How well does JPEG compress images?
Very well indeed, when working with its intended type of image (photographs and suchlike).
For full-color images, the uncompressed data is normally 24 bits/pixel. The best known
lossless compression methods can compress such data about 2:1 on average. JPEG can typi-
cally achieve 10:1 to 20:1 compression without visible loss, bringing the effective storage
requirement down to 1 to 2 bits/pixel. 30:1 to 50:1 compression is possible with small to
moderate defects, while for very-low-quality purposes such as previews or archive indexes,
100:1 compression is quite feasible. An image compressed 100:1 with JPEG takes up the
same space as a full-color one-tenth-scale thumbnail image, yet it retains much more detail
than such a thumbnail.
For comparison, a GIF version of the same image would start out by sacrificing most of the
color information to reduce the image to 256 colors (8 bits/pixel). This provides 3:1 com-
pression. GIF has additional "LZW" compression built in, but LZW doesn't work very well
on typical photographic data; at most you may get 5:1 compression overall, and it's not at all
uncommon for LZW to be a net loss (i.e., less than 3:1 overall compression). LZW *does*
work well on simpler images such as line drawings, which is why GIF handles that sort of
image so well. When a JPEG file is made from full-color photographic data, using a quality
setting just high enough to prevent visible loss, the JPEG will typically be a factor of four or
five smaller than a GIF file made from the same data.
Gray-scale images do not compress by such large factors. Because the human eye is much
more sensitive to brightness variations than to hue variations, JPEG can compress hue data
more heavily than brightness (gray-scale) data. A gray-scale JPEG file is generally only
about 10%-25% smaller than a full-color JPEG file of similar visual quality. But the uncom-
pressed gray-scale data is only 8 bits/pixel, or one-third the size of the color data, so the cal-
culated compression ratio is much lower. The threshold of visible loss is often around 5:1
compression for gray-scale images.
The exact threshold at which errors become visible depends on your viewing conditions.
The smaller an individual pixel, the harder it is to see an error; so errors are more visible on a
computer screen (at 70 or so dots/inch) than on a high-quality color printout (300 or more
dots/inch). Thus a higher-resolution image can tolerate more compression ... which is
fortunate considering it's much bigger to start with. The compression ratios quoted above
are typical for screen viewing. Also note that the threshold of visible error varies
considerably across images.
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