How to write or modify EXIF data for an existing image on the filesystem, without loading the image?

Question

I've asked similar questions before, but have not received a definitive answer. Seems that there must be a way to simply add/modify metadata to an image without loading the image into memory, without having to deal with directly reading bits.

Seems like ways exist when using CMSampleBufferRefs, but I need to be able to do this with a regular image already saved to disk.
For instance, given a very large png at /Documents/photo.png, I want to modify its exif metadata without having to load that image.

Related: [Is there any easy way to edit jpeg marker data in-place?](http://stackoverflow.com/q/10508410/85950) — blahdiblah, Aug 01 '12 at 23:23
Since @H2CO3 provided you with perhaps the only solution you can use (I didn't have one), please accept his answer. — David H, Aug 07 '12 at 11:16
Please also note that if you don't accept any answer, you still won't get the bounty's value back. So it would only be a benefit for one of the answerers if you accepted one of the answers. :) — , Aug 08 '12 at 09:30

score 9 · Accepted Answer · answered Aug 01 '12 at 05:39

9

You can use libexif - I've had success with compiling it for iOS before. With libexif, you can modify any image's EXIF metadata.

answered Aug 01 '12 at 05:39

Sure, but does this framework load the image into memory first? – akaru Aug 01 '12 at 21:46
That's good. Could you provide an example of such usage with libexif? i.e. loading the metadata and writing it back to the file. – akaru Aug 01 '12 at 22:44
@akaru this is how I implemented EXIF metadata reading and writing in my file manager app: https://github.com/H2CO3/MyFile/blob/master/src/MFImageMetadataController.m – Aug 01 '12 at 22:50
Thanks, @H2CO3, I'll look into this. If you happen to know where I can grab an iOS build or XCode project of libexif for iOS, let me know. – akaru Aug 03 '12 at 07:24
@akaru I don't know about one, but I think the source of libexif should compile out of the box in Xcode. – Aug 03 '12 at 07:29
3

Btw I'd appreciate having accepted my answer if it was helpful for you :) – Aug 03 '12 at 07:30
@akaru Did you manage to build libexif for iOS? I'm having difficulty doing so. – Sebastian Dwornik Oct 13 '12 at 15:33
@SebastianDwornik [here is](http://apt.thebigboss.org/onepackage.php?bundleid=libexif) an iOS build of libexif by me. – Oct 13 '12 at 15:37
@H2CO3 Thank you. How did you built it by the way? Was it difficult? – Sebastian Dwornik Oct 13 '12 at 15:44
@SebastianDwornik Not difficult at all if you have experience crossing stuff. Because the Apple toolchain is not entirely GNU, the standard configure - make - make install workflow won't work, but in general I just manually patch `config.h` then compile all the c files together using a manually created Makefile. – Oct 13 '12 at 15:52
And here I was thinking I could do it simply through XCode. I'm still trying to extract that .DEB file you provided. Perhaps the Makefile might be easier. ;) – Sebastian Dwornik Oct 13 '12 at 15:55
@SebastianDwornik I never use Xcode :) And use `dpkg -x filename.deb directory` to extract the Debian package. – Oct 13 '12 at 15:56
Extracted finally just before your comment. :) – Sebastian Dwornik Oct 13 '12 at 15:58
@H2CO3 Just looking at the code, doesn't `[MFPictureMetadata save]` load the entire image into memory before writing the EXIF? You have a call there to `jpeg_data_load_file()` – Eric Brochu Sep 27 '13 at 18:34
1

@EricBrochu (OMG that code of mine from some 2 years ago is so horrible I can't even express. Look at that chained if-else and the superfluous local `malloc()`s, aaargh...!) Indeed, it does, but I'm not sure it's a must. In fact, I'm pretty confident that's not how it should be done. Seeing the quality of that piece of code of mine... I don't even assume I got the usage of the library right. – Sep 27 '13 at 18:42

score 0 · Answer 2 · answered Aug 07 '12 at 17:33

If you know how to modify the EXIF, you can modify the binary data directly from the file. Just replace in the image the binary portion with the new one. I don't know if objective-c permit this, but in ansi c should be simple. The complicate part is to identify the exact part to change.

How to write or modify EXIF data for an existing image on the filesystem, without loading the image?

2 Answers2

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