Type
Description
BMP
BMP is the native Windows bitmap format. BMP supports 8-bit and 24-bit images. The format is perfect for storing small bitmaps that will only be used on a Windows computer. This is the only format you should use for WinHelp authoring. The BMP format does not usually use compression, although a compression technique called Run Length Encoding (or RLE) is sometimes used.
GIF
GIF is an excellent way to store small graphics, especially for distribution over the Internet. GIF uses LZW compression, so that the file size is reduced without loss of detail in the image. GIF files are limited to 256 colours, so they are of limited use for photographs. The current version of GIF, which is GIF 89a, provides some multimedia features that modern web browsers can exploit. Among these are:
Interlaced GIFs, which allow web browsers to load the graphic in stages, so the recipient sees a low-resolution graphic that fills in the details bit by bit.
Transparent GIFs, which allow a background graphic to show through portions of the bitmap that have been defined as invisible. This is good for product logos.
Animated GIFs, which are layered bitmaps that show one layer at a time. These can be used for little movies or animations on web pages
The main use for GIF files these days is on web pages. They handle images with areas of solid colour very well, but photographs less well.
JPEG
JPEG is exclusively for storage of photographs. The format supports True Colour only. Since JPEG is a lossy compression, it can add unwanted artifacts and noise to graphics, if used improperly. However, this noise is far less noticeable in a real photo than in a purely computer-generated image. If you want to put a photo of yourself on a web page, JPEG is the best choice. If you are putting a diagram or schematic on your web page, use GIF.
TIFF
TIFF is one of the oldest and most flexible bitmap formats. It supports True Colour, and uses lossless compression. It can contain images of unlimited size, and of any bit depth (from 1-bit to 24-bit and any other value in between). It is often used for photographs that are in the process of being retouched, because no details are lost when images are compressed as TIFF. TIFF files are usually much bigger than JPEG files, but smaller than BMP. The format is used for printing, not multimedia.
WMF
WMF is native only to Windows. It is useful for embedding small drawings or bitmaps in Word documents and online help, but not for much else. Large WMF files take time to draw on screen, making them cumbersome for publishing work. And since WMF is not a page-description language like Postscript, it does not provide consistent results when printing at high resolution. You should avoid using WMF in printed documentation. Use TIFF and EPS instead.
PICT
PICT is native only to Macintosh. It is somewhat more flexible than WMF, and has support for features that make it useful for multimedia production. However, for printing, TIFF and EPS are preferable.
PNG
PNG stands for Portable Network Graphic. This format has been around for five years or so, and support for it has arrived very slowly. This is unfortunate, because PNG has great potential.
PNG is a lossless format similar to TIFF. It has all the features of TIFF, and compresses images into smaller files. In addition, it supports transparencies, so it has built-in support for things like masking, and drop shadows, and other cool effects.
PNG is ideal for web sites, but suport for the format in modern browsers has arrived slowly. All browsers now have full support for PNG, with the very significant exception of Internet Explorer 6.