage has the initializer property with which i can create a color overlay over the detected edge. What i want to do is instead of creating colored area I want to create bordered area but in real time. What is the best way to do that?

fileprivate func overlayImageForFeatureInImage(_ image: CIImage, feature: CIRectangleFeature) -> CIImage! {

    var overlay = CIImage(color: CIColor(color: self.borderDetectionFrameColor))

    overlay = overlay.cropping(to: image.extent)
    print("the extent of image is \(image.extent)")

    overlay = overlay.applyingFilter("CIPerspectiveTransformWithExtent", withInputParameters: ["inputExtent":     CIVector(cgRect: image.extent),
                                                                                               "inputTopLeft":    CIVector(cgPoint: feature.topLeft),
                                                                                               "inputTopRight":   CIVector(cgPoint: feature.topRight),
                                                                                               "inputBottomLeft": CIVector(cgPoint: feature.bottomLeft),
                                                                                               "inputBottomRight": CIVector(cgPoint: feature.bottomRight)])

    return overlay.compositingOverImage(image)
}