Research On The Median Filtering Algorithm For Suppressing High-ratio Impulse Noise In Digital Images

Digital images are often corrupted by the impulse noise in the image acquisitions and the image transmission progress because of the sensor noise and channel noise. This may result in difficulties in the subsequent image processing, such as edge detection, image segmentation and object recognition, therefore, detection and removal of the noise are inevitable in the image processing. The Gauss noise and impulse noise are the two most commonly used noise models. This thesis is mainly concerned in the development on the filtering algorithms for the impulse noise.Median filter and its improved algorithms are normally considered to have good performances on removing impulse noise. After the corrupted images are filtered by the median filter, the smooth areas in the filtered images will contain no or little noise. However, the existing median filtering algorithms normally have poor performances on protecting the image details. Especially these algorithms may cause serious distortion in the images which are highly corrupted by the impulse noise.In this thesis two improved median filtering algorithms are presented to remove high-ratio impulse noise. One filter (named HSADMF) is focused on filtering color images highly corrupted by the salt-and-pepper impulse noise. This algorithm firstly detects and processes the salt-and-pepper impulse noise using point to point method according to the characteristics of this noise. Then HSADMF takes full use of the oriental image information to filter the corrupted one. The performances of HSADMF on filtering gray and color images are compared with those of some classic filters. The comparison shows that HSADMF has better performances on detail protection and higher restoration ratio. The other filter (named SADMF-VMF) is focused on filtering color images highly corrupted by the random impulse noise. SADMF-VMF can be divided into two steps to remove random impulse noise, firstly it used HSADMF to remove salt and pepper noise which put a greater impact on the quality of the image, secondly it suppress other corrupted pixels by VMF. The performances of SADMF-VMF on filtering color images are compared with those of some classic filters. The comparison shows that SADMF-VMF has better performances on detail protection and higher restoration ratio.