The simplest way to resize an UIImage?

Question

In my iPhone app, I take a picture with the camera, then I want to resize it to 290*390 pixels. I was using this method to resize the image :

UIImage *newI         


        

Answer 1

Here's a modification of the category written by iWasRobbed above. It keeps the aspect ratio of the original image instead of distorting it.

- (UIImage*)scaleToSizeKeepAspect:(CGSize)size {
    UIGraphicsBeginImageContext(size);

    CGFloat ws = size.width/self.size.width;
    CGFloat hs = size.height/self.size.height;

    if (ws > hs) {
        ws = hs/ws;
        hs = 1.0;
    } else {
        hs = ws/hs;
        ws = 1.0;
    }

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(context, 0.0, size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextDrawImage(context, CGRectMake(size.width/2-(size.width*ws)/2,
        size.height/2-(size.height*hs)/2, size.width*ws,
        size.height*hs), self.CGImage);

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return scaledImage;
}

Answer 2

If you want to make a thumbnail of a UIImage (with proportional resizing or maybe some cropping involved), check out UIImage+Resize category that allows you to use concise, ImageMagick-like syntax:

UIImage* squareImage       = [image resizedImageByMagick: @"320x320#"];

Answer 3

Swift 3.0 with failsafe option (returns the original image in case of error):

func resize(image: UIImage, toSize size: CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(size,false,1.0)
    image.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
    if let resizedImage = UIGraphicsGetImageFromCurrentImageContext() {
        UIGraphicsEndImageContext()
        return resizedImage
    }
    UIGraphicsEndImageContext()
    return image
}

Answer 4

This improvement to Paul's code will give you a sharp high res image on an iPhone with a retina display. Otherwise when scaling down it's blurry.

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
    if ([[UIScreen mainScreen] scale] == 2.0) {
        UIGraphicsBeginImageContextWithOptions(newSize, YES, 2.0);
    } else {
        UIGraphicsBeginImageContext(newSize);
    }
} else {
    UIGraphicsBeginImageContext(newSize);
}
[image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
UIGraphicsEndImageContext();
return newImage;
}

Answer 5

I found a category for UIImage in Apple's own examples which does the same trick. Here's the link: https://developer.apple.com/library/ios/samplecode/sc2273/Listings/AirDropSample_UIImage_Resize_m.html.

You'll just have to change the call:

UIGraphicsBeginImageContextWithOptions(newSize, YES, 2.0);

in imageWithImage:scaledToSize:inRect: with:

UIGraphicsBeginImageContextWithOptions(newSize, NO, 2.0);

In order to consider the alpha channel in the image.

Answer 6

For Swift 5:

extension UIImage {
  func resized(to newSize: CGSize) -> UIImage? {
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0)
    defer { UIGraphicsEndImageContext() }

    draw(in: CGRect(origin: .zero, size: newSize))
    return UIGraphicsGetImageFromCurrentImageContext()
  }
}