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In signal processing, particularly digital image processing, ringing artifacts are artifacts that appear as spurious signals near sharp transitions in a signal. Visually, they appear as bands or "ghosts" near edges; audibly, they appear as "echos" near transients, particularly sounds from percussion instruments; most noticeable are the pre-echos. The term "ringing" is because the output signal oscillates at a fading rate around a sharp transition in the input, similar to a bell after being struck. As with other artifacts, their minimization is a criterion in filter design. == Introduction == The main cause of ringing artifacts is due to a signal being bandlimited (specifically, not having high frequencies) or passed through a low-pass filter; this is the frequency domain description. In terms of the time domain, the cause of this type of ringing is the ripples in the sinc function,〔, section I.6, Enhancement: Frequency Domain Techniques, (p. 16 )〕 which is the impulse response (time domain representation) of a perfect low-pass filter. Mathematically, this is called the Gibbs phenomenon. One may distinguish overshoot (and undershoot), which occurs when transitions are accentuated – the output is higher than the input – from ringing, where ''after'' an overshoot, the signal overcorrects and is now below the target value; these phenomena often occur together, and are thus often conflated and jointly referred to as "ringing". The term "ringing" is most often used for ripples in the ''time'' domain, though it is also sometimes used for ''frequency'' domain effects:〔(Digital Signal Processing ), by J.S.Chitode, Technical Publications, 2008, ISBN 978-81-8431-346-8, (4 - 70 )〕 windowing a filter in the time domain by a rectangular function causes ripples in the ''frequency'' domain for the same reason as a brick-wall low pass filter (rectangular function in the ''frequency'' domain) causes ripples in the ''time'' domain, in each case the Fourier transform of the rectangular function being the sinc function. There are related artifacts caused by other frequency domain effects, and similar artifacts due to unrelated causes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ringing artifacts」の詳細全文を読む スポンサード リンク
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