In a four-dimensional image composed of time-varying three-dimensional images or an N-dimensional image produced with N (‰¥4) independent parameters as a base, a four-dimensional continuous domain or an N-dimensional continuous domain is detected. Moreover, a four-dimensional spatial filter or an N-dimensional spatial filter is convoluted to the four-dimensional image or N-dimensional image in order to convert image quality. When the four-dimensional image is four-dimensionally scanned or the N-dimensional image is N-dimensionally scanned, continuity centered on a pixel concerned that is being scanned is checked in a three-dimensional space or an N-1-dimensional space. At the same time, continuity is checked along a time axis or an N-dimensional axis. The same label number is assigned to continuous four-dimensional domains or N-dimensional domains. Moreover, the value of the pixel concerned is converted or processed based on the relationship between the value of the pixel concerned that is being scanned and the values of pixels contained in a neighboring local domain.
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