Compression 1. A method to store text, data, or images in fewer bits. Rather than store every pixel of a blue square, the computer could store only one blue pixel and the dimensions of the square. Several compression standards exist: JPEG, JBIG, MPEG, GIF, TIFF, PICT, ZIP. 2. Lossy compression. A form of compression in which some data is discarded to allow much smaller file sizes. In image compression, lossy techniques such as those uses in the JPEG format.

Metafile file that may contain either bitmapped or vector graphics data.

Joint Photographic Experts Group (for still images, looks for sets of pixels containing similar or identical data--throws away redundant pixels, notes what was done). Can be the most efficient image storage method of all - at a price. First to the name. Everybody might call these images JPEG, but that just describes the type of compression used for the data; it doesn't describe how the compressed data is sorted and stored. The idea of JPEG is that as it compresses the data it throws some of it away - technically, this is called "lossy compression". You can configure how lossy you want your JPEGs to be; 100% quality gives you almost exactly the same result as the original picture. 10% quality takes up much less space but looks dodgy. You have to strike a balance. JPEG can store up to 24 bit color, so it's suitable for professional use, and it can do interlaced display like GIF (called "progressive" JPEG), which along with its small file sizes makes it the standard format for Web graphics. Like GIF, JPEG is a data-stream format - you can view images before you've received all of the data. Also like GIF, JPEG supports interlacing. There's also SPF or SPIFF, Still Picture Interchange File Format, the "official" International Standards organization Joint Photographic Experts Group (ISO JPEG) image format defined in the recent Part 3 extensions to the JPEG standard. SPIFF offers more features than the current JPEG standard and is backwards compatible, but has not yet achieved much popularity. SPIFF files may also be suffixed .JPG. An image format optimized for "natural" images, JPEGs are probably second only to GIFs in level of acceptance. JPEGs manage to capture wonderfully detailed images in millions of colors in minimal space by taking advantage of limitations with human vision, and performing little lossy compressions. This means that each time a JPEG is saved, it will lose a little more quality, although each individual loss will be nearly invisible to the human eye. If care is not taken, however, a JPEG image can become worse looking than a color-limited GIF. JPEGs are also not good at storing cartoon-like images or line drawings; for these cases either GIFs or PNGs or X-bitmaps are better choices. JPEGs also cannot contain a transparent color. If a transparent color is needed, GIFs, PNGs, and X-bitmaps are better choices. JPEG has nothing to do with either JBIG or MPEG in spite of the similarity of names.

GIF pronounced "JIF", by the format's creator, but many prefer GIF as in gift Graphics Interchange Format is a very efficient, and popular picture format. There are two versions of GIF, the old 87 and the newer 89a. 89a adds several extra features like transparency (so background graphics can "show through" selected colors) and animation. GIF animations are a very popular form of Web multimedia, because they're small and display on all current graphical browsers without needing a special plug-in or taking up much CPU time. Unfortunately, GIF pictures can only have 256 colors, or 256 shades of grey. 256 greys is photo quality so GIF is fine for any monochrome image, and 256 color looks OK for many pictures, but it's no use for professional imaging. GIF images can also be interlaced, so that you can see a low resolution version of the picture before downloading very much of it. GIF interlacing has four passes, which show one out of every eight lines, then another eighth of the image, then another quarter, then the remaining half. GIF is a data-stream type format, so you can view partially downloaded images whether or not they're interlaced, without interlacing, a 25% downloaded picture gives you the first 25% of the lines, starting at the top. PNG, pronounced "ping", is the Portable Network Graphics format, and was created as a free replacement for GIF, whose LZW compression is owned by Unisys and which can't be included in commercial software without paying license fees to the owners. It handles 1 to 48 bit images, and is a lossless, well-compressed format like GIF. PNG is designed to replace the GIF. In fact, PNG is sometimes jokingly said to really stand for "PNG's not GIF". PNG is completely lossless and can handle millions of colors; it is not limited to a palette of two-hundred fifty-six like GIF. It also has full support for transparent colors. Its only real disadvantage is that right now few browsers support it directly. It has been recommended by the W3C, though, so odds are pretty good that future versions of browsers will start to provide direct inline support for PNG, and PNG images will start to become more commonplace on the web. MNG, pronounced "ming", is the proposed Multiple Network Graphics format and is a multi-image extension of the existing PNG format.

Bmp Bitmapped files, also known as raster files, contain graphics information described as pixels, such as photographic images. The computer assigns a value to each pixel, from one bit of information (black or white), to as much as 24 bits per pixel for full color images.

TIFF or TIF stands for Tag Image File Format; TIFF was a large, unwieldy, 24 bit format until version 6 came out, which supported compression and made it less painful. The fact that its compression was somewhat broken and might or might not be compatible with different programs on different computers somewhat reduced the bonus. The compression is LZW and thus owned and licensed out by Unisys (see GIF) is another problem. TIFF is, nonetheless, a very popular professional graphics format. A TIFF file permits the image to be edited in other applications (ie QuarkXpress, and Macromedia Freehand)

PCX ZSoft Paint format, occasionally suffixed .PCC, is ancient but still fairly widely used, simply because everybody understands it. There are three common versions, 0, 2 and 5; 0 is the original two color one (small but not useful), 2 only does 16 colors and is hence also of little interest to owners of rather old video cards, and 5 does 24 bit. All are large for what they do, but fast to load on elderly computers. PCX is the IBM equivalent of Amiga IFF. The size listed is for v5, at full 24 bit; v2 scored 216k and v0 48.1k.

PICT Pict is the Apple Quickdraw metaformat and the Macintosh bitmap picture format. This is a fairly simple bitmap format that can be viewed on many different platforms but is directly supported by very few browsers. It is not used too much on the web with GIFs, JPEGs, PNGs, and even X-bitmaps being preferred. It is somewhat similar to the bmp format, although utilizes compression and so boasts smaller file sizes. It can include bitmapped or vector images, and can use different compression schemes. Avoid using PICT as a file format if you need to color-separate your output or if the image contains PostScript text or graphics. With PICT, what you see onscreen is not always what you get on paper. The more recent PICT2 format supports 24-bit color.

.WAV The standard PC audio-file format for everything from system and game sounds to CD-quality audio. Created by Microsoft, the format, which requires a lot of storage space, is usable on PCs and other computer platforms.

mpeg, mpg, mpeg2, mpeg3, mp2, mp3, & m3u The Moving Pictures Expert Group devised formats for storing both movies and sounds. A file with this extension could thus be either an audio file or a movie file. Both types are supported by many different platforms, with perhaps the audio type being a little more recognized. Both types also utilize lossy compression that is designed to take advantage of the limitations of human vision and hearing. Regardless the compression still occasionally does weird things producing "MPEG compression funnies", particularly during periods of rapid movement in video. MPEGs are not limited to computers; some digital satellite television transmissions use an MPEG format, and DVDs use an MPEG format. MPEG has nothing to do with either JPEG or JBIG in spite of the similarity of names. MPEG does have some relation to QuickTime, though, and the next version of each will work to merge the two. If there is a number at the end of the extension, it refers to the specific generation of the MPEG format in use within the file.

MP3 (MPEG-1 Audio Layer-3) The de facto standard for downloadable - and occasionally streaming - digital music. In .MP3 format, audio files are compressed into about one-twelfth the size of the original file. Utilizing CD-ripping software, data can be extracted from an audio CD and turned into MP3 files. Despite controversy - such as Metallica's war with Napster - some music publishers provide Web-based sample cuts in .MP3 format as a way to spark music buyers' interest in new releases. The ID3 Tag is an MP3 add-on, the ID3 Tag enables the user to type in up to 128 bytes of track data per MP3 file, including song title, artist name, album title, year of release, comments and genre.

Ogg Vorbis Equivalent to MP3, Ogg Vorbis is an emerging free, open-source, general-purpose compressed audio format for high-quality digital-audio encoding and playback. According to its developers, Ogg Vorbis - unlike MP3 - is entirely patent and royalty free. The format creates the file extension .OGG.

MIDI Musical Instrument Digital Interface A protocol designed for recording and playing back music on PC sound card digital synthesizers. The MIDI format generates conveniently small files but lacks specific sound control.

aiff & aif The audio interchange file format originated on Macs but has since been ported to several other platforms as well.

.txt This is a generic sort of extension indicating a simple text file (usually ASCII). It can be readily used on virtually any computer, although sometimes character set differences will require slight conversions. A file with this extension will not typically have significant formatting as that would decrease its simplicity and portability.

.rtf Normally used as a well-understood cross-platform word processing document format, but which can store pictures as well as text. As image storage formats go, though, this one is as inefficient as Postscript.

.doc This is a rather generic sort of extension indicating some sort of document, usually in simple ASCII; of particular note though is the fact that both FrameMaker and MS-Word often save their native format files with this extension, and such a file may only be read with FrameMaker or FrameViewer (if a Frame document) or MS-Word (if an MS-Word document). Neither of these programs are available for all platforms, and even when they are available are usually not free. To complicate matters even further, there are several different versions of MS-Word in common use that cannot read each others' files. Thus in general this format should not be viewed as portable, and chances of reading it on any particular platform are slim. Documents saved from either FrameMaker or MS-Word that are meant for other machines should probably be saved in MIF or RTF formats (respectively) in any case.

.wpd Correl Word Perfect word processor format

comma delimited CDF uses commas between fields and a carriage return between records

tab delimited TDF uses tabs instead of commas.

.xls Microsoft Excel

PS Adobe Systems Postscript isn't an image format, per se - it's a page description language, originally conceived so computers could send very accurate page descriptions to the then new high resolution laser printers. You can save black and white or even color pictures as Postscript, but you'll end up with a very large file. Postscript is not a very efficient format, but its advantage is all plain text - you can modify a Postscript file with any text editor, if you know what you're doing.

PSD Adobe Photoshop's native format, which stores all of its layer and selection and miscellaneous other image data.

EPS or Encapsulated PostScript a type of Postscript which can be imported into page layout and vector graphics programs.

RAW often a Photoshop RAW file, which is a PSD file with no identifying header. Or it may be a minimally formatted image data dump.

AI Adobe Illustrator's metafile format, which is actually a type of Encapsulated Postscript.

.ra Real Audio (often streamed)

.qt Apple QuickTime