如何编写自定义的元数据，以在iOS的PNG图像(How to write custom metada

我的应用程序应该能够编写自定义元数据项PNG图像出口到UIPasteboard。

关于这一主题拼凑不同的岗位，我已经能够拿出下面给出源的类。

触发copyPressed方法有一个按钮，我可以设置自定义元数据与JPG图片（EXIF）：

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;
    };
}

虽然我能够读取PNG元就好了，我似乎无法对其进行写入操作：

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，没有在文件中表明，这应该失败，许多存在PNG特定的元数据常量表明，它应该会成功。

我的应用程序应该使用PNG避免JPG的有损压缩 。

我为什么不能设置自定义元数据的iOS在内存中的PNG图像？

注：我已经看到了这太问题 ，但它不会在这里讨论的问题，这就是如何将元数据写入PNG图像明确。

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

Answer 1:

如果你尝试保存图像与修改的元数据

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

而不是查看它的属性在Finder中。 你会看到， kCGImagePropertyPNGDescription领域已成功设置好的了。

但是，如果你会尝试读取这个新的文件的元数据， kCGImagePropertyPNGDescription将会丢失。

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

经过一番研究，我发现，PNG不包含元数据。 但是，它可能包含XMP元数据 。 不过好像ImageIO不与XMP工作。
也许你可以尝试使用ImageMagic或libexif 。

相关链接：
PNG规范
读/ iPhone上/ Objective-C的写作图像XMP
是否像作者，相机型号等PNG支持元数据字段？
是否PNG包含类似JPG EXIF数据？
libexif.sourceforge.net

文章来源: How to write custom metadata to PNG images in iOS