Graphic Interchange Format (GIF)

Image File Formats - LegalScans

(.GIF file extension)

GIF was developed by CompuServe to show images online (in 1987 for 8 bit video boards, before JPG and 24 bit color was in use). GIF uses indexed color, which is limited to a palette of only 256 colors (next page). GIF was a great match for the old 8 bit 256 color video boards, but is inappropriate for today's 24 bit photo images.

GIF files do NOT store the image's scaled resolution ppi number, so scaling is necessary every time one is printed. This is of no importance for screen or web images. GIF file format was designed for CompuServe screens, and screens don't use ppi for any purpose. Our printers didn't print images in 1987, so it was useless information, and CompuServe simply didn't bother to store the printing resolution in GIF files.

GIF is still an excellent format for graphics, and this is its purpose today, especially on the web. Graphic images (like logos or dialog boxes) use few colors. Being limited to 256 colors is not important for a 3 color logo. A 16 color GIF is a very small file, much smaller, and more clear than any JPG, and ideal for graphics on the web.

Graphics generally use solid colors instead of graduated shades, which limits their color count drastically, which is ideal for GIF's indexed color. GIF uses lossless LZW compression for relatively small file size, as compared to uncompressed data. GIF files offer optimum compression (smallest files) for solid color graphics, because objects of one exact color compress very efficiently in LZW. The LZW compression is lossless, but of course the conversion to only 256 colors may be a great loss. JPG is much better for 24 bit photographic images on the web. For those continuous tone images, the JPG file is also very much smaller (although lossy). But for graphics, GIF files will be smaller, and better quality, and (assuming no dithering) pure and clear without JPG artifacts.

If GIF is used for continuous tone photo images, the limited color can be poor, and the 256 color file is quite large as compared to JPG compression, even though it is 8 bit data instead of 24 bits. Photos might typically contain 100,000 different color values, so the image quality of photos is normally rather poor when limited to 256 colors. 24 bit JPG is a much better choice today. The GIF format may not even be offered as a save choice until you have reduced the image to 256 colors or less.

So for graphic art or screen captures or line art, GIF is the format of choice for graphic images on the web. Images like a company logo or screen shots of a dialog box should be reduced to 16 colors if possible and saved as a GIF for smallest size on the web. A complex graphics image that may look bad at 16 colors might look very good at say 48 colors (or it may require 256 colors if photo-like). But often 16 colors is fine for graphics, with the significance that the fewer number of colors, the smaller the file, which is extremely important for web pages.

GIF optionally offers transparent backgrounds, where one palette color is declared transparent, so that the background can show through it. The GIF File - Save As dialog box usually has an Option Button to specify which one GIF palette index color is to be transparent.

Interlacing is an option that quickly shows the entire image in low quality, and the quality sharpens as the file download completes. Good for web images, but it makes the file slightly larger.

For GIF files, a 24 bit RGB image requires conversion to indexed color. More specifically, this means conversion to 256 colors, or less. Indexed Color can only have 256 colors maximum. There are however selections of different ways to convert to 256 colors.