- Homomorphic filtering
Homomorphic filtering is a generalized technique for signal and image processing, involving a nonlinear mapping to a different domain in which linear filter techniques are applied, followed by mapping back to the original domain. This concept was developed in the 1960s by
Thomas Stockham, Alan V. Oppenheim, and Ronald W. Schaferat MIT.
Homomorphic filter is sometimes used for
image enhancement. It simultaneously normalizes the brightness across an image and increases contrast. Here homomorphic filtering is used to remove multiplicative noise. Illumination and reflectance are not separable, but their approximate locations in the frequency domain may be located. Since illumination and reflectance combine multiplicatively, the components are made additive by taking the logarithmof the image intensity, so that these multiplicative components of the image can be separated linearly in the frequency domain. Illumination variations can be thought of as a multiplicative noise, and can be reduced by filtering in the log domain.
To make the illumination of an image more even, the high-frequency components are increased and low-frequency components are decreased, because the high-frequency components are assumed to represent mostly the reflectance in the scene (the amount of light reflected off the object in the scene), whereas the low-frequency components are assumed to represent mostly the illumination in the scene. That is,
high-pass filtering is used to suppress low frequencies and amplify high frequencies, in the log-intensity domain. [cite book | url = http://books.google.com/books?id=sNEodegXy2MC&pg=PT1120&dq=%22homomorphic+filtering%22+stockham+oppenheim&as_brr=3&ei=sfriRo3QJqPIowLzvYXuAw&sig=Ut69Hq1Xnr8J_lTAclOxXZ295vI#PPT1120,M1 | title = Digital signal processing handbook | author = Douglas B. Williams and Vijay Madisetti | publisher = CRC Press | year = 1999 | isbn = 0849321352 ]
Audio and speech analysis
Homomorphic filtering is used in the log-spectral domain to separate filter effects from excitation effects, for example in the computation of the
cepstrumas a sound representation; enhancements in the log spectral domain can improve sound intelligibility, for example in hearing aids. [cite book | title = Readings in Speech Recognition | author = Alex Waibel and Kai-Fu Lee | isbn = 1558601244 | year = 1990 | publisher = Morgan Kaufmann | url = http://books.google.com/books?id=IJoOLVYD8KQC&pg=PA64&dq=%22homomorphic+filtering%22+stockham+oppenheim&as_brr=3&ei=sfriRo3QJqPIowLzvYXuAw&sig=UMPo33j2ppYSISVSgJXrM6-eP90 ]
* [http://www.cs.sfu.ca/~stella/papers/blairthesis/main/node35.html Overview of homomorphic filtering]
Wikimedia Foundation. 2010.
Look at other dictionaries:
Digital image processing — This article is about mathematical processing of digital images. For artistic processing of images, see Image editing. Paranal Observatory landscape after application of the city globe image technique. The Digital image processing is the use of… … Wikipedia
Image editing — For the uses, cultural impact, and ethical concerns of image editing, see Photo manipulation. For the process of culling and archiving images, see Digital asset management. A colorized version of originally black and white photo, colorized using… … Wikipedia
гомоморфная фильтрация — Метод нелинейной фильтрации, в основе которой лежит идея использования линейной фильтрации логарифма спектра сигнала (кепстра), полученного с помощью обратного преобразования Фурье. Удобство такого преобразования для анализа и синтеза речи… … Справочник технического переводчика
Cepstrum — Cepstral redirects here. For the software company based in Pennsylvania, see Cepstral (company). A cepstrum /ˈkɛps … Wikipedia