可以设置VNRequestRevision1来获取紧密贴合的检测矩形。这个方法校正了iOS 11 Vision框架返回的不准确的检测矩形。以下是代码实现：

let request = VNDetectRectanglesRequest(completionHandler: { (request, error) in

    guard error == nil else {
        print("Error detecting rectangles: \(error!.localizedDescription)")
        return
    }

    if let observations = request.results as? [VNRectangleObservation] {
        for observation in observations {
            let correctedBoundingBox = observation.boundingBox.applying(transform)
            let outline = createRectangle(boundingBox: correctedBoundingBox, color: UIColor.red)
            self.drawLayer.addSublayer(outline)
        }
    }
})
request.revision = VNRequestRevision1

其中，createRectangle方法是用来创建红色的矩形框。