Appendix A
Graphic File Formats
Communique! tools support a wide variety of graphic file formats for loading and saving images. By supporting most popular file formats Communique! tools can work seamlessly with many different applications, operating systems, and computing platforms. If you are unfamiliar with some of the formats supported by Communique! tools, this section will briefly explain each of them.
File Formats
The following file formats are supported by Communique!
BMP
The Microsoft Windows Bitmap (BMP) format is used primarily on MS-DOS, Microsoft Windows, and Windows NT platforms. Most applications running on PCs support the Windows Bitmap format.
The BMP format can store images with up to a 24-bit color depth (16.7 million colors). Communique! uses the simplest, uncompressed form of Windows Bitmap to ensure compatibility with other applications. This form decodes quickly but can produce files of considerable size, especially for 24-bit images.
EPS
Encapsulated PostScript (EPS) files are a special subset of PostScript files. Most desktop publishing packages on PCs, Macintoshes, and UNIX workstations will allow you to insert EPS images in their documents.
GIF
The Graphics Interchange Format (GIF) was created by CompuServe and is now a widely supported format. Most PC, Macintosh, and UNIX programs that read and write graphics data support GIF.
One limitation to GIF is that files are limited to 8 bit color (256 colors). If you save a 24-bit image in GIF, the image will be dithered down to an 8-bit image with 256 colors. GIF uses a moderate compression scheme that produces files much smaller then Windows BMP, but in most cases larger than files created by the JPEG file format.
JPEG/JFIF
JPEG or JFIF files use the JPEG compression algorithm developed by the Joint Photographic Experts Group. The JPEG file format is not as widely supported as GIF and TIFF, but it is becoming more popular.
The JPEG format can store 24-bit color images (16.7 million colors) and achieves incredible compression ratios. There are two drawbacks to using the JPEG format for your files, however.
First, JPEG employs lossy compression. To achieve such dramatic compression levels, JPEG removes image data which may result in a slight decrease in image clarity. Typically, this loss in quality is not noticed by the unaided eye, even for photographic or real world images, but you will not want to use JPEG when fine image details should not be lost.
The second drawback to JPEG is speed. JPEG files take longer to load and save than with other formats. This delay may become an issue if you are using a relatively slow computer or are working with large images.
PCX
The PCX format developed by ZSoft is used primarily on MS-DOS, Microsoft Windows, and Windows NT platforms. Most applications running on PCs support the PCX format.
PCX can store up to 24-bit color images (16.7 million colors). It employs a simple, lossless compression scheme that is very fast to decode but can produce files of considerable size, especially for 24-bit images.
SGI
The SGI image library format is used primarily on Silicon Graphics UNIX workstations. Most applications running on SGI machines will support the SGI format, but applications designed for other platforms are unlikely to support it.
The SGI image library format can store 24-bit images (16.7 million colors). Communique! writes the simplest form of the SGI image library format without using file compression to ensure compatibility with other applications. This can produce files of considerable size, especially for 24-bit images.
Sun Raster
The Sun Raster format is used primarily on Sun Microsystems UNIX workstations. Most applications running on Sun machines, and many on other UNIX platforms, support the Sun Raster format; but PC and Macintosh applications are unlikely to support it.
The Sun Raster format can store 24-bit color images (16.7 million colors). Communique! uses the simplest form of Sun Raster file compression to ensure compatibility with other applications. This method employs a lossless run length encoded compression scheme which is very fast to decode but can produce files of considerable size, especially for 24-bit images.
TARGA
The Targa (or TGA) format is used primarily on MS-DOS, Microsoft Windows, and Windows NT platforms. The Targa format is supported mainly by high end graphics applications, especially those packages dealing with scanners, frame grabbers, and ray tracing.
The Targa format can store up to 32-bit color images. It employs a simple, lossless run length encoded compression scheme that is very fast to decode but can produce files of considerable size, especially for 24 and 32-bit images.
TIFF
The Tagged Image File Format (TIFF) is a widely supported format storing up to 24-bit color images (16.7 million colors). Communique! uses the simplest form of TIFF file compression to assure compatibility with other applications. This form employs a simple, lossless compression scheme that is very fast to decode but can produce files of considerable size, especially for 24-bit images.
XWD
The X Window Dump (XWD) format is used by UNIX workstations running X Windows. Communique! writes files in the XWD format and will read XWD files created by other applications, such as the UNIX command xwd.