为什么我的弦乐图像之一被拍打而不被拍打？

Question

我正在使用Rob Mayoff在对this SO question的回答中提供的代码，作为创建向UIImage（不一定是字符串）添加浮雕功能的起点。我正在尝试不同的实现，但遇到了麻烦。在我的代码中（见下文），我有两个不同版本的函数，该函数为字符串生成掩码。 mask1（Rob's）和mask2（mine）。他们似乎都正常工作。但是，当我将蒙版传递给求反函数时，Rob的蒙版正确取反，而我的蒙皮被翻转了。

This SO question似乎提供了解释，但我无法完全解释，因为它仅发生在其中一张蒙版图像上。

模拟器屏幕截图包含四个图像视图。从顶部开始：1）Rob的面具，2）Rob的面具的底片，3）我的面具，4）我的面具的底片。

我已将项目放在github

有人可以帮我了解发生了什么事吗？

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var imageView1: UIImageView!
    @IBOutlet weak var imageView2: UIImageView!
    @IBOutlet weak var imageView3: UIImageView!
    @IBOutlet weak var imageView4: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()

        let text = "Verticon"
        let font = UIFont(name: "Helvetica", size: 100)!
        let size = imageView1.bounds.size

        let maskA = mask1(from: text, drawnWith: font, andSize: size)
        imageView1.image = maskA

        let negativeA = negative(of: maskA)
        imageView2.image = negativeA

        let maskB = mask2(from: text, drawnWith: font, andSize: size)
        imageView3.image = maskB

        let negativeB = negative(of: maskB)
        imageView4.image = negativeB
    }
}

extension ViewController {

    // Draw the specified string using the provided attributes and return the resultant image (which has only an alpha channel, no color information).
    func mask1(from string: String, drawnWith font: UIFont, andSize size: CGSize) -> UIImage {
        let scale = UIScreen.main.scale
        let colorSpace = CGColorSpaceCreateDeviceGray()
        let bitmapInfo = CGImageAlphaInfo.alphaOnly.rawValue

        guard let context = CGContext(data: nil, width: Int(size.width * scale), height: Int(size.height * scale), bitsPerComponent: 8, bytesPerRow: Int(size.width * scale), space: colorSpace, bitmapInfo: bitmapInfo) else { fatalError("mask1() - cannot create a graphics context") }
        context.scaleBy(x: scale, y: scale)

        UIGraphicsPushContext(context)
        let rect = CGRect(origin: CGPoint.zero, size: size)
        let attributes: [NSAttributedStringKey : Any] = [.font : font]
        string.draw(in: rect, withAttributes: attributes)
        UIGraphicsPopContext()

        guard let cgImage = context.makeImage() else { fatalError("mask1() - could not get image from context.") }
        return UIImage(cgImage: cgImage, scale: scale, orientation: .downMirrored)
    }

    // Draw the specified string using the provided attributes and return the resultant image.
    func mask2(from string: String, drawnWith font: UIFont, andSize size: CGSize) -> UIImage {
        let renderer = UIGraphicsImageRenderer(size: size)
        return renderer.image { ctx in
            let rect = CGRect(origin: CGPoint.zero, size: size)
            let attributes: [NSAttributedStringKey : Any] = [.font : font, .foregroundColor : UIColor.white]
            string.draw(in: rect, withAttributes: attributes)
        }
    }

    // Return a black rectangle wherein the area corresponding to the specified image has been cleared (made transparent).
    func negative(of: UIImage) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(of.size, false, of.scale)

        let rect = CGRect(origin: .zero, size: of.size)
        UIColor.black.setFill()
        UIRectFill(rect)
        guard let cgImage = of.cgImage else { fatalError("negative() - could not obtain CGImage.") }
        guard let context = UIGraphicsGetCurrentContext() else { fatalError("negative() - could not obtain CGContext.") }
        context.clip(to: rect, mask: cgImage)
        context.clear(rect)
        guard let negative = UIGraphicsGetImageFromCurrentImageContext() else { fatalError("invert() could not obtain negative.") }

        UIGraphicsEndImageContext();

        return negative;

    }
}