One of the reasons, why it is so hard to repair damaged images and why there are so few specialists in image restoring, is the big number of possible photo corruptions. These corruptions vary from file to file and every failure must be treated in a different way.
Sometimes the photo header is corrupt, or the JPG markers are invalid, or there are bit errors in JPEG scan. We offer here a brief overview of the most frequent image faults.
One of the worst problems with images are single or multiple bit and byte errors in JPEG scans. In this case a file is for the most part in order, but because of the problems with storage medium or because of transmission errors some pieces of data in the image are wrong.
Such corruptions are especially hard because JPEG format does not "pardon" any bit faults (at least in the basic version of JPEG, the so-called Baseline). JPEG scans have a feature of sequential compression. This means, a data region in the image file is compressed with the help of data from the previous data region in this file. When a certain piece of data in a JPEG scan is wrong, decoding will result in erroneous data for this data region itself and for all successive data regions in the photo file. Because the parts of an image are coded horizontally, there appear the so feared faded lines in wrong colour. The image to the right shows a typical JPG file with such an error.
A JPG file can be roughly divided into two parts.
If the header of a image file is invalid, the restoring is often impossible. The photo header contains all the data necessary for decoding a JPG scan. A JPEG header is built in such a way that even a small error makes the whole header invalid. When a header is corrupt, it can still be possible to use a header of a different photo file instead of the original header, provided the JPG files are technical similar. Theoretically, a photo file achieves its best compression results only if its header is adjusted specially for the image, but the majority of digital cameras do not optimize the header. The reason is: without header optimization the photo compression is faster. If the header is not optimized, the standard header setting, recommended by JPEG (Joint Photographic Experts Group, a united Committee of ISO and ITU-T, which developed the standards JPEG and JPEG 2000), are used.
Here it should be mentioned, that the photo header makes only 0.05% of the file (see above). So a situation when a header is corrupt and the rest of the JPG file is valid is very improbable. This can only happen if a coding program of a poor quality (for instance, the program in the memory of a digital camera) has created a broken header. Because of that such errors still occur.
We often receive question like this:
"I can yet see the thumbnail of my photo file, but I can't open the file itself."
This Problem is easy to explain. According to JPEG format, a thumbnail of the image is saved in the header of the file to quickly enable preview of the picture. When a digital camera saves the shot picture in the image file, a miniature of this image is saved in the header of the file. This JPEG preview can be shown in the browser or on the camera display without loading and decoding the whole file. Some errors in images result in the situation when the JPG preview can be displayed but the JPG scan itself is invalid. More than that: some digital cameras save even two thumbnails in a file, and a JPEG file has than three images inside. Below we show sections of a typical JPEG file.
A JPG file contains:
The transmission of images from a digital camera to a computer or to some other storage medium often results in a data failure. If the reason for the failure is a physical defect of the storage medium, a part of the original data can be for ever lost. At the same time it is not possible to view even the rest of the image because graphics software requires a valid ending of the JPG file.