如何在Swift中计算颜色合成?

我正在尝试为我的照片编辑器应用程序创建橡皮擦工具 Photoshop (或者可以从图像中删除已定义的颜色),例如,用户将在图像上绘制具有所需颜色的线条,我的功能将在触摸立即结束后处理图像。

我创建了一个方法,它可以很好地与一些颜色,如白色,黑色,黄色等,但当我选择颜色与0.5阿尔法它不工作的理想!

extension UIImage {

func getImageRemovedDefinedColor(choosedColor: UIColor) -> UIImage? {

    let image = UIImage(data: self.jpegData(compressionQuality: 1.0)!)!
    let rawImageRef: CGImage = image.cgImage!

    let components = choosedColor.cgColor.components!

    var colorMasking: [CGFloat] = []
    var red: CGFloat = 0.0, green: CGFloat = 0.0, blue: CGFloat = 0.0, alpha: CGFloat = 0.0

    if choosedColor.isEqual(UIColor.white) {

        colorMasking = [222, 255, 222, 255, 222, 255]

    } else if choosedColor.isEqual(UIColor.black) {

        colorMasking = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

    } else {

        if components.count > 3 {

            red = components[0]
            green = components[1]
            blue = components[2]
            alpha = components[3]

            red = red * 255.0
            green = green * 255.0
            blue = blue * 255.0
            alpha = alpha * 255.0

            let redRange: [CGFloat] = {
                return [max(red - (alpha / 2.0), 0.0), min(red + (alpha / 2.0), 255.0)]
            }()

            let greenRange: [CGFloat] = {
                return [max(green - (alpha / 2.0), 0.0), min(green + (alpha / 2.0), 255.0)]
            }()

            let blueRange: [CGFloat] = {
                return [max(blue - (alpha / 2.0), 0.0), min(blue + (alpha / 2.0), 255.0)]
            }()

            colorMasking = [redRange[0], redRange[1], greenRange[0], greenRange[1], blueRange[0], blueRange[1]]
        }
    }

    print(colorMasking)

    UIGraphicsBeginImageContext(image.size);

    let maskedImageRef = rawImageRef.copy(maskingColorComponents: colorMasking)
    UIGraphicsGetCurrentContext()?.translateBy(x: 0.0,y: image.size.height)
    UIGraphicsGetCurrentContext()?.scaleBy(x: 1.0, y: -1.0)
    UIGraphicsGetCurrentContext()?.draw(maskedImageRef!, in: CGRect.init(x: 0, y: 0, width: image.size.width, height: image.size.height))
    let result = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return result

}

注: 也许我做这个工具的方式是完全错误的,如果你有关于“我们怎样才能用Swift创建橡皮擦工具”的另一个想法,请分享。