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14-September-2008 18:02:38 - Compression artifact Original image, with good color grade. Original image, with good color grade. Loss of edge clarity and tone fuzziness in heavy JPEG compression. Loss of edge clarity and tone fuzziness in heavy JPEG compression. This article or section is missing citations or needs footnotes. Using inline citations helps guard against copyright violations and factual inaccuracies. September 2007 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. Contents 1 Compression artifacts in picture coding 1.1 Image artifact reduction 2 Compression artifacts in audio coding 3 See also 4 External links Compression artifacts in picture coding This image shows the accentuated difference between a jpg-saved image and the original. Note especially the changes apparent on sharp edges. This image shows the accentuated difference between a jpg-saved image and the original. Note especially the changes apparent on sharp edges. 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. 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 ing programs, for instance, have proprietary JPEG artifact reduction algorithms built-in. 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 96 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. Please help improve this section by expanding it. Further information might be found on the talk page or at requests for expansion. June 2008 See also Transparency data compression Lossy data compression JPEG JPEG 2000 Generation loss Artifact error 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 v d e Data compression methods Lossless Theory Entropy · Complexity · Redundancy Entropy encoding Huffman · Adaptive Huffman · Arithmetic Shannon-Fano · Range · Golomb · Exp-Golomb · Universal Elias · Fibonacci Dictionary RLE · DEFLATE · LZ Family LZ77/78 · LZSS · LZW · LZWL · LZO · LZMA · LZX · LZJB · LZT Others CTW · BWT · PPM · DMC Audio Theory Convolution · Sampling · Nyquist-Shannon theorem Audio codec parts LPC LAR · LSP · WLPC · CELP · ACELP · A-law · μ-law · MDCT · Fourier transform · Psychoacoustic model Others Dynamic range compression · Speech compression · Sub-band coding Image Terms Color space · Pixel · Chroma subsampling · Compression artifact Methods RLE · Fractal · Wavelet · EZW · SPIHT · DCT · KLT Others Bit rate · Test images · PSNR quality measure · Quantization Video Terms Video Characteristics · Frame · Frame types · Video quality Video codec parts Motion compensation · DCT · Quantization Others Video codecs · Rate distortion theory CBR · ABR · VBR Timeline of information theory, data compression, and error-correcting codes See Compression Formats and Standards for formats and Compression Software Implementations for codecs Retrieved from http://en..org/wiki/Compression_artifact Categories: Audio engineering | Computer graphic artifacts | Data compressionHidden categories: Articles with statements since September 2007 | All articles with statements | Articles to be expanded since June 2008 | All articles to be expanded Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Dansk Deutsch Italiano Magyar МакедонÑ?ки ‪Norsk bokmÃ¥l‬ Português РуÑ?Ñ?кий Svenska This page was last modified on 15 August 2008, at 06:4
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