A smooth portion is efficiently extracted from an image. There is provided
an entropy filter in which a noted pixel (x, y) is determined in an
original image, said image is partitioned by a window of size (width,
height)=(A, B) from the noted pixel, entropy of the partitioned window is
calculated, and obtained entropy value is saved on the coordinates (x, y)
of a resulting image. Here, in a noted pixel (x, y), x is ranged from 0
to (an image width minus a window width) and y is ranged from 0 to (an
image height minus a window height). By using an entropy filter, a smooth
portion in image quality is extracted from an original image. A preferred
example of the smooth portion is a cell nucleus.