.JPG, JPEG
Joint Photographic Experts Group's still-image format that
supports "lossy"
compression/decompression (codec) scheme of between 4-to-1
and 10-to-1. Specification is a "baseline" system that uses
the two-dimensional DCT (discrete cosine transform)
algorithm to achieve high compression ratios and maintain
image quality. Originally developed for colored still-images
or photographs, primarily it condenses information within
each frame.
JPEG images ("artifacts") appear onscreen as made up of many square tiles, a "chunky" effect. Sharp edges, which are rare in scanned photographs, appear blurred without a high quality setting. At least 16 gray levels are required before JPEG is useful for gray scale images. Continuous-color images are represented more faithfully in JPEG than in the competing (8 bits /pixel) GIF format; while images with few distinct colors have a greater compression in GIF than in JPEG.
PICT and TIFF images (of 8-bit grayscale or 8-bit, 16-bit, or 32-bit color) can be stored in the compressed JPEG file format. The JPEG image may then be opened (decompressed) and compared with its original TIFF or PICT image (if there is sufficient RAM and data-storage capacities). Some applications can accurately edit high-qualityty images in the compressed JPEG file format. Images saved in PICT or TIFF format, may be accessible by other applications using the same operating system.
Converting from JPEG to TIFF or PICT file formats, image quality is maintained; from TIFF or PICT to JPEG file format, image quality is related to its compression ratio. Although this "lossy" ISO standard supports compressions up to 100:1 (100 bytes represented by 1 byte), images degrade noticeably in most applications if the compression ratio exceeds 20:1. A low ratio maintains image quality, but the file size is large.
JPEG color images can be quantized to precisely match the viewer's display hardware and software algorithms. Upgrading both can result in a better view; however, the JPEG compressors used by some board manufacturers may be incompatible with the decompressors of other board manufacturers. Test to be sure. Note, JPEG files are compatible with QuickTime PICT Compressor. DOS viewer utilities: CompuShow and DVPEG. JPEG is also supported by NetScape and OS/2 WebExplorer.
Windows files must have this extension: .JPG .
Motion-JPEG, derived from the still-image codec, allows easy random access to any frame in a digitized sequence. It can be used for capturing full-screen, full-rate (30-frames-per- second) video. Compression time increases as the image increases in size on screen. Audio is not supported.
.KF A
SpDE (back annotation) QuickLogic delay-scaling information file used for intergraph computer circuitry simulation.
LAW
A data compression technique. Variations are used in AU, AIFF and MOD sound file formats.
LBM, IFF, ILBM
Electronic Arts' DOS/Windows and Amiga data-formats. Supports 17 million colors for IFF, ILBM, LBM and PBM files; 4096 colors for HAM files. Also supports SHAM and DHAM files. File translations may be limited to 256 colors. These files may have a filename extension of: .LBM.
Linear 8 bit 2's complement
A SND/AU MacOS sound file format similar to µ-law and A-law.
.LOF
A SpDE File-Export LOF file. It can be used by with Data I/O Unisite System Software. (The file should be "zipped" with PKZIP.)
.LOG
A Silos III computer circuitry simulation log file.
Lossless / Lossy
"Lossless" data compression reduces the size of a file by as much as 25% without losing data. Typically this scheme is used for text and numerical data. "Lossy" data compression results in much smaller file sizes by allowing some loss of data. Typically this scheme is used for image files because sizes can be reduced by sacrificing some image quality. (Text or numerical data would be rendered useless by this compression scheme.)
Lotus™ PIC
Lotus 123™'s PrintGraph data-format for DOS/Windows which supports graphics with a maximum 256 colors.
Low-Res
Low Resolution, low-quality reproduction results from using a small number of dots per inch (dpi), e.g., 53 dpi, in representing (horizontally and vertically) a file's text and/or images onscreen or in print.
Luminance
A video signal's brightness.
.LZH
LZH file compression application. Compressed files have a filename extension of .LZH. Use same application to decompress archived files under DOS/Windows. Use LHarc, by Kazuaki Ishizaki, to decompress files under MacOS.
LZ, LZW
Lempel-Ziv-Welch document compression/decompression (codec) format which supports a compression ratio of 2 bytes to 1 (2:1). In the document, redundant strings of ASCII code, such as those representing the words: the, then, they, those, etc., are each replaced with a single, unique, fixed-length code.
To keep track of codes, LZ encoding builds a special dynamic data dictionary based on the actual frequency of occurence of characters in the data stream making these algorithms ideal for networks supporting a mix of data that changes over time. A larger dictionary usually results in more effective compression, but also requires the use of more RAM capacity by the compression device. However, smaller dictionaries can impose more latency, especially as the incoming data rate rises.
Some LZ algorithms have been designed to support "packetized" LAN and WAN network communications with continuous mode and packet mode compressions. Continuous mode compressions creates and maintains a data dictionary from the continuous stream of characters crossing packet boundaries.
The dictionary must be synchronized via a reliable data link protocol such as reliable-mode PPP or X.25. Compression ratios are large because of the dictionary's larger data sampling. Packet mode compressions creates a data dictionary for each packet. Synchronization is handled by the existing network transport layer protocol. Compression ratios are smaller than under Continuous mode; RAM requirements are also smaller because it is flushed after each packet, rather than maintained.
Encoding slows down (latency increases) as packet rates increase. Networked LZ compression ratios are generally in the 4:1 range for text and fax. Voice data can be compressed to 8:1 or higher using a parameteric or similar algorithm that modifies the human voice tract.
References are samples only. Each one is presented in greater detail in the
Technical Research Assistant for 2001
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