平滑地应用渐变到UIImage

Question

I am trying to apply gradient to UIImage using CoreGraphic; however, the result I got is not very nice. I want to create a black to transparent gradient at the bottom of the image to create a contrast for me to place some text. However, the gradient I was able doesn't blend well with the image; you can clearly see the separation in the center . The result I am looking for is like this application: http://capptivate.co/2014/02/17/yummly-3/ How should I apply the gradient to achieve this? ( I have to apply this to large quantity of images ).

My Result:

Here is my code:

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

I tried this out in a Swift playground, I couldn't replicate the exact issue you are describing, however here are some suggestions that you could try.

You could change the way you set up the stops in the gradient, I think it would make more sense to instead of shifting the gradient using the stops, shift the start point, ie instead of starting from the top edge, start from the vertical center, something like this

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()

Notice locations is now 0 and 1, while the startPoint y coordinate has shifted downwards.

Alternatively, look at passing kCGGradientDrawsBeforeStartLocation as an option to CGContextDrawLinearGradient , as that could also make a difference.