Can deleted image data be recovered?

December 19, 2012 8:41 AM

Is it possible to recover portions of an image that have been cropped or cut away? And if so, how to prevent that?

Every so often I will put a picture online with potentially sensitive info removed, things like receipts on a shopping site, or maybe an item with a serial number on ebay/craigslist.
I usually use a program called irfanview to cut/crop, here's an example of such an image:

http://imgur.com/d7Q1h

Can the text that was in the black bar be undeleted? I'm pretty confident the answer is no, but I want to be sure. (the sample text is random/not sensitive).

Thanks

posted by aerotive to Computers & Internet (7 answers total)

Best answer: When you use a format such as JPEG, which throws away visual data that is not visible in the final image, that data cannot be recovered.

There are at least three other things to watch out for when redacting data in image format:

1) Avoid using a weak redaction method such as a blur overlay; letters and numbers can often be probabilistically reconstructed. 100% opacity solid color areas are fine.

2) Avoid using "package" file formats that may preserve thumbnails of unredacted images; avoid using image formats that don't flatten layers and may preserve original image data in a layer beneath the added redaction.

3) Be sure to strip EXIF and other metadata as appropriate to avoid conveying information relevant to that which you have successfully redacted from the image data itself.
posted by Inspector.Gadget at 8:46 AM on December 19, 2012 [5 favorites]

EXIF data is your main risk. EXIF data can also include preview thumbnails of varying size. These thumbnails aren't always updated by your image editor when you modify the image. If the resolution is high enough it's possible details that you have cropped could be recovered.

For an example, save this image to your hard drive and select a view where you can see the thumbnail in your explorer/finder, etc. Compare that to the image if you open it in an editor.
posted by rocketpup at 8:56 AM on December 19, 2012

OH yeah, lot's of phones will also save location data into your EXIF data. I would definitely make it a practice to strip metadata from your photos before posting.
posted by rocketpup at 8:58 AM on December 19, 2012

Best answer: One low-tech solution is to make the required edits to the picture, and then take a screenshot of the picture (e.g. Command-Shift-4 on a Mac) and use that instead.
posted by googly at 9:38 AM on December 19, 2012 [2 favorites]

To follow up on this question, how does one strip EXIF data?
posted by medusa at 10:10 AM on December 19, 2012

Response by poster: Thanks everyone, I had though thought of layers and exif but not package formats. And taking a screenshot of the edited picture is a good idea.
posted by aerotive at 1:22 PM on December 19, 2012

Taking a screenshot of the image is going to kill quality because you have let another lossy hoop to jump through when you save that as a jpeg. On top of it, are you ignoring the EXIF in the screenshot now? That may hold information you may not want to share as well.

EXIF editing is trivial. Find an application you like and just verify before sending.
posted by damn dirty ape at 1:42 PM on December 19, 2012