1. A computer-implemented image signal processing method for converting an input image signal denoting a first two-dimensional image into an output image signal denoting a second two-dimensional image which is visually enhanced relative to the first two-dimensional image, said input image signal containing plural pixel signals each defining a relative image density of a corresponding pixel of the first two-dimensional image, said method comprising: a first step of storing in a memory the plural pixel signals contained by said input image signal; a second step of extracting, from among said plural pixels stored in the memory, a pixel as a target pixel and 4n pixels (n=1 or 2) located adjacent to said target pixel in 4.times.n radial directions of said first two-dimensional image each having an equal angular pitch of (360.degree./4n) with respect to said target pixel, a third step of subjecting the target pixel to an enhancement process by obtaining 4.times.n first differences for said 4.times.n radial directions, each of said 4.times.n first differences being a difference between said target pixel and each of said 4.times.n pixels, and then enhancing said target pixel with 4n enhancement factors, each of said 4n enhancement factors being defined as a product of said 4.times.n first differences and an enhancement coefficient which is determined according to an absolute value of a corresponding one of said 4.times.n first differences; a fourth step of subjecting the target pixel to a smoothing process by obtaining 4.times.n mean values for said 4.times.n pixels, each of said 4.times.n mean values being defined as a mean value of a pair of two pixels selected from among said 4.times.n pixels in such a manner that said two pixels are located in two radial directions of said two-dimensional image orthogonal to each of said 4.times.n radial directions, obtaining 4.times.n second differences for said 4.times.n radial differences, each of said 4.times.n second differences being a difference between said target pixel and each of said 4.times.n mean values, and then smoothing said target pixel with 4.times.n smoothing factors, each of said 4.times.n smoothing factors being defined as a product of one of said 4.times.n second differences and a corresponding one of said enhancement coefficients obtained in said enhancement process; a fifth step of repeating said second through fourth steps with respect to each remaining pixel from among said plural pixel signals stored in said memory so as to subject each said remaining pixel to said enhancement and smoothing processes to obtain corresponding enhanced and smoothed image pixel signals; a sixth step of accumulating the enhanced and smoothed image pixels and forming the output image signal containing the enhanced and smoothed image pixel signals each defining an image density of a corresponding pixel of the second two-dimensional image.