如何用Cocoa压缩jpeg图像？

Question

我有一个 jpeg 图像，我希望能够使用 Cocoa/Core Image/Core Graphics 增量压缩它。例如，我有A.jpg（3MB），我压缩A并得到B.jpg（1MB），压缩B并得到C.jpg（400KB），依此类推，直到图像无法再压缩。

我尝试使用 NSBitmapImageReprepresentationUsingType:properties 但没有取得太大成功。我使用从压缩图像数据创建的图像代表来尝试进一步压缩，但如果我使用与创建初始压缩数据相同的压缩因子，则效果不佳。

这是我一直在争论的代码

NSString * fileName = @"largeImage.jpg";

NSLog(@"== Initializing ==\n");

NSData * imageData = [NSData dataWithContentsOfFile:fileName];
NSLog(@"imageData length: %d\n", [imageData length]);

NSImage * image = [[NSImage alloc] initWithData:imageData];
NSBitmapImageRep * bitmapRep = [[image representations] objectAtIndex:0];

NSData * data = [bitmapRep representationUsingType:NSJPEGFileType properties:nil];
NSLog(@"data length: %d\n", [data length]);

NSDictionary * dict = [NSDictionary dictionaryWithObject: [NSNumber numberWithFloat:0.5] forKey:NSImageCompressionFactor];
NSData * data5 = [bitmapRep representationUsingType:NSJPEGFileType properties:dict];
NSLog(@"data5 length: %d\n", [data5 length]);

NSLog(@"== End ==\n\n");

NSLog(@"== Image from Data5 ==\n");
NSImage * imageFromData5 = [[NSImage alloc] initWithData:data5];
NSBitmapImageRep * bitmapRepFromData5 = [[imageFromData5 representations] objectAtIndex:0];

NSData * dataFromImageFromData5 = [bitmapRepFromData5 representationUsingType:NSJPEGFileType properties:nil];
NSLog(@"dataFromImageFromData5 length: %d\n", [dataFromImageFromData5 length]);

dict = [NSDictionary dictionaryWithObject: [NSNumber numberWithFloat:0.5] forKey:NSImageCompressionFactor];
NSData * dataFiveFromImageWithData5 = [bitmapRepFromData5 representationUsingType:NSJPEGFileType properties:dict];
NSLog(@"dataFiveFromImageWithData5 length: %d\n", [dataFiveFromImageWithData5 length]);
NSLog(@"== End ==\n\n");

这是我的结果

> == Initializing ==
> imageData length: 1882694
> data length: 1888365
> data5 length: 1102461
> == End ==

> == Image from Data5 ==
> dataFromImageFromData5 length: 1646137
> dataFiveFromImageWithData5 length: 1102115
> == End ==


Try a different image
> == Initializing ==
> imageData length: 3620257
> data length: 2889458
> data5 length: 1404750
> == End ==

> == Image from Data5 ==
> dataFromImageFromData5 length: 2007951
> dataFiveFromImageWithData5 length: 1405398
> == End ==

Answer 1

每次都需要减小该系数。使用相同的质量级别重新压缩未经编辑的 JPEG 图像实际上根本不会改变结果太多，这应该可以解释为什么您没有看到尺寸有任何减小。

（如果您想确保质量不会降低超过必要的程度，则可以用递减的因子一遍又一遍地重新压缩原始图像，而不是像示例中那样链接它们。）