Damaged JPEG files

Repairing of damaged JPEG files

One of the reasons, why it is so hard to repair damaged JPEG files and why there are so few specialists in JPG repairing, is the big number of possible JPG corruptions. These corruptions vary from file to file and every failure must be treated in a different way.

Sometimes the JPG 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 JPG corruptions.

Damaged JPG header

A JPG file can be roughly divided into two parts.

1. JPEG-Header (~ 0.05 %)
2. JPEG-Scan (~ 99.95 %)

If the header of a JPG file is invalid, the repairing is often impossible. The JPG 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 damaged, it can still be possible to use a header of a different JPG file instead of the original header, provided the JPEG files are technical similar. Theoretically, a JPG 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 JPG 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 JPG 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 damaged header. Because of that such errors still occur.

We offer:

• We can find damaged JPG headers and completely or partially replace a damaged header with a header from a sample file.
• Even when the replacement of a damaged JPG header is not successful, our staff will try to manually adjust the damaged header.

The thumbnail of a JPEG can be yet seen

We often receive question like this:

"I can yet see the thumbnail of my JPG 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 JPG 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 corruptions in JPEG files 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:

1. JPEG-Header
   • Thumbnail of 160 x 120 pixel
   • Thumbnail of 640 x 480 pixel
2. JPEG-Scan
   • Original image with the resolution of 2576 x 1932 pixel

We offer:

• We can recognise thumbnails in every resolution in valid and damaged JPEG files. These thumbnails will be shown in the list of the repaired JPEG files.

Incomplete JPEG scan (grey area in the JPG)

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.

We offer:

• We determine incomplete JPEG scans and modify the file so that the other graphics programs can open it.
• Highlight: even when both the beginning and the ending of a JPG file are missing (i.e. the JPG has no header), we can make the rest of the JPG visible using a sample file for the repairing.

Bit error in a JPEG scan (Color - Error)

One of the worst problems with JPEG files 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 JPG are wrong.

Such corruptions are especially hard because JPEG format does not "pardon" any bit corruptions (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 JPG 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 JPG 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.

We offer:

• We have already fixed lots of such scan corruptions. Nevertheless, byte corruptions can also be irreparable. Special manual analysis of the file contents is needed in each case. We offer these examinations at Repair JPEG files.