Question

我想使用CG创建缩略图。它会创建缩略图。

这里我希望缩略图的大小为1024（纵横比）。是否可以直接从CG获得所需大小的缩略图？

在选项字典中，我可以传递可以创建的最大尺寸，但有没有办法让最小尺寸相同..？

 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

如果你想要一个大小为1024（最大尺寸）的缩略图，你应该传递1024而不是2048.另外，如果你想确保根据你的规格创建缩略图，你应该要求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 = [
        kCGImageSourceShouldAllowFloat as String: true as NSNumber,
        kCGImageSourceCreateThumbnailWithTransform as String: true as NSNumber,
        kCGImageSourceCreateThumbnailFromImageAlways as String: true as NSNumber,
        kCGImageSourceThumbnailMaxPixelSize as String: max as NSNumber
    ] as CFDictionary

    guard let thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options)
        else {
            return nil
    }

    return UIImage(cgImage: thumbnail)
}

CGImage创建具有所需大小的缩略图图像

2 个答案: