The GIF filter imports and exports bitmap images from CompuServe Graphics Interchange Files .GIF.
File Description
The Graphics Interchange Format .GIF is an image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability. Graphics interchange files are suitable for sharp-edged line art (such as logos) with a limited number of colors. This takes advantage of the format's lossless compression, which favors flat areas of uniform color with well defined edges (in contrast to JPEG, which favors smooth gradients and softer images). The 256 image palette color limitation makes the GIF format unsuitable for reproducing color photographs and other images with continuous color, but it is well-suited for simpler images such as graphics or logos with solid areas of color. The GIF format was originally designed to facilitate image transfer and online storage for use by CompuServe and its customers, GIF is primarily an exchange and storage format. GIF is a well-defined, well-documented format in wide use, which is quick, easy to read, and reasonably easy to uncompress. It lacks, however, support for the storage of deep-pixel images.
GIF images are compressed using the Lempel-Ziv-Welch (LZW) lossless data compression technique to reduce the file size without degrading the visual quality. This compression technique was patented in 1985. Controversy over the licensing agreement between the patent holder, Unisys, and CompuServe in 1994 inspired the development of the Portable Network Graphics (PNG) standard; since then all the relevant patents have expired.
Usage
· Sharp-edged line art (such as logos) with a limited number of colors. This takes advantage of the format's lossless compression, which favors flat areas of uniform color with well defined edges (in contrast to JPEG, which favors smooth gradients and softer images).
· Used to store low-color sprite data for games.
· Used for small animations and low-resolution film clips.
· In view of the general limitation on the GIF image palette to 256 colors, it is not usually used as a format for digital photography. Digital photographers use image file formats capable of reproducing a greater range of colors, such as TIFF, RAW or the lossy JPEG, which is more suitable for compressing photographs.
· The PNG format is a popular alternative to GIF images since it uses better compression techniques and does not have a limit of 256 colors, but PNGs do not support animations.
File Format
The format supports up to 8 bits per pixel, allowing a single image to reference a palette of up to 256 distinct colors chosen from the 24-bit RGB color space. It also supports animations and allows a separate palette of 256 colors for each frame. The color limitation makes the GIF format unsuitable for reproducing color photographs and other images with continuous color, but it is well-suited for simpler images such as graphics or logos with solid areas of color.
GIF is different from many other common bitmap formats in the sense that it is stream-based. It consists of a series of data packets, called blocks, along with additional protocol information. Because of this arrangement, GIF files must be read as if they are a continuous stream of data. The various blocks and sub-blocks of data defined by GIF may be found almost anywhere within the file. This uncertainty makes it difficult to encapsulate every possible arrangement of GIF data in the form of C structures.
There are a number of different data block categories, and each of the various defined blocks falls into one of these categories. In GIF terminology, a Graphics Control Extension block is a type of Graphics Control block, for instance. In like manner, Plain Text Extension blocks and the Local Image Descriptor are types of Graphic Rendering blocks. The bitmap data is an Image Data block. Comment Extension and Application Extension blocks are types of Special Purpose blocks.
Blocks, in addition to storing fields of information, can also contain sub-blocks. Each data sub-block begins with a single count byte, which can be in the range of 1 to 255 and indicates the number of data bytes that follow the count byte. Multiple sub-blocks may occur in a contiguous grouping (count byte, data bytes, count byte, data bytes, and so on). A sequence of one or more data sub-blocks is terminated by a count byte with a value of zero.
The GIF format is capable of storing bitmap data with pixel depths of 1 to 8 bits. Images are always stored using the RGB color model and palette data. GIF is also capable of storing multiple images per file, but this capability is rarely utilized, and the vast majority of GIF files contain only a single image. Most GIF file viewers do not, in fact, support the display of multiple image GIF files or may display only the first image stored in the file. For these reasons, we recommend not creating applications that rely on multiple images per file, even though the specification allows this.
File Name Extensions
.GIF
Import Method
Choose the File | Import command.
Export Method
Choose the File | Export command or select an image module and choose the File | Save Data command.
Export Options
See Export Options - Size and Color page.
See GIF Export Options dialog.
Export Restrictions/Limitations
The .GIF file format is limited to paletted images with 8 or fewer bits per pixel. If the export source contains more than 8 bits per pixel, it is quantized down to 8 bits per pixel during the export procedure. The .GIF file format is set to 72 Pixels per inch for all .GIF images and cannot be changed. GIF images are always 72 DPI by definition. For higher quality images, it is suggested that PNG, TIFF, or BMP is used instead of GIF.
See Also