Photo recovery

Photo recovery is the process of salvaging digital photographs from damaged, failed, corrupted, or inaccessible secondary storage media when it cannot be accessed normally. Photo Recovery can be considered a subset of the overall Data Recovery field.

Photo loss or deletion failures may be due to both hardware or software failures.

An excellent explanation of hardware failures is provided in the section for data recovery. Typically, if your drive or card is so badly damaged that your computer can not recognize that a drive/card has been connected, you will need to consult a data recovery service provider.

Logical Damage or the inability to view photos can occur due to many reasons. The most common reasons are:

The majority of photo recovery programs work by using a technique called file carving (data carving). There are many different file carving techniques that are used to recover photos. Most of these techniques fail in the presence of file system fragmentation. Simson Garfinkel showed that on average 16% of JPEGs are fragmented, which means on average 16% of jpegs are recovered partially or appear corrupt when recovered using techniques that can't handle fragmented photos.

In Header-Footer Carving, a recovery program attempts to recover photos based on the standard starting and ending byte signature of the photo format. To take an example, all JPEGs always begin with the hex sequence "FFD8" and they must end with the hex sequence "FFD9".

Header-Footer Carving cannot be used to recover fragmented photos, and fragmented photos will appear to be partially recovered or corrupt if incorrect data is added. Header-Footer Carving, along with Header-Size Carving, are by far the most common techniques for photo recovery. One of the first non-gui/console based programs to use this technique is PhotoRec. Use of footers can often truncate a photo, as many JPEGs contain thumbnails as an embedded object. If a file is terminated with a FFD9 it will be corrupted, unless nested FFD8/FFD9s are counted.

In Header-Size Carving, a recovery program attempts to recover photos based on the standard starting byte signature of the photo format, along with the size of the photo that is either derived or explicitly stated in the photo format. To take an example all 24-bit Windows Bitmaps (*.bmp), begin with the letters "BM", and store the size of the file in the header. Header-Size Carving cannot be used to recover fragmented photos, and fragmented photos will appear to be partially recovered or corrupt if incorrect data is added.

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Wikipedia