CGImage create thumbnail image with desired size

Question

I want to create the thumbnail using the CG. It creates the thumbnails.

Here i want to have the thumbnail with the size 1024 (with aspect ratio.) Is it possible to get the desired size thumbnail directly from the CG?

In the options dictionary i can pass the max size of the thumnail can be created, but is there any way to have min size for the same..?

 NSURL * url = [NSURL fileURLWithPath:inPath];
 CGImageSourceRef source = CGImageSourceCreateWithURL((CFURLRef)url, NULL);
 CGImageRef image=nil;
 if (source)
 {
  NSDictionary* thumbOpts = [NSDictionary dictionaryWithObjectsAndKeys:
           (id) kCFBooleanTrue, (id)kCGImageSourceCreateThumbnailWithTransform, 
           (id)kCFBooleanTrue, (id)kCGImageSourceCreateThumbnailFromImageIfAbsent,
           [NSNumber numberWithInt:2048],  kCGImageSourceThumbnailMaxPixelSize,

           nil];

  image = CGImageSourceCreateThumbnailAtIndex(source, 0, (CFDictionaryRef)thumbOpts);   

  NSLog(@"image width = %d %d", CGImageGetWidth(image), CGImageGetHeight(image));
  CFRelease(source);
 }

Answer 1

If you want a thumbnail with size 1024 (maximum dimension), you should be passing 1024, not 2048. Also, if you want to make sure the thumbnail is created to your specifications, you should be asking for kCGImageSourceCreateThumbnailFromImageAlways, not kCGImageSourceCreateThumbnailFromImageIfAbsent, since the latter might cause an existing thumbnail to be used, and it could be smaller than you want.

So, here's code that does what you ask:

NSURL* url = // whatever;
NSDictionary* d = [NSDictionary dictionaryWithObjectsAndKeys:
                   (id)kCFBooleanTrue, kCGImageSourceShouldAllowFloat,
                   (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailWithTransform,
                   (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailFromImageAlways,
                   [NSNumber numberWithInt:1024], kCGImageSourceThumbnailMaxPixelSize,
                   nil];
CGImageSourceRef src = CGImageSourceCreateWithURL((CFURLRef)url, NULL);
CGImageRef imref = CGImageSourceCreateThumbnailAtIndex(src, 0, (CFDictionaryRef)d);
// memory management omitted

Answer 2

Swift 3 version of the answer:

func loadImage(at url: URL, maxDimension max: Int) -> UIImage? {
    
    guard let imageSource = CGImageSourceCreateWithURL(url as CFURL, nil)
        else {
            return nil
    }

    let options: [CFString: Any] = [
        kCGImageSourceShouldAllowFloat: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceThumbnailMaxPixelSize: max
    ]
    
    guard let thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options as CFDictionary)
        else {
            return nil
    }
    
    return UIImage(cgImage: thumbnail)
}