如何在iOS中将UIImage保存为渐进式JPEG格式

Question

在iOS中，要将UIImage保存为JPEG，我使用UIImageJPEGRepresentation，但它不接受压缩比以外的选项。我想将UIImage保存为progressive JPEG格式，有简单的方法吗？

在OS X中，似乎有一个NSImageProgressive选项可以将NSImage保存为渐进格式。

Answer 1

我认为您可以使用ImageIO框架来完成，如下所示：

#import <UIKit/UIKit.h>
#import <ImageIO/ImageIO.h>
#import <MobileCoreServices/MobileCoreServices.h>
#import "AppDelegate.h"
int main(int argc, char *argv[])
{
    @autoreleasepool {
        UIImage *sourceImage = [UIImage imageNamed:@"test.jpg"];
        CFURLRef url = CFURLCreateWithString(NULL, CFSTR("file:///tmp/progressive.jpg"), 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:.6], kCGImageDestinationLossyCompressionQuality,
            jfifProperties, kCGImagePropertyJFIFDictionary,
            nil];
        CGImageDestinationAddImage(destination, sourceImage.CGImage, (__bridge CFDictionaryRef)properties);
        CGImageDestinationFinalize(destination);
        CFRelease(destination);
        return 0;
    }
}

Preview.app说输出文件是渐进的，ImageMagick的identify命令说它有"Interlace:JPEG"。

Answer 2

此代码在设备上确实有效-关键是指定所有密度值（注意它使用了新的文字语法）：

- (UIImage *)imager
{
    UIImage *object = [UIImage imageNamed:@"Untitled.jpg"];
    NSString *str = [[self applicationDocumentsDirectory] stringByAppendingPathComponent:@"Tester.jpg"];
    NSURL *url = [[NSURL alloc] initFileURLWithPath:str];
    CGImageDestinationRef destination = CGImageDestinationCreateWithURL((__bridge CFURLRef)url, kUTTypeJPEG, 1, NULL);
    NSDictionary *jfifProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                    @72, kCGImagePropertyJFIFXDensity,
                                    @72, kCGImagePropertyJFIFYDensity,
                                    @1, kCGImagePropertyJFIFDensityUnit,
                                    nil];
    NSDictionary *properties = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithFloat:.5f], kCGImageDestinationLossyCompressionQuality,
                                jfifProperties, kCGImagePropertyJFIFDictionary,
                                nil];
    CGImageDestinationAddImage(destination, ((UIImage*)object).CGImage, (__bridge CFDictionaryRef)properties);
    CGImageDestinationFinalize(destination);
    CFRelease(destination);
    return [UIImage imageWithContentsOfFile:str];
}