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Image File Types

    Most of the important differences in image file types are related to whether or not the image data are compressed, and if so, how. The other major difference is whether the image is represented as "raster" data — bit after bit indicating the color and luminance of each screen pixel, versus "vector" or "object" data — line/square/circle/text, etc. encoded as mathematical formulas. Some formats include both raster and object data.

    As Nikon's Scanning Essentials states, "If u cn rd ths, thn u undrstnd cmprsn." Compression types fall into two main categories: "lossless", meaning none of the original image data is discarded in the process of creating the compressed version, and "lossy", in which some information is actually lost. The common lossless schemes are Run-Length Encoding (RLE), Limpel Ziv Welch (LZW), and Huffman encoding. Joint Photographics Expert Group (JPEG) is the most common lossy compression scheme. There are many file formats but they tend to use one or more of these formulas for compression.

    Here is a brief summary of various image files types:

File TypeExtensionCompressionData TypeComments

BitmapbmpRGB
or
RLE (lossless)
rasterThis is the basic screen-display format for Windows and OS/2, can be 2 to 16.7 million colors; generally used uncompressed. The Windows clipboard uses this format. RLE compression not supported by all display adapters; rarely used.

CDR - CorelDrawcdr?vectorProprietary format

DCS - Desktop Color Separationdcslosslessraster/objectVariant of EPS used in desktop publishing.

EPS - Encapsulated Postscriptepslosslessraster/objectUsed across platforms; often used by printing services?

GIF - CompuServe Graphics Interchange FormatgiflosslessrasterSo-named because it was developed by CompuServe to transmit image files in smaller sizes over modem connections. Limited to maximum 256 colors, though which 256 varies according to the palette chosen. Allows for one color to be "transparent" — i.e., not displayed, allowing background to show through. Also can contain multiple images to be displayed in sequence to create animation. Widely used on the Internet.

JFIF - JPEG File Formatjif?rasterThis is the most common implementation of the JPEG specification.

JPEG - Joint Photographic Experts GroupjpglossyrasterVariable compression ratios allow great reduction in file sizes while preserving most image information without noticeable image degradation, versus very high compression with significant loss of quality. 2 to 16.7 million colors. Very commonly used, especially on the Internet and in digital cameras.

PCX - ZSoft Paintbrushpcxnone or lossless?rasterCommonly used

PICTpct?rasterThis is to Macintosh computers what BMP is to Windows

PNG - Portable Network Graphicspnglosslessraster Saves 16M colors plus transparency. Compression not as high as JPEG, but lossless. Supported by all major Web browsers.

TIFF - Tagged Image File Formattif, tiffnone or losslessraster Most widely used format, most portable between applications and across platforms (e.g Windows to Mac). Generally does not achieve high compression ratios.

TGA - TARGAtga?rasterI don't know about this one but it's often supported by graphics software

WMF - Windows Metafilewmflosslessraster/objectplatform-dependent (Windows). Useful for scaling images. Also used on the Windows clipboard.

WPG - WordPerfect Graphicswpg?raster/vectorProprietary WordPerfect format, but most graphics applications can read it if not write to it

    Obviously this is very superficial. Paint Shop Pro has a little information on file formats in its help file. If you want more detailed information on any of these, I'll try to find it for you.

* Disclaimer: I am in no way an authority on this subject. This information may not be entirely accurate. wrp

This page created by William R. Parke with ParkeNet HTML Editor

Last modified Sunday, October 27, 2002

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