What JPG is
One of the two most common types of image formats used on the World Wide Web (the other being GIF). The shorter extension, JPG (without the E), is usually used in association with PC platform files.
A standardized image compression mechanism, JPG is named after the original name of the committee that wrote the standard, the "Joint Photographic Experts Group." JPG is designed for compressing either full-color or gray-scale images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings. JPG handles only still images, but there is a related standard called MPEG for motion pictures. JPG is "lossy," meaning that the decompressed image isn't quite as sharp as the one you started with. (There are "lossless" image compression algorithms, but JPG achieves much greater compression than is possible with lossless methods.) JPG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. Thus, JPG is intended for compressing images that will be looked at by humans. If you plan to machine-analyze your images, the small errors introduced by JPG may be a problem, even if they are invisible to the eye.
On the Web, there are image archives set up for Web designers to acquire graphics easily. These sites usually display thumbnail versions of images that may be GIFs or JPGs. Clicking on these thumbnails will usually link you to the larger, higher-resolution version of the same image.
The meaning of GIF (Graphics Interchange Format)
Developed by CompuServe, using file compression technology from Unisys, this is a graphics file format used on the Internet. It was originally referred to as a "CompuServe Bitmap" but it stands for "Graphics Interchange Format." You may hear GIF mentioned as "GIF format" (this is redundant, but it is widely used). On Web pages, the images (or pictures) you see are usually in GIF because the files are small and can be downloaded quickly. Another type of graphics format commonly used online is JPG; these files download even faster and contain a better resolution. However, JPGs cannot be "interlaced," so many Web authors use GIFs instead, to get that "melting onto the screen" effect that happens with interlaced images. Following are special kinds of GIF:
Animated GIF - a series of static images are displayed one after another or on top of each other, giving the effect of motion or animation.
Interlaced GIF - these appear first with poor resolution and then gain resolution once the entire image has arrived, as opposed to arriving linearly from the top row to the bottom row. This type of format is good for giving the user a quick idea of what the entire image will look like while they're waiting for the rest of the image to load.
Transparent GIF - useful because they appear to blend in smoothly with the Web page's background, even if the user has set a background color that differs from the one the developer expected. To make a GIF see-through or appear to be the same color as the background, you must assign one color to be transparent; if the Web browser supports transparency, that color will be replaced by the browser's background color, whatever it may be.
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