Compression artifact
If the compressor cannot store enough data in the compressed version, the result is a loss of quality, or introduction of artifacts.
The compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the user.
However, artifacts are occasionally intentionally produced for artistic purposes, a style known as glitch art[4] or datamoshing.
When performing block-based discrete cosine transform (DCT)[1] coding for quantization, as in JPEG-compressed images, several types of artifacts can appear.
Other lossy algorithms, which use pattern matching to deduplicate similar symbols, are prone to introducing hard to detect errors in printed text.
No post-processing technique has been shown to improve image quality in all cases; consequently, none has garnered widespread acceptance, though some have been implemented and are in use in proprietary systems.
Many photo editing programs, for instance, have proprietary JPEG artifact reduction algorithms built-in.
[10] Boundary artifact in JPEG can be turned into more pleasing "grains" not unlike those in high ISO photographic films.
Where gross errors have occurred in the bit-stream, decoders continue to apply updates to the damaged picture for a short interval, creating a "ghost image" effect, until receiving the next independently compressed frame.
Video compression artifacts include cumulative results of compression of the comprising still images, for instance ringing or other edge busyness in successive still images appear in sequence as a shimmering blur of dots around edges, called mosquito noise, as they resemble mosquitoes swarming around the object.
[14][15] The so-called "mosquito noise" is caused by the block-based discrete cosine transform (DCT) compression algorithm used in most video coding standards, such as the MPEG formats.
Lossy audio compression typically works with a psychoacoustic model—a model of human hearing perception.
In general, musical tones have repeating waveforms and more predictable variations in volume, whereas applause is essentially random, therefore hard to compress.
[19] The technique was pioneered by artists Bertrand Planes in collaboration with Christian Jacquemin in 2006 with DivXPrime,[20] Sven König, Takeshi Murata, Jacques Perconte and Paul B. Davis in collaboration with Paperrad, and more recently used by David OReilly and within music videos for Chairlift and by Nabil Elderkin in the "Welcome to Heartbreak" music video for Kanye West.
[21][22] There is also a genre of internet memes where often nonsensical images are purposefully heavily compressed sometimes multiple times for comedic effect.
