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JPEG File Repair

(October 2002) posted on Wed Oct 09, 2002

Tired of spending hours trying to clean up low-quality JPEG files so that they're suitable for separating and printing?


By Mark A. Coudray

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File compression for digital images follows one of two schemes. The method is either lossy or lossless. With a lossless format, there is no damage to the file on compression, and all digital information remains intact. Lossy format, on the other hand, will allow for greater levels of compression, which results in smaller files on your hard drive. But you pay a higher price for the extra compression.

 

Lossy formats introduce artifacts that irreversibly damage the image. The greater the level of compression, the more artifacts that are introduced. Worse yet is the fact that this can be a progressive degradation, meaning that the more often you open and resave an image file in a format that employs lossy compression, the more damage you do to the image. When this process is repeated enough, the damage becomes so great that you can no longer use the file as the basis for printing.

 

The most commonly used lossy file format is JPEG. It is the default file-compression format for almost all digital cameras today. You might ask why companies like Nikon, Epson, or Olympus would use a file format that damages the image irreversibly. It has to do with economy and the average level of image-quality acceptance on the part of the consumer. It may also have to do with the fact that most images captured on digital cameras are not used for commercial print reproduction. If they are used for anything commercial, it is usually for posting on the Web, where image quality is not a big issue and economical file size generally takes precedence.

 

For the prepress technician, color separating and printing JPEG image files is becoming a daily issue. Sales people and clients will typically e-mail you the designs they want printed in the form of JPEG files. That would be fine if these files were just samples for evaluation. But more often than not, these images become the artwork you're expected to separate and print.

 

The real problem comes when you need to enlarge the damaged files to the scale at which they'll be printed. Now the artifacts are magnified, and the very, very painful task of cloning and repainting begins. I have seen prepress technicians working for hours trying to repair a single JPEG file.

 


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