Question

I wanted to resize a UIImage to a certain width and height keeping the proportion in place. The simplest way to do this is:

 CGSize newSize = CGSizeMake(726, 521);
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
    UIImage * newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();

Does the method above have any drawbacks in terms of image quality and the time it takes? What is a better way than the method above?

EDITED:

How about this one:

- (UIImage *)imageByScalingProportionallyToSize:(CGSize)targetSize {

    UIImage *sourceImage = self;
    UIImage *newImage = nil;

    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;

    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;

    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;

    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

    if (CGSizeEqualToSize(imageSize, targetSize) == NO) {

        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;

        if (widthFactor < heightFactor) 
            scaleFactor = widthFactor;
        else
            scaleFactor = heightFactor;

        scaledWidth  = width * scaleFactor;
        scaledHeight = height * scaleFactor;

        // center the image

        if (widthFactor < heightFactor) {
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5; 
        } else if (widthFactor > heightFactor) {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }


    // this is actually the interesting part:

    UIGraphicsBeginImageContext(targetSize);

    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width  = scaledWidth;
    thumbnailRect.size.height = scaledHeight;

    [sourceImage drawInRect:thumbnailRect];

    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    if(newImage == nil) NSLog(@"could not scale image");


    return newImage ;
}

Answer 1

In a performance sample of Image Resizing Techniques the score was

• 0.1420 UIKit
• 0.1722 Core Graphics
• 0.1616 Image I/O
• 2.4983 Core Image
• 2.3126 vImage

so the fastest and simplest way is:

let renderer = UIGraphicsImageRenderer(size: size)
let resized = renderer.image { (context) in
    image.draw(in: CGRect(origin: .zero, size: size))
}

Resize a UIImage the right way is from 2009 but contains useful talk about interpolation quality and preserving the aspect ratio.

---

Additionally, to preserve the ratio within a max size:

let image: UIImage = ...
let maxSize = CGSize(width: 726, height: 521)
let newSize = AVMakeRect(aspectRatio: image.size, insideRect: CGRect(origin: .zero, size: maxSize)).size
let resized = UIGraphicsImageRenderer(size: newSize).image { _ in
    image.draw(in: CGRect(origin: .zero, size: newSize))
}

To change the pixels per point density (default is the ppp of the current screen):

let format = UIGraphicsImageRendererFormat()
format.scale = 1
UIGraphicsImageRenderer(size: newSize, format: format)
...