JPEG (Joint Photographic Experts Group)
JPEG is a standardised image compression mechanism. JPEG is designed for compressing either full-colour (24 bit) or grey-scale digital images of "natural" (real-world) scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or black-and-white line drawings (files come out very large). JPEG handles only still images, but there is a related standard called MPEG for motion pictures.JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in. The algorithm achieves much of its compression by exploiting known limitation of the human eye, notably the fact that small colour details aren't perceived as well as small details of light-and-dark. Thus, JPEG is intended for compressing images that will be looked at by humans.
A lot of people are scared off by the term "lossy compression". But when it comes to representing real-world scenes, no digital image format can retain all the information that impinges on your eyeball. By comparison with the real-world scene, JPEG loses far less information than GIF.
GIF (Graphics Interchange Format)
GIF, like JPG, is an older filetype, and one generally associated with the internet as opposed to photography. GIF stands for “Graphics Interchange Format” and employs the same lossless LZW compression that TIFF images use. This technology was once controversial (for patent enforcement issues) but has become an accepted format since all patents have expired.The Graphics Interchange Format was developed in 1987 at the request of Compuserve, who needed a platform independent image format that was suitable for transfer across slow connections. It is a compressed (lossless) format (it uses the LZW compression) and compresses at a ratio of between 3:1 and 5:1
It is an 8 bit format which means the maximum number of colours supported by the format is 256.
There are two GIF standards, 87a and 89a (developed in 1987 and 1989 respectively). The 89a standard has additional features such as improved interlacing, the ability to define one colour to be transparent and the ability to store multiple images in one file to create a basic form of animation.
Both Mosaic and Netscape will display 87a and 89a GIFs, but while both support transparency and interlacing, only Netscape supports animated GIFs.
PNG (Portable Network Graphics)
PNG stands for Portable Network Graphics (or, depending on whom you ask, the recursive “PNG-Not-GIF”). It was developed as an open alternative to GIF, which used the proprietary LZW compression algorithm discussed earlier. PNG is an excellent filetype for internet graphics, as it supports transparency in browsers with an elegance that GIF does not possess. Notice how the transparent color changes and blends with the background. Right-click the image to see. This is actually one image that is on four different background colors.PNG supports 8-bit color like GIF, but also supports 24-bit color RGB, like JPG does. They are also non-lossy files, compressing photographic images without degrading image quality. PNG tends to be the biggest of the three filetypes and isn’t supported by some (usually older) browsers.
In addition to being an excellent format for transparency, the non-lossy nature of 24-bit PNG is ideal for screenshot software, allowing pixel for pixel reproduction of your desktop environment.
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JPG: Unfortunately, JPGs don’t support transparency. JPG files have an unlimited color palette, but they blend pixels together to reduce the size of the image.
SUMMARY:
- PNG is good option for transparency and non-lossy, smaller files. Larger files, not so much, unless you demand non-lossy images.
- GIF is largely a novelty and only useful for animation, but can produce small 8-bit images.
- JPG is still the king for photographs and photo-like images on the internet, but be careful, as your file can degrade with every save.
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