JPEG Printing Knowledge
JPEG

The JPEG (Joint Photographic Experts Group) format/compression technique was developed specifically for photographs. JPEG is utilized to gain high compression levels for photographic images without sacrificing the image quality. It is used exclusively for the compression of 24-bit images and it will not work for images less than 24-bit. It also does not work very well for non-photographic images such as illustrations, cartoons, flat color areas, or images with defined edges. JPEG is much more suitable for images that contain irregularities and soft edges rather than images with many straight lines and hard edges. The irregularities cause the pixels to be less well defined, which decreases the size of the file. The more irregular the image is, the better suited it is for JPEG.

Note: The JPEG format is used mostly for the web and for PhotoCDs. Images that will be used in a page layout program and printed on a press should NOT be saved as JPEGs. 

 
24-bit JPEG images look great on 24-bit monitors, but may not look so good on 8-bit or 16-bit systems. The colors in the 24-bit image that are not contained in the 8-bit or 16-bit palette of the computer system, will be dithered. Even if flat areas of color in the JPEG image are among the colors in the 8-bit or 16-bit color palette, there could still be problems with the JPEG image when viewed on a lower bit depth system. The JPEG compression process introduces elements into the solid color areas that make the images look muddy or blurry.

JPEG compression is known as "lossy compression", which means that non-essential data is lost during the compression. JPEG images may be compressed at several levels. The way the compression works is that the image data is separated into levels of importance. The more the image is compressed, the more levels of information are thrown out, which creates a smaller file, and along with it, the loss of image detail. The loss of this data is permanent and it cannot be restored. If the image is not compressed by too great a factor, the overall quality does not suffer that much. With JPEG, you have the choice of compressing an image without sacrificing too much in the way of image quality, or you can have the advantage of having a greatly reduced file size, but a resulting image of much poorer quality.

100% quality = 78.81k

 25% quality = 6.73k

Even though the image on the right has been compressed to 25% of the quality of the original image on the left, the quality of the right image is still tolerable and the file size has been reduced to less than one-tenth of the original.

 

 


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