it-e-69 Image Compression

Why is image compression so important? Image files, in an uncompressed form, are very large.
And the Internet, especially for people using a 56k dialup modem, can be pretty slow.[1]This
combination could seriously limit one of the Web's most appreciated aspectsits ability to present
images easily.
JPEG (Joint Photographic Experts Group)compression is currently the best way to compress
PHOTOGRAPHIC IMAGES for the web. Other forms of image compression, including GIF and
PNG, are best used for other purposes on the web.
GIF (Graphics Interchange Format) is best used for graphics that have a limited color pallet
and large areas of flat tone, like cartoons or banners. Although it has several remarkable features,
such as transparency and the ability to present animated images, it is not well suited for the
presentation of continuous tone images, such as photographs, due to its limit of 256 colors.
PNG (Portable Network Graphics) is a relatively new format with a lot of potential but, until
all browsers can see images compressed in PNG form, it is not a good idea to use it.
JPEG, or JPG, is an evolving format that is universal in its use as a means of compressing
continuous tone photographs for speedy transmission over the Internet. Photographs compressed
using the JPEG format look good because JPEG supports millions of colors, so you can see the
gradation of tones.
A Bitmap is a simple series of pixels all stacked up. But the same image saved in GIF or
JPEG format uses less bytes to make up the file. How? Compression.
"Compression" is a computer term that represents a variety of mathematical formats used to
compress an image's byte size. Let's say you have an image where the upper right-hand corner
has four pixels all the same color. Why not find a way to make those four pixels into one?[2]That
would cut down the number of bytes by three-fourths, at least in the one corner. That's a
compression factor.
Bitmaps can be compressed to a point. The process is called "run-length encoding." Runs of
pixels that are all the same color are all combined into one pixel. [3]The longer the run of pixels,

the more compression. Bitmaps with little detail or color variance will really compress. Those
with a great deal of detail don't offer much in the way of compression. Bitmaps that use the
run-length encoding can carry either the common ".bmp" extension or ".rle". Another difference
between the two files is that the common Bitmap can accept 16 million different colors per pixel.
Saving the same image in run-length encoding knocks the bits-per-pixel down to 8. That locks
the level of color in at no more than 256.
So, why not create a single pixel when all of the colors are close? You could even lower the
number of colors available so that you would have a better chance of the pixels being close in
color. Good idea. The people at CompuServe felt the same way.
GIF, which stands for "Graphic Interchange Format," was first standardized in 1987 by
CompuServe, although the patent for the algorithm (mathematical formula) used to create GIF
compression actually belongs to Unisys. The first format of GIF used on the Web was called
GIF87a, representing its year and version. It saved images at 8 bits-per-pixel, capping the color
level at 256. That 8-bit level allowed the image to work across multiple server styles, including
CompuServe, TCP/IP.
CompuServe updated the GIF format in 1989 to include animation, transparency, and interlacing.
They called the new format, you guessed it: GIF89a.
There's no discernable difference between a basic (known as non-interlaced) GIF in 87 and
89 formats.

 

1, continuous tone  
连续色调

2, gradation  [ɡrə'deiʃən]
n. (色彩、颜色、次序、音调等的)渐变;分等级;(各种状态、性质等的)分阶段渐变;元音交替

3, discernable  [di'sə:nəbl, -'zə:-]
adj. 可辨别的;可认识的

4, interlaced  [,intə'leist]
a. 交织的,交错的

5, interlacing  [intə(:)'leisiŋ]
n. 交错,隔行;隔行扫描
v. 交错,交织(interlace现在分词)


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