How to set the alpha of an UIImage in SWIFT programmatically?

Question

I found a lot solutions here but not for Swift, and I know you can do this with a UIImageView, but in my case i need an programmatically set alpha transparent background image for an UIButton. Best would be an UIImage extension!

let img = UIImage(named: "imageWithoutAlpha")
var imgInsets = UIEdgeInsetsMake(0, 24, 0, 24)
image = image!.resizableImageWithCapInsets(imgInsets)

let myButton = UIButton(frame: CGRect(x: 50, y: 50, width: img!.size.width, height: img!.size.height))
myButton.setBackgroundImage(img, forState: UIControlState.Normal)
myButton.contentEdgeInsets = UIEdgeInsetsMake(0, 20, 0, 20)
myButton.setTitle("Startbutton", forState: UIControlState.Normal)
myButton.setTitleColor(UIColor.blackColor(), forState: UIControlState.Normal)
myButton.sizeToFit()
view.addSubview(myButton)

Current result:

current

Desired result:

desired

Peter Kreinz · Accepted Answer · 2017-09-29T09:16:20.627

96

Luckily I was able to help myself and would like to share with you my solution:

Swift 3

// UIImage+Alpha.swift

extension UIImage {  

    func alpha(_ value:CGFloat) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, false, scale)
        draw(at: CGPoint.zero, blendMode: .normal, alpha: value)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!   
    }
}

The above new Swift extension I added to my project and then I changed the UIButton example as follows, to have an alpha transparent background image with a transparency of 50%.

let img = UIImage(named: "imageWithoutAlpha")!.alpha(0.5)
let myButton = UIButton()

myButton.setBackgroundImage(img, for: .normal)

edited Sep 29 '17 at 09:16

answered Feb 14 '15 at 16:52

Peter Kreinz

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12

In swift 2 I had to replace kCGBlendModeMultiply with CGBlendMode.Multiply, see http://stackoverflow.com/questions/31309521/error-use-of-unresolved-identifier-kcgblendmodemultiply – ChucK Oct 09 '15 at 20:12
can you post an swift 3 version? – Patonz Sep 29 '16 at 12:50
1

swift 3 version added! – Peter Kreinz Sep 30 '16 at 22:37
1

This seems to ignore color. I have an image with colors in it and when I apply this method, the resulting image is gray (but it does have the appropriate alpha). – Ethan G Nov 06 '17 at 15:59
In reading some of https://developer.apple.com/library/content/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_paths/dq_paths.html#//apple_ref/doc/uid/TP30001066-CH211-TPXREF101 it seems like .normal should work fine. Anyone else experiencing this "gray-scale" effect? – Ethan G Nov 06 '17 at 16:26

score 35 · Answer 2 · edited Sep 21 '17 at 19:35

35

The easiest way is to put your UIImage inside a UIImageView and set the alpha there.

let image = UIImage(named: "imageWithoutAlpha")
let imageView = UIImageView(image: image)
imageView.alpha = 0.5

myButton.setBackgroundImage(image, forState: UIControlState.Normal)

edited Sep 21 '17 at 19:35

Mo Abdul-Hameed

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answered Feb 14 '15 at 17:19

leonardo

1,686
15
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sorry, but that makes no sense! – Peter Kreinz Feb 15 '15 at 14:30
What exactly doesn't make sense? – leonardo Feb 15 '15 at 14:35
I tried your solution, but the transparency/opacity of the background image will not change – Peter Kreinz Feb 15 '15 at 16:07
3

Well, try again because it works 100%. Test properly before downvoting answers. – leonardo Feb 15 '15 at 16:51
18

Answer to another question. Question is not "how to change opacity of UIImageView". Question is "how to change alpha of UIImage" – Add080bbA Jan 19 '16 at 22:24
3

You are creating `imageView` but then never using it. You can't set UIView via `UIButton.setBackgroundImage`. – gondo May 13 '20 at 07:30
I have to agree with gondo here. – Jan Nash Jun 26 '21 at 10:22

score 15 · Answer 3 · answered Sep 05 '20 at 18:30

Swift 5, iOS 10+

If your app doesn't support operating systems prior to iOS 10 then you can take advantage of the more robust UIGraphicsImageRenderer UIKit drawing API.

extension UIImage {
    func withAlpha(_ a: CGFloat) -> UIImage {
        return UIGraphicsImageRenderer(size: size, format: imageRendererFormat).image { (_) in
            draw(in: CGRect(origin: .zero, size: size), blendMode: .normal, alpha: a)
        }
    }
}

Usage

let img = UIImage(named: "someImage")?.withAlpha(0.5)

score 9 · Answer 4 · answered Mar 12 '15 at 09:49

9

I was able to set the alpha using the following code:

self.imageView.image = UIImageView(image: "image.png")
imageView.alpha = 0.5

answered Mar 12 '15 at 09:49

Bigfoot11

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Not sure why this was down-voted: it proved an easy fix for me. – dumbledad Sep 28 '15 at 10:41
14

Answer to another question. Question is not "how to change opacity of UIImageView". Question is "how to change alpha of UIImage" – Add080bbA Jan 19 '16 at 22:26

How to set the alpha of an UIImage in SWIFT programmatically?

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