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
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.