Method of and apparatus for establishing highlight and shadow densities

DETAILED DESCRIPTION The present invention is directed to a method of and an apparatus for establishing a gradation control density X.
sub.
c employed as a highlight density or a shadow density on a gradation correction curve F(X) used in gradation correction of image data expressing density of each pixel in an image.
The method comprises the steps of: (1) on the basis of the image data, generating a statistical curve Y=S(X) expressing a cumulative density distribution on the image, where the statistical curve Y=S(X) is defined on a X-Y coordinate plane having an X-axis representing density levels and a Y-axis representing a cumulative number of pixels in an arbitrary scale; (2) providing a threshold density X.
sub.
TH on the X-axis; (3) finding a terminal value Y.
sub.
m on the Y-axis at which the statistical curve Y=S(X) is terminated; (4) providing a first reference value Y.
sub.
1R on the Y-axis, (5) finding a terminal density X.
sub.
m on the X-axis at which the statistical curve Y=S(X) reaches the terminal value Y.
sub.
m ; (6) converting the first reference value Y.
sub.
1R through the statistical curve Y=S(X), to thereby obtain a first reference density X.
sub.
1R ; (7) comparing the first reference density X.
sub.
1R with the threshold density X.
sub.
TH, to thereby designate the statistical curve Y=S(X) as a first type corresponding to an extraordinarily-bright image or an extraordinarily-dark image when the first reference density X.
sub.
1R is farther from the terminal density X.
sub.
m than the threshold density X.
sub.
TH is, or as a second type corresponding to an ordinary image other than said extraordinarily-bright image and said extraordinarily-dark image when the first reference density X.
sub.
1R is closer to the terminal density X.
sub.
m than the threshold density X.
sub.
TH is; (8) selecting a first procedure for setting the gradation control density X.
sub.
c at a terminal reference density X.
sub.
TR predetermined between the terminal density X.
sub.
m and the first reference density X.
sub.
1R when the statistical curve Y=S(X) is the first type; or selecting a second procedure other than the first procedure for setting the gradation control density X