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An introduction to the GIF file format

Propeties of the data format

The GIF file format was developed by CompuServe considerung the needs of online applications. It allows image representation with a color palette of the a maximum of 256 colours. LZW must be used as the only compression scheme. Several fframes can be combined to an animation in one GIF file.

When saved in interlaced mode, the data produces a discernable image faster than non-interlaced images, especially when images are to be transmitted over land lines (modems) or slow network connections. For this purpose, the data is saved in several runs, first only each eighths. line (0.,8.,16.,24.,) then the data of the intervening lines (4.,12.,20....) and so forth.

For every frame of a GIF file, one colour can be defined as representing "transparent" regions.

GIF files can contain comments.

LZW patent problems

The LZW technique used as compression patent protected legally by Unisys. Since Unisys has raised the claim only since 1995, it is yet the most important graphics format in addition to JPEG in the internet. In newer applications GIF should be avoided as far as possible and should be replaced by the PNG format, which allows for higher compression rates and has no patent problem.

Implementation in DaVinci

If you do not specify the flag IPF_ALLOWLZW when exporting as a GIF file, DaVinci creates an uncompressed image as a GIF file which is compatible for every standard GIF viewer program. This process is not based on patented technologies.

DaVinci can't combine animations from several "frames". When importing an animation, only the last frame is accessible. The access to comments is not supported. Transparent figures are not supported.

If a DIB with more than 256 colours is exported, ipExportInd uses the function DitherTo8 (lpbmi, 6, 0, NULL) to perform the colour reduction.

Þ http://www.gnu.org/philosophy/gif.html

Þ c't 3/95 S.29 Volker König, "Lizenzquerelen: Lizenzgebühren für GIF-Format erhoben"