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Asked: 2026-05-26T19:44:10+00:00

I have successfully been using CGImageSource to read an image in (mostly JPEG) and

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I have successfully been using CGImageSource to read an image in (mostly JPEG) and CGImageDestination to write it back out. It works but my image comes out a lot smaller than it was before (a 3.9Mb image will become a 2.1 Mb image).

I have been playing around with kCGImageDestinationLossyCompressionQuality and while it does affect the size of the file, I don’t understand the scale it uses.

E.g. that same 3.7Mb file will change its size to:

  • 1.9Mb with a compression quality of 0.7

  • 2.4Mb with a compression quality of 0.8

  • 3.0Mb with a compression quality of 0.9

  • 7.4Mb with a compression quality of 1.0

I tried everything (going to 6 decimals using dichotomy) to find the sweet spot to get back to that magic 3.9Mb but it jumps from 3.3Mb to 7.4Mb with seemingly no way to get it to stay in between these 2 numbers.

Is there any other Objective-C library I can use to modify EXIF data that leaves the compression (and thus the file size) alone?

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1 Answer

  1. Editorial Team

    Using CGImageDestination you are creating a new image, and it’s very difficult that the size of the newly created image matches the original one. (depends on the compression level).

    If you need just to modify the metadata in the original picture, without changing the image information, you should use another library. I know two of them which can be used in a cocoa app:

    • libexif C++ library. Supports writing jpgs. LGPL license.
    • exiv2 C++. Supports writing to a lot of formats. But it neeeds a
      payed license if you are using it in a commercial app.

    Another option is to use exiftool. It’s a perl scripts that has become the de-facto standard for changing metadata information. You could include it in the resources folder of your app, and invoke it using NSTask to change the metadata of the pictures. Quite easy to do, and by far the best tool of the three. (Only for mac, not sure if you’re targetting iphone or Mac)

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