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 Type Extension Compression Data Type Comments

Bitmap bmp RGB
or
RLE (lossless) raster This 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 - CorelDraw cdr ? vector Proprietary format

DCS - Desktop Color Separation dcs lossless raster/object Variant of EPS used in desktop publishing.

EPS - Encapsulated Postscript eps lossless raster/object Used across platforms; often used by printing services?

GIF - CompuServe Graphics Interchange Format gif lossless raster So-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 Format jif ? raster This is the most common implementation of the JPEG specification.

JPEG - Joint Photographic Experts Group jpg lossy raster Variable 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 Paintbrush pcx none or lossless? raster Commonly used

PICT pct ? raster This is to Macintosh computers what BMP is to Windows

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

TIFF - Tagged Image File Format tif, tiff none or lossless raster Most widely used format, most portable between applications and across platforms (e.g Windows to Mac). Generally does not achieve high compression ratios.

TGA - TARGA tga ? raster I don't know about this one but it's often supported by graphics software

WMF - Windows Metafile wmf lossless raster/object platform-dependent (Windows). Useful for scaling images. Also used on the Windows clipboard.

WPG - WordPerfect Graphics wpg ? raster/vector Proprietary WordPerfect format, but most graphics applications can read it if not write to it

File Type	Extension	Compression	Data Type	Comments
Bitmap	bmp	RGB or RLE (lossless)	raster	This 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 - CorelDraw	cdr	?	vector	Proprietary format
DCS - Desktop Color Separation	dcs	lossless	raster/object	Variant of EPS used in desktop publishing.
EPS - Encapsulated Postscript	eps	lossless	raster/object	Used across platforms; often used by printing services?
GIF - CompuServe Graphics Interchange Format	gif	lossless	raster	So-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 Format	jif	?	raster	This is the most common implementation of the JPEG specification.
JPEG - Joint Photographic Experts Group	jpg	lossy	raster	Variable 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 Paintbrush	pcx	none or lossless?	raster	Commonly used
PICT	pct	?	raster	This is to Macintosh computers what BMP is to Windows
PNG - Portable Network Graphics	png	lossless	raster	Saves 16M colors plus transparency. Compression not as high as JPEG, but lossless. Supported by all major Web browsers.
TIFF - Tagged Image File Format	tif, tiff	none or lossless	raster	Most widely used format, most portable between applications and across platforms (e.g Windows to Mac). Generally does not achieve high compression ratios.
TGA - TARGA	tga	?	raster	I don't know about this one but it's often supported by graphics software
WMF - Windows Metafile	wmf	lossless	raster/object	platform-dependent (Windows). Useful for scaling images. Also used on the Windows clipboard.
WPG - WordPerfect Graphics	wpg	?	raster/vector	Proprietary 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