ios - 使用 cgImage.mask 屏蔽两个 UIImage 不起作用，但适用于 imageview.layer.mask

Question

我有两张图像，它们都有 Alpha channel 。我想把它们结合在一起。它确实适用于 UIImageView，但我想通过使用 cgimage 来实现，而无需创建 UIImageView。 我尝试了 cgimage，这样不起作用:

    func  combine3(_ bg: UIImage, cover: UIImage) -> UIImage?{
        let size = CGSize(width: self.view.frame.width, height: self.view.frame.height)
        UIGraphicsBeginImageContext(size)
        
        let maskRef = cover.cgImage!
        let mask = CGImage.init(maskWidth: maskRef.width, height: maskRef.height, bitsPerComponent: maskRef.bitsPerComponent, bitsPerPixel: maskRef.bitsPerPixel, bytesPerRow: maskRef.bytesPerRow, provider: maskRef.dataProvider!, decode: nil, shouldInterpolate: false)
        let masked = bg.cgImage?.masking(mask!)
        let outPutImage = UIImage(cgImage: masked!)
        
        UIGraphicsEndImageContext()
        return outPutImage
    }

但是对于 UIImageView 来说，它的效果非常好:

        let bg = creatBGImageFinal()!
        bgIV.image = bg

        let cover = createCoverImageFinal(progress: 0)!
        let layer = CALayer()
        layer.contents = cover.cgImage
        layer.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)
        maskIV.layer.mask = layer

背景: cover，是一张中间有白圈的图片，其他都是透明的: 结果:

Answer 1

首先，反转蒙版:

cgImage屏蔽的规则是Dst = 1 - Src。

然后将蒙版图像绘制到具有不透明度的图像上下文上:

func combine3(_ bg: UIImage, cover: UIImage, size: CGSize) -> UIImage? {
    UIGraphicsBeginImageContextWithOptions(size, true, 1)
    defer {
        UIGraphicsEndImageContext()
    }
    let context = UIGraphicsGetCurrentContext()!
    let maskRef = cover.cgImage!
    let mask = CGImage.init(maskWidth: maskRef.width, height: maskRef.height, bitsPerComponent: maskRef.bitsPerComponent, bitsPerPixel: maskRef.bitsPerPixel, bytesPerRow: maskRef.bytesPerRow, provider: maskRef.dataProvider!, decode: nil, shouldInterpolate: false)!
    let masked = bg.cgImage!.masking(mask)!
    // adjust for lower-left-origin CG coordinates
    context.translateBy(x: 0, y: size.height)
    context.scaleBy(x: 1, y: -1)
    context.draw(masked, in: CGRect(origin: .zero, size: size))
    return UIGraphicsGetImageFromCurrentImageContext()
}

结果: