Transparent GIFGIF stands for Graphics Interchange format, a compressed graphics file format patented by Unisys that is widely used in HTML documents for inline graphics elements. A transparent GIF image blends in with any background on a Web page, whether explicitly defined, or supplied by default (a standard neutral gray). Basically, the transparency applies to that portion of the image which is not foreground material-- part of the frame that encloses the image, but does not belong to the image. Most modern browsers support transparent GIFs. It's easy to recognize non-transparent GIFs because their backgrounds do not blend in with the Web page on display.
Please consult the topic, "Creating Transparent GIFs". Transparent GIFs are useful because they appear to blend in smoothly with the user's display, even if the user has set a background color that differs from that the developer expected. They do this by assigning 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.
Interlaced GIFs appear first with poor resolution and then improve in resolution until the entire image has arrived, as opposed to arriving linearly from the top row to the bottom row. This is great to get a quick idea of what the entire image will look like while waiting for the rest. This doesn't do much for you if your Web browser doesn't support progressive display as the image is downloaded, but non-progressive-display Web browsers will still display interlaced GIFs once they have arrived in their entirety.
You can make transparent and interlaced GIFs through the Web without running any utility software on your own system through the Visioneering image manipulation page which will access your image through the Web and produce an enhanced version for you to save.
To create transparent and interlaced GIFs under UNIX, check out David Koblas' giftool, a program which can manipulate those options and many more aspects of your GIF file.
For Windows PCs, try Lview Pro, version 1A or later, available by anonymous FTP from oak.oakland.edu in the directory SimTel/win3/graphics: ftp://oak.oakland.edu/SimTel/win3/graphics, as well as from many mirror sites.
Adobe Photoshop users will be interested in PhotoGIF, Boxtop Software's plug-in to add sophisticated GIF support to Photoshop. PhotoGIF can save transparent and interlaced GIFs, as well as optimizing GIF images in other ways.
You can also create transparent and interlaced GIFs using the widely available NETPBM tools (an enhanced version of the older pbmplus tools, which do not support these options).
In addition, there is a document explaining transparent GIFs available at the URL http://melmac.corp.harris.com/transparent_images.html. You can fetch the program giftrans.
There are also several utilities for the Macintosh: