平滑地应用渐变到UIImage

Question

我正在尝试使用CoreGraphic将渐变应用于UIImage;但是，我得到的结果并不是很好。我想在图像的底部创建一个黑色到透明的渐变，为我创建一个对比，让我放置一些文本。然而，渐变我能够与图像很好地融合; 您可以清楚地看到中心的分离。我要找的结果就像这个应用程序：http://capptivate.co/2014/02/17/yummly-3/ 我应该如何应用渐变来实现这一目标？ （我必须将它应用于大量图像）。

我的结果：

这是我的代码：

func imageWithGradient(img:UIImage!) -> UIImage{

    UIGraphicsBeginImageContext(img.size)
    var context = UIGraphicsGetCurrentContext()

    img.drawAtPoint(CGPointMake(0, 0))

    let colorSpace = CGColorSpaceCreateDeviceRGB()
    let locations:[CGFloat] = [0.50, 1.0]
    //1 = opaque
    //0 = transparent
    let bottom = UIColor(red: 0, green: 0, blue: 0, alpha: 0.5).CGColor
    let top = UIColor(red: 0, green: 0, blue: 0, alpha: 0).CGColor

    let gradient = CGGradientCreateWithColors(colorSpace,
        [top, bottom], locations)


    let startPoint = CGPointMake(img.size.width/2, 0)
    let endPoint = CGPointMake(img.size.width/2, img.size.height)

    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0)

    let image = UIGraphicsGetImageFromCurrentImageContext()

    UIGraphicsEndImageContext()

    return image
}

Answer 1

SWIFT 3

func imageWithGradient(img:UIImage!) -> UIImage {

    UIGraphicsBeginImageContext(img.size)
    let context = UIGraphicsGetCurrentContext()

    img.draw(at: CGPoint(x: 0, y: 0))

    let colorSpace = CGColorSpaceCreateDeviceRGB()
    let locations:[CGFloat] = [0.0, 1.0]

    let bottom = UIColor(red: 0, green: 0, blue: 0, alpha: 0.5).cgColor
    let top = UIColor(red: 0, green: 0, blue: 0, alpha: 0).cgColor

    let colors = [top, bottom] as CFArray

    let gradient = CGGradient(colorsSpace: colorSpace, colors: colors, locations: locations)

    let startPoint = CGPoint(x: img.size.width/2, y: 0)
    let endPoint = CGPoint(x: img.size.width/2, y: img.size.height)

    context!.drawLinearGradient(gradient!, start: startPoint, end: endPoint, options: CGGradientDrawingOptions(rawValue: UInt32(0)))

    let image = UIGraphicsGetImageFromCurrentImageContext()

    UIGraphicsEndImageContext()

    return image!
}

Answer 2

我在Swift游乐场尝试了这个，我无法复制你所描述的确切问题，但是这里有一些你可以尝试的建议。

你可以改变你在渐变中设置光圈的方式，我认为更有意义的是不是使用光圈移动渐变，而是移动起点，即不是从顶边开始，而是从垂直中心，像这样

UIGraphicsBeginImageContext(image.size)
var context = UIGraphicsGetCurrentContext()

image.drawAtPoint(CGPointMake(0, 0))

let colorSpace = CGColorSpaceCreateDeviceRGB()
let locations:[CGFloat] = [0.0, 1.0]
let bottom = UIColor(red: 0, green: 0, blue: 0, alpha: 0.5).CGColor
let top = UIColor(red: 0, green: 0, blue: 0, alpha: 0).CGColor

let gradient = CGGradientCreateWithColors(colorSpace,
               [top, bottom], locations)

let startPoint = CGPointMake(image.size.width / 2, image.size.height / 2)
let endPoint = CGPointMake(image.size.width / 2, image.size.height)
CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0)

let finalImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()

通知位置现在为0和1，而startPoint y坐标向下移动。

或者，请将kCGGradientDrawsBeforeStartLocation作为选项传递给CGContextDrawLinearGradient，因为这也可能有所作为。