|
A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy data compression. Lossy data compression involves discarding some of the media's data so that it becomes simplified enough to be stored within the desired disk space or be transmitted (or streamed) within the bandwidth limitations (known as a data rate or bit rate for media that is streamed). If the compressor could not reproduce enough data in the compressed version to reproduce the original, the result is a diminishing of quality, or introduction of artifacts. Alternatively, the compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the viewer. Compression artifacts occur in many common media such as DVDs, common computer file formats such as JPEG, MP3, or MPEG files, and some alternatives to the compact disc, such as Sony's MiniDisc format. Uncompressed media (such as on Laserdiscs, Audio CDs, and WAV files) or losslessly compressed media (such as FLAC or PNG) do not suffer from compression artifacts. The minimization of perceivable artifacts is a key goal in implementing a lossy compression algorithm. However, artifacts are occasionally ''intentionally'' produced for artistic purposes, a style known as glitch art or datamoshing. Technically speaking, a compression artifact is a particular class of data error that is usually the consequence of quantization in lossy data compression. Where transform coding is used, they typically assume the form of one of the basis functions of the coder's transform space. == Images == When performing block-based coding for quantization, as in JPEG-compressed images, several types of artifacts can appear. * Ringing * Contouring * Posterizing * Staircase noise (aliasing) along curving edges * Blockiness in "busy" regions (block boundary artifacts, sometimes called macroblocking, quilting, or checkerboarding) Other lossy algorithms, which use pattern matching to deduplicate similar symbols, are prone to introducing hard to detect errors in printed text. For example the numbers "6" and "8" may get replaced. This has been observed to happen with JBIG2 in certain photocopier machines.〔(【引用サイトリンク】title=Xerox scanners/photocopiers randomly alter numbers in scanned documents )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「compression artifact」の詳細全文を読む スポンサード リンク
|