Top Document: JPEG image compression FAQ, part 1/2 Previous Document: [3] When should I use JPEG, and when should I stick with GIF? Next Document: [5] What are good "quality" settings for JPEG? See reader questions & answers on this topic! - Help others by sharing your knowledge 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 typically 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 compression. 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 uncompressed gray-scale data is only 8 bits/pixel, or one-third the size of the color data, so the calculated 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. User Contributions: 1 chloroquineorigin ⚠ aralen for sale https://chloroquineorigin.com/ - aralen where to buy malarone buy chloroquine uk https://chloroquineorigin.com/ 2 eyytfb ⚠ May 6, 2022 @ 9:21 pm hydroxychloroquine classification https://keys-chloroquinehydro.com/ Comment about this article, ask questions, or add new information about this topic:Top Document: JPEG image compression FAQ, part 1/2 Previous Document: [3] When should I use JPEG, and when should I stick with GIF? Next Document: [5] What are good "quality" settings for JPEG? Part1 - Part2 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jpeg-info@uunet.uu.net
Last Update March 27 2014 @ 02:11 PM
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