swift - 在 CGContext 中重新创建 UIImageView 的 CGAffineTransform

Question

我在 UIVIew 中使用 clipsToBounds = true 转换了 UIImage，并且我想使用 重新创建(重绘)它>CGContext。很难解释，代码如下:

class ViewController: UIViewController {

    @IBOutlet weak var topView: UIView!
    @IBOutlet weak var topImageView: UIImageView!
    @IBOutlet weak var bottomImageView: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()

        // the example image
        let image = UIImage(named: "image")!

        // topImageView is embedded inside topView so that the image is being clipped
        topView.clipsToBounds = true
        topImageView.image = image
        topImageView.contentMode = .center // I set this to center because it seems to be easier to draw it then

        // The transformation data
        let size = CGSize(width: 800, height: 200)
        let scale: CGFloat = 1.5
        let offset: (x: CGFloat, y: CGFloat) = (x: 0.0, y: 0.0)

        // transform the imageView
        topImageView.transform = CGAffineTransform(translationX: offset.x, y: offset.y).scaledBy(x: scale, y: scale)

        // Now try to recreate the transform by using a transformed CGContext
        UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
        let context = UIGraphicsGetCurrentContext()!

        // transform the context
        // Almost the same as above, but I additionally move the context so that the middle of the image is in the middle of the size defined above
        context.concatenate(CGAffineTransform(translationX: (-image.size.width / 2 + size.width / 2) + offset.x, y: (-image.size.height / 2 + size.height / 2) + offset.y).scaledBy(x: scale, y: scale))

        image.draw(in: CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height))
        bottomImageView.image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
    }


}

只有当我将 scale 设置为 1.0(因此没有比例)时，此功能才有效。否则，我无法获得正确的图像。这是一个屏幕截图，以便您查看:

这不一样。很难弄清楚如何正确地转换上下文，我不完全理解它。有什么想法吗？

谢谢!

Answer 1

这是一个谜。我真的花了几个小时试图把这件事做好。发布这个问题5分钟后，我解决了。如果你把它画出来的话，其实并不难(我用Photoshop来可视化坐标系并玩了一下。应该早点做这个..)

UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
let context = UIGraphicsGetCurrentContext()!

context.concatenate(CGAffineTransform.identity.scaledBy(x: scale, y: scale).translatedBy(x: size.width / (2 * scale), y: size.height / (2 * scale)).translatedBy(x: offset.x / scale, y: offset.y / scale))

image.draw(in: CGRect(x: -image.size.width / 2, y: -image.size.height / 2, width: image.size.width, height: image.size.height))
bottomImageView.image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()