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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 which is determined by an algorithm to be of lesser importance to the overall content but which is nonetheless discernible and objectionable to the user. Artifacts in time-dependent data such as audio or video are often a result of the latent error 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.
Compression artifacts in picture coding
When using the Discrete Cosine Transform for block-based coding, as in JPEG-compressed images, several types of artifacts can appear, including contouring in smooth regions, staircase noise along curving edges, mosquito noise around edges, and/or checker boarding in "busy" regions.
Where predictive coding of motion pictures is used, as in MPEG-1, compression artifacts tend to remain on several generations of decompressed frames, leading to a "painting" effect being seen, as if the picture were being painted by an unseen artist's paint-brush.
Where motion prediction is used, as in MPEG-2 or MPEG-4, compression artifacts tend to 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.
Errors in the 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, it is not unknown for decoders to continue to apply 'painting' updates to the damaged picture, creating "ghost image" effects.
To stop the build-up of compression artifacts, most compression systems occasionally send an entire compressed frame without prediction or differencing, usually at the start of a shot and at regular intervals thereafter. In MPEG picture coding, these are known as "I-frames", with the 'I' standing for "intraframe compression."
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.Compression artifacts in audio coding
Compression artifacts in audio depend upon the lossy codec used. In MPEG audio (particularly MP3), artifacts typically present as a metallic ringing or warbling that in severe instances may be described as "underwater"-sounding. MP3 artifacts are usually more common in the higher frequency range (greater than 16 kHz) due to design deficiencies in the format.
Ogg Vorbis tends to produce artifacts that are described as sounding more analog, such as hissing, "graininess", reduced soundstage and in some instances pre-echo (this is commonly demonstrated with castanets over a silent background).
See also
External links
Audio engineering | Computer graphics | Data compression
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"Compression artifact"