Compression artifact
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A compression artifact (or artefact) is the result of an aggressive data compression scheme applied to an image, audio, or video that discards some data that may be too complex to store in the available data-rate, or may have been incorrectly determined by an algorithm to be of little subjective importance, but is in fact objectionable to the user. Artifacts are often a result of the latent errors inherent in lossy data compression.
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.
Compression artifacts occur in many common media such as DVDs, common computer file formats such as JPEG, MP3, or MPEG files, and Sony's ATRAC compression algorithm. Uncompressed media (such as on Laserdiscs, Audio CDs, and WAV files) or losslessly compressed media (FLAC, PNG, etc.) do not suffer from compression artifacts.
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[edit] Compression artifacts in picture coding
When using quantization with block-based coding, as in JPEG-compressed images, several types of artifacts can appear, including contouring and posterizing in otherwise smooth gradients, staircase noise along curving edges, "mosquito noise" around edges, and/or blockiness in "busy" regions (sometimes called quilting or checkerboarding).
Where 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."
[edit] Image artifact reduction
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.
[edit] Compression artifacts in audio coding
Lossy audio compression typically works with a psychoacoustic model, using perceptual coding. Lossy audio formats typically involve a time/frequency domain transform, such as an MDCT. 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 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 fails, 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, drop-outs, 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 (eg 92 kbit/sec) MP3. In general, music has repeating structures and more predicable variations in volume, whereas applause is essentially random, therefore hard to compress. This highly compressed track of applause will illustrate the "metallic ringing" and other compression artifacts very well.
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[edit] See also
[edit] External links
- JPEG Tutor, an interactive applet allowing you to investigate the effects of changing the quantization matrix.
- JPEG deringing and deblocking: Matlab software and Photoshop plug-in
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