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 (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 which may be 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[1] or datamoshing.[2]
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.
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When performing block-based coding for quantization, as in JPEG-compressed images, several types of artifacts can appear.
Various approaches have been proposed to reduce the effects of image compression, but in order to utilize standardized compression/decompression techniques and to retain the benefits of the compression (for instance, lower transmission and storage costs), many of these methods have focused on "post-processing"—that is, processing the images when they are received or viewed. 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, including Paint Shop Pro1.
When motion prediction is used, as in MPEG-1, MPEG-2 or MPEG-4, compression artifacts tend to remain on several generations of decompressed frames, and move with the optic flow of the image, leading to a peculiar effect, part way between a painting effect and "grime" that moves with objects in the scene.
Data errors in the compressed bit-stream can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to "break-up" of the picture. 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. In MPEG picture coding, these are known as "I-frames", with the 'I' standing for "intra".
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| Static illustration of edge busyness;[3] in video these artifacts vary, looking like swarming mosquitoes. | |
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.[3][4]
Lossy audio compression typically works with a psychoacoustic model—a model of human hearing perception. Lossy audio formats typically involve the use of a time/frequency domain transform, such as a modified discrete cosine transform. With the psychoacoustic model, masking effects such as frequency masking and temporal masking are exploited, so that sounds that should be imperceptible are not recorded. For example, in general, human beings are unable to perceive a quiet tone played simultaneously with a similar but louder tone. A lossy compression technique might identify this quiet tone and attempt to remove it. Also, quantization noise can be "hidden" where they would be masked by more prominent sounds. With low compression, a conservative psy-model is used with small block sizes.
When the psychoacoustic model is inaccurate, when the transform block size is restrained, or when aggressive compression is used, this may result in compression artifacts. Compression artifacts in compressed audio typically show up as ringing, pre-echo, "birdie artifacts", drop-outs, rattling, warbling, metallic ringing, an underwater feeling, hissing, or "graininess".
A good way to observe compression artifacts in audio is to listen to the applause in a relatively highly compressed audio file (e.g. 96 kbit/sec MP3). In general, musical tones have repeating waveforms and more predictable variations in volume, whereas applause is essentially random, therefore hard to compress. A highly compressed track of applause will illustrate the "metallic ringing" and other compression artifacts very well.
Compression artifacts may intentionally be used as a visual style, sometimes known as glitch art.
In still images, an example is Jpegs by German photographer Thomas Ruff,[5][6] which uses intentional JPEG artifacts as the basis of the picture's style.
In video art, one technique is datamoshing, where two videos are interleaved so intermediate frames are interpolated from two separate sources. Another technique involves simply transcoding from one lossy video format to another, which exploits the difference in the how the separate video codecs process motion and color information.[7] The technique was pioneered by artists 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 Kanye West.[8][9]
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