使用 ImageIO 在 iOS 上创建渐进式 jpeg 会在设备上产生块状结果

问题描述

我正在尝试从 UIImage 对象创建渐进式 jpeg，这是我的代码

I'm trying to create a progressive jpeg from a UIImage object, this is the code i'm

NSMutableData *data = [NSMutableData data];

NSString *path = [NSHomeDirectory() stringByAppendingPathComponent: @"Library/Caches/test.jpg"];

CFURLRef url = CFURLCreateWithString(NULL, (CFStringRef)[NSString stringWithFormat:@"file://%@", path], NULL);
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(url, kUTTypeJPEG, 1, NULL);
CFRelease(url);

NSDictionary *jfifProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                (__bridge id)kCFBooleanTrue, kCGImagePropertyJFIFIsProgressive,
                                nil];

NSDictionary *properties = [NSDictionary dictionaryWithObjectsAndKeys:
                            [NSNumber numberWithFloat:.7], kCGImageDestinationLossyCompressionQuality,
                            jfifProperties, kCGImagePropertyJFIFDictionary,
                            nil];

CGImageDestinationAddImage(destination, ((UIImage*)object).CGImage, (__bridge CFDictionaryRef)properties);
CGImageDestinationFinalize(destination);
CFRelease(destination);

这在模拟器中运行时效果很好，但不幸的是在设备上产生了块状/块状结果:

This works great when running in simulator, but unfortunately produces chunky/blocky results on device:

对正在发生的事情有什么想法吗?我会恢复使用 UIImageJPEGRepresentation 作为最后的手段，我真的需要渐进式 JPEG.

Any ideas on what's going on? i'd revert to using UIImageJPEGRepresentation as a last resort, I really need progressive JPEGs.

推荐答案

好消息:我刚刚在 iPhone 4 上测试了这个，图像看起来很棒:

Good news: I just tested this on an iPhone 4 and the image looks great:

NSDictionary *jfifProperties = [NSDictionary dictionaryWithObjectsAndKeys:
@72, kCGImagePropertyJFIFXDensity,
@72, kCGImagePropertyJFIFYDensity,
@1, kCGImagePropertyJFIFDensityUnit,
nil];

(使用新的文字语法).

(using new literal syntax).

很好的JFIF 参考，了解这些密度选项的含义.

A good JFIF reference for what these density options mean.

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