Transparent (alpha) gradient with Core Graphic iPhone

Question

anyone knows how to programmatically create a transparent gradient (alpha gradient), over an image with Core Graphic? I need it both for display an image and save it.

An example of what I want to obtain:

Answer 1

You can use QuartzCore to apply a mask (with gradient transparency) to the UIImageView:

UIImageView *myImageView = ["the image view"];
CAGradientLayer *l = [CAGradientLayer layer];
l.frame = myImageView.bounds;
 l.colors = [NSArray arrayWithObjects:(id)[[UIColor colorWithRed:0 green:0 blue:0 alpha:0] CGColor], (id)[[UIColor colorWithRed:0 green:0 blue:0 alpha:1] CGColor], nil];

l.startPoint = CGPointMake(0.0, 0.0f);
l.endPoint = CGPointMake(1.0f, 1.0f);

//you can change the direction, obviously, this would be top to bottom fade
myImageView.layer.mask = l; 

To save it you can find the answer easily here on Stack Overflow, just by searching for how to save the contents of the canvas to a UIImage.

Or Swift version as requested by @Zazu:

// Whatever you image view is, obviously not hardcoded like this
let imageView = UIImageView.init(image: UIImage(named: "Image"))
imageView.frame = CGRectMake(0, 0, 320, 480)
self.view.addSubview(imageView)
// Create the gradient layer
let gradientLayer = CAGradientLayer.init()
gradientLayer.frame = imageView.bounds
gradientLayer.colors = [
    UIColor.init(colorLiteralRed: 0, green: 0, blue: 0, alpha: 0).CGColor,
    UIColor.init(colorLiteralRed: 0, green: 0, blue: 0, alpha: 1).CGColor]
// Whatever direction you want the fade. You can use gradientLayer.locations
// to provide an array of points, with matching colors for each point, 
// which lets you do other than just a uniform gradient.
gradientLayer.startPoint = CGPointMake(1.0, 0.0);
gradientLayer.endPoint = CGPointMake(0.0, 0.0);
// Use the gradient layer as the mask
imageView.layer.mask = gradientLayer;

Answer 2

Swift 3 - This is what worked for me. I needed only 20% of bottom image to be transparent with gradient.

    // Create the gradient layer
    let gradientLayer = CAGradientLayer()
    gradientLayer.frame = imageView.bounds

    gradientLayer.colors = [
        UIColor.white.withAlphaComponent(1).cgColor,
        UIColor.white.withAlphaComponent(0).cgColor]
    // Whatever direction you want the fade. You can use gradientLayer.locations
    // to provide an array of points, with matching colors for each point,
    // which lets you do other than just a uniform gradient.
    gradientLayer.startPoint = CGPoint(x: 0.0, y: 0.80)
    gradientLayer.endPoint = CGPoint(x: 0.0, y: 1.0)
    // Use the gradient layer as the mask
    imageView.layer.mask = gradientLayer

Answer 3

I don't know a core graphics solution for the gradient but if you can make an image in photoshop which starts white on the right and fades to alpha on the left you could overlay that on top of the content image

to overlay the image you would do something like

+ (UIImage*) addFadeOutToImage:(UIImage*)sourceImage
{
   UIGraphicsBeginImageContext(sourceImage.size);

   [sourceImage drawAtPoint:CGPointZero];

   UIImage* fadeAlphaImage = [UIImage imageNamed:@"fadedAlphaImage.png"];

   CGPoint fadeOutStartPoint = CGPointMake(sourceImage.size.width - fadeAlphaImage.size.width, 0);
   //assumes the fade image is the same height as the source
   [fadeAlphaImage drawAtPoint:fadeOutStartPoint];

   UIImage* fadeOutImage = UIGraphicsGetImageFromCurrentImageContext();
   UIGraphicsEndImageContext();

   return fadeOutImage;
}

this code adapted from this question

Or Swift version as requested by @Zazu:

// Whatever you image view is, obviously not hardcoded like this
let imageView = UIImageView.init(image: UIImage(named: "Image"))
imageView.frame = CGRectMake(0, 0, 320, 480)
self.view.addSubview(imageView)
// Create the gradient layer
let gradientLayer = CAGradientLayer.init()
gradientLayer.frame = imageView.bounds
gradientLayer.colors = [
    UIColor.init(colorLiteralRed: 0, green: 0, blue: 0, alpha: 0).CGColor,
    UIColor.init(colorLiteralRed: 0, green: 0, blue: 0, alpha: 1).CGColor]
// Whatever direction you want the fade. You can use gradientLayer.locations
// to provide an array of points, with matching colors for each point, 
// which lets you do other than just a uniform gradient.
gradientLayer.startPoint = CGPointMake(1.0, 0.0);
gradientLayer.endPoint = CGPointMake(0.0, 0.0);
// Use the gradient layer as the mask
imageView.layer.mask = gradientLayer;