如何在iOS中向PNG图像编写自定义元数据

Question

我的应用程序应该能够将自定义元数据条目写入PNG图像，以便导出到UIPasteboard。

通过组合有关此主题的各种帖子，我已经能够得出下面给出的类作为源代码。

通过使用按钮触发copyPressed方法，我可以设置JPG图像（EXIF）的自定义元数据。

Image[6101:907] found jpg exif dictionary
Image[6101:907] checking image metadata on clipboard
Image[6101:907] {
    ColorModel = RGB;
    Depth = 8;
    Orientation = 1;
    PixelHeight = 224;
    PixelWidth = 240;
    "{Exif}" =     {
        ColorSpace = 1;
        PixelXDimension = 240;
        PixelYDimension = 224;
        UserComment = "Here is a comment";
    };
    "{JFIF}" =     {
        DensityUnit = 0;
        JFIFVersion =         (
            1,
            1
        );
        XDensity = 1;
        YDensity = 1;
    };
    "{TIFF}" =     {
        Orientation = 1;
    };
}

虽然我能够很好地读取PNG元数据，但似乎无法对其进行写入：

Image[6116:907] found png property dictionary
Image[6116:907] checking image metadata on clipboard
Image[6116:907] {
    ColorModel = RGB;
    Depth = 8;
    PixelHeight = 224;
    PixelWidth = 240;
    "{PNG}" =     {
        InterlaceType = 0;
    };
}

然而，文档中没有任何提示表明这应该失败，很多PNG特定的元数据常量的存在表明它应该成功。

我的应用程序应该使用PNG以避免JPG的有损压缩。

为什么我不能在iOS中的内存PNG图像上设置自定义元数据？

注意：我看到了这个SO问题，但它并没有解决这里的问题，即如何特别地将元数据写入PNG图像。

IMViewController.m

#import "IMViewController.h"
#import <ImageIO/ImageIO.h>

@interface IMViewController ()

@end

@implementation IMViewController

- (IBAction)copyPressed:(id)sender
{
//    [self copyJPG];
    [self copyPNG];
}

-(void)copyPNG
{
    NSData *pngData = UIImagePNGRepresentation([UIImage imageNamed:@"wow.png"]);
    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)pngData, NULL);
    NSDictionary *metadata = (__bridge NSDictionary *) CGImageSourceCopyPropertiesAtIndex(source, 0, NULL);
    NSMutableDictionary *mutableMetadata = [metadata mutableCopy];
    NSMutableDictionary *dict = [[mutableMetadata objectForKey:(NSString *) kCGImagePropertyPNGDictionary] mutableCopy];

    if (dict) {
        NSLog(@"found png property dictionary");
    } else {
        NSLog(@"creating png property dictionary");
        dict = [NSMutableDictionary dictionary];
    }

    // set values on the root dictionary
    [mutableMetadata setObject:@"Name of Software" forKey:(NSString *)kCGImagePropertyPNGDescription];
    [mutableMetadata setObject:dict forKey:(NSString *)kCGImagePropertyPNGDictionary];

    // set values on the internal dictionary
    [dict setObject:@"works" forKey:(NSString *)kCGImagePropertyPNGDescription];

    CFStringRef UTI = CGImageSourceGetType(source);
    NSMutableData *data = [NSMutableData data];
    CGImageDestinationRef destination = CGImageDestinationCreateWithData((__bridge CFMutableDataRef) data, UTI, 1, NULL);

    if (!destination) {
        NSLog(@">>> Could not create image destination <<<");

        return;
    }

    CGImageDestinationAddImageFromSource(destination, source, 0, (__bridge CFDictionaryRef) mutableMetadata);

    BOOL success = CGImageDestinationFinalize(destination);

    if (!success) {
        NSLog(@">>> Error Writing Data <<<");
    }

    UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];

    [pasteboard setData:data forPasteboardType:@"public.png"];
    [self showPNGMetadata];
}

-(void)copyJPG
{
    NSData *jpgData = UIImageJPEGRepresentation([UIImage imageNamed:@"wow.jpg"], 1);
    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef) jpgData, NULL);
    NSDictionary *metadata = (__bridge NSDictionary *) CGImageSourceCopyPropertiesAtIndex(source, 0, NULL);
    NSMutableDictionary *mutableMetadata = [metadata mutableCopy];
    NSMutableDictionary *exif = [[mutableMetadata objectForKey:(NSString *)kCGImagePropertyExifDictionary] mutableCopy];

    if (exif) {
        NSLog(@"found jpg exif dictionary");
    } else {
        NSLog(@"creating jpg exif dictionary");
    }

    // set values on the exif dictionary
    [exif setObject:@"Here is a comment" forKey:(NSString *)kCGImagePropertyExifUserComment];
    [mutableMetadata setObject:exif forKey:(NSString *)kCGImagePropertyExifDictionary];

    CFStringRef UTI = CGImageSourceGetType(source);
    NSMutableData *data = [NSMutableData data];
    CGImageDestinationRef destination = CGImageDestinationCreateWithData((__bridge CFMutableDataRef) data, UTI, 1, NULL);

    if(!destination) {
        NSLog(@">>> Could not create image destination <<<");

        return;
    }

    CGImageDestinationAddImageFromSource(destination,source, 0, (__bridge CFDictionaryRef) mutableMetadata);

    BOOL success = CGImageDestinationFinalize(destination);

    if (!success) {
        NSLog(@">>> Could not create data from image destination <<<");
    }

    UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];

    [pasteboard setData:data forPasteboardType:@"public.jpeg"];
    [self showJPGMetadata];
}

-(void)showJPGMetadata
{
    NSLog(@"checking image metadata on clipboard");

    UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];
    NSData *data = [pasteboard dataForPasteboardType:@"public.jpeg"];

    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    NSDictionary *metadata = (__bridge NSDictionary *) CGImageSourceCopyPropertiesAtIndex(source,0,NULL);

    NSLog(@"%@", metadata);
}

-(void)showPNGMetadata
{
    NSLog(@"checking image metadata on clipboard");

    UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];
    NSData *data = [pasteboard dataForPasteboardType:@"public.png"];

    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    NSDictionary *metadata = (__bridge NSDictionary *) CGImageSourceCopyPropertiesAtIndex(source,0,NULL);

    NSLog(@"%@", metadata);
}

@end

- Rich Apodaca

Answer 1

如果您尝试使用修改后的元数据保存图像

[data writeToFile:[NSTemporaryDirectory() stringByAppendingPathComponent:@"test.png"]   
       atomically:YES];

然后在Finder中查看其属性。您会发现kCGImagePropertyPNGDescription字段已成功设置。

但是如果您尝试读取这个新文件的元数据，kCGImagePropertyPNGDescription 将会丢失。

ColorModel = RGB;
Depth = 8;
PixelHeight = 1136;
PixelWidth = 640;
"{PNG}" =     {
    InterlaceType = 0;
};

经过一些研究，我发现PNG文件不包含元数据。但是它可能包含XMP元数据。然而似乎ImageIO无法处理XMP。
也许你可以尝试使用ImageMagic或libexif。

有用的链接：
PNG规范
iPhone/Objective-c上的读写图像XMP
PNG是否支持作者、相机型号等元数据字段？
PNG是否包含像JPG一样的EXIF数据？
libexif.sourceforge.net

- Sergey Kuryanov

---

好知道。显然，iOS无法从使用我的方法的文件中读取PNG元数据。如果我有一种读取PNG元数据的方法，我可能会有一个解决方案。鉴于iOS具有像kCGImagePropertyPNGDescription这样的常量，我宁愿不使用外部库，而是更想知道如何仅使用iOS库来实现这一点。 - Rich Apodaca

想象一下，您能够向图像添加元数据，您确定其他软件会读取此信息吗？为什么要添加元数据？ - Sergey Kuryanov

你找到答案了吗？ - Crashalot