Graphics
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Graphics |
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The following brief format descriptions are the most commonly used formats. Many other independent formats exist. These descriptions are non-technical and are only intended to give the reader a feeling for the basis of the format. In some cases a technical description document link is included for those students with the necessary background and interest.
Postscript/Encapsulated Postscript
In addition to representing text documents postscript files may contain graphics and images. The postscript language was designed as a typesetting language for both text and graphics. Although used primarily for vector/structured graphics, postscript contains a mechanism for storing bit-mapped images. Several popular drawing programs (Illustrator, FreeHand), use the postscript language as for graphic representation, (i.e. as their imaging model). Postscript/Encapsulated does not provide compression which causes files to be quite large when stored in this format. Since no compresssion is provided of course postscript is a lossless format.
JPEG
JPEG is a standard created by the Joint Photographics Experts Group. It utilizes compression techniques that are specifically designed for photographic images. It takes advantage of limitations in the human vision system to achieve high rates of compression. JPEG is a lossy format which allows a user to set the desired level of quality maintained in the image. Higher qualities requires more storage, but is only needed for particular images. Allowing users to control the quality/compression decision is a superb advantage of JPEG since it utilizes a person's innate vision processing, their knowledge of the application of an image and their knowledge of the available memory of the particular storage system.
The UNISYS Corp. and Compuserve, (a commercial network), devised the Graphics Interchange Format (GIF), to facillatate transmission of graphical images over phone lines via modems. Due to the relatively slow transfer rate of modems, file size was the over-riding design goal behind GIF. The Lempel-Ziv Welch algorithm, (a form of Huffman Coding), modified slightly for image scan line packets, (line grouping of pixels), yields the high compression rates that the GIF format achieves. Part of the reason for small GIF file sizes is due to the limitation of the format to only storing 8-bit color images. Because Compuserve is accessed by users of a wide variety of different computer systems the format was also designed for portability.
TIFF
The Tagged Image File Format (TIFF), provides a mechanism for storing many different types of images (e.g., monochrome, grayscale, 8-bit & 24-bit RGB, etc.). TIFF tags the type of data in the file that describes the image type. The latest revisions to the specification provides tags for compression formats, but this is not supported by many programs that implement TIFF. TIFF is a lossless format (when not utilizing the new JPEG tag which allows for JPEG compression). The TIFF format was developed by the Aldus Corp. in the 1980's and later supported by the Microsoft Corp. Since TIFF does not provide any major advantages over JPEG and is not as user-controllable it appears to be declining in popularity.
Many graphical/imaging applications create their own file format particular to the systems they are executed upon. Far too many formats exist to be covered in this document, (for a fairly comprehensive listing check CICA at Indiana Univ.). This short section will only mention each of the predominant types that are standard within the specific operating system.
Windows/DOS: bmp
The Microsoft Corporation not only brought graphics to the DOS world with the Windows operating system, but also a system standard graphics file format (bmp). The bmp format (short for bitmap), is implemented in the PC Paintbrush program. Unlike GIF it is capable of storing 24-bit bitmap images.
Macintosh: paint and pict
Apple by any reasonable argument created the first commercially successful image format (paint). The paint format was used in the original MacPaint program. The format was initially used only for storing 1-bit monochrome images. Although Apple has since extended the format, many supporting programs still limit the format to the original 1-bit specification. Apple also produced one of the first popular vector based drawing programs. MacDraw utilized the pict format for storing structured graphics and it still remains a popular translation format on the Macintosh platforms.
X-windows: xbm The primary graphics format for the X Window system. It is a 24-bit color bitmap capable format. There exist many public domain graphic editors for the xbm format. The most popular of which is the xv application. This format is used in X Windows for storing icons, pixmaps, backdrops, etc.
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