CGImage：创建所需尺寸的缩略图

Question

我想使用 Core Graphics 创建缩略图。缩略图的大小应为 1024 [原文如此]，且宽高比与原始图像相同。是否可以直接在 Core Graphics 中设置缩略图所需的大小？

在下面的选项字典中，我可以传递要创建的缩略图的最大尺寸，但是有什么方法可以传递最小尺寸吗？

 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

如果您想要尺寸为 1024（最大尺寸）的缩略图，则应该传递 1024，而不是 2048。此外，如果您想确保按照您的规范创建缩略图，您应该要求 kCGImageSourceCreateThumbnailFromImageAlways，而不是 kCGImageSourceCreateThumbnailFromImageIfAbsent，因为后者可能会导致使用现有的缩略图，并且它可能比您想要的要小。

所以，这里的代码可以满足您的要求：

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版本的答案：

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)
}