impression

n 1: a vague idea in which some confidence is placed; "his

impression of her was favorable"; "what are your

feelings about the crisis?"; "it strengthened my belief

in his sincerity" [syn: {feeling}, {belief}, {notion}]

2: an outward appearance; "he made a good impression"; "I

wanted to create an impression of success"; "she retained

that bold effect in her reproductions of the original

painting" [syn: {effect}]

3: a clear and telling mental image; "he described his mental

picture of his assailant"; "he had no clear picture of

himself or his world"; "the events left a permanent

impression in his mind" [syn: {mental picture}, {picture}]

4: a concavity in a surface produced by pressing; "he left the

impression of his fingers in the soft mud" [syn: {depression},

{imprint}]

5: a symbol that is the result of printing; "he put his stamp

on the envelope" [syn: {stamp}]

6: all the copies of a work printed at one time; "they ran off

an initial printing of 2000 copies" [syn: {printing}]

7: (dentistry) an imprint of the teeth and gums in wax or

plaster; "the dentist made an impression for use in

preparing an inlay"

8: an impressionistic portrayal of a person; "he did a funny

impression of a politician"

compression

1. (Or "compaction") The coding of data to save

storage space or transmission time. Although data is already

coded in digital form for computer processing, it can often be

coded more efficiently (using fewer bits). For example,

{run-length encoding} replaces strings of repeated characters

(or other units of data) with a single character and a count.

There are many compression {algorithms} and utilities.

Compressed data must be decompressed before it can be used.

The standard {Unix} compression utilty is called {compress}

though {GNU}'s {gzip} is better. Other compression utilties

include {pack}, {zip} and {PKZIP}.

When compressing several similar files, it is usually better

to join the files together into an {archive} of some kind

(using {tar} for example) and then compress them, rather than

to join together individually compressed files. This is

because some common compression {algorithm}s build up tables

based on the data from their current input which they have

already compressed. They then use this table to compress

subsequent data more efficiently.

See also {TIFF}, {JPEG}, {MPEG}, {Lempel-Ziv Welch}.

There is a compression {FAQ} at

{(ftp://rtfm.mit.edu/pub/usenet/news.answers/compression-faq/)}.

{Usenet} newsgroups: {news:comp.compression},

{news:comp.compression.research}.

2. Reducing the dynamic range of an audio signal,

making quiet sounds louder and loud sounds quieter. Thus,

when discussing digital audio, the preferred term for reducing

the total amount of data is "compaction". Some advocate this

term in all contexts.

(1999-10-22)