Method of and an apparatus for generating a normalizing curve

DETAILED DESCRIPTION The present invention is directed to a method of converting original color image signals D.
sub.
x into converted color image signals D.
sub.
Nx suitable for image processing in a color image processor.
The original color image signals D.
sub.
x represent optical densities of an original color image for each pixel and for respective color components.
According to the present invention, the method comprises the steps of: (a) specifying gray pixels on the original color image; (b) averaging the original color image signals D.
sub.
x for respective color components in each gray pixel to obtain an average signal D.
sub.
M for each gray pixel; (c) dividing a range of the average signal D.
sub.
M into a plurality of classes D.
sub.
Mi ; (d) classifying the average signal D.
sub.
M representative of the gray pixels into the plurality of classes D.
sub.
Mi ; (e) accumulating the original color signals D.
sub.
x for each color component in each class D.
sub.
Mi of the average signal D.
sub.
M to obtain accumulated density signal .
SIGMA.
D.
sub.
x for each color component; (f) generating a sum signal .
SIGMA.
(.
SIGMA.
D.
sub.
x).
sub.
M representative of a sum of the accumulated density signal .
SIGMA.
D.
sub.
x for each color component within a predetermined intermediate density section D.
sub.
Mk .
ltoreq.
D.
sub.
M .
ltoreq.
D.
sub.
Mh ; (g) counting number .
SIGMA.
N.
sub.
pn of pixels whose original color signals D.
sub.
x are included in the intermediate density section D.
sub.
Mk .
ltoreq.
D.
sub.
M .
ltoreq.
D.
sub.
Mh ; (h) dividing the sum signal .
SIGMA.
(.
SIGMA.
D.
sub.
x).
sub.
M for each color component by the number .
SIGMA.
N.
sub.
pn of pixels to obtain accumulated average signals M.
sub.
dx for respective color components; (i) averaging the accumulated average signals M.
sub.
dx for respective color components to obtain a gray average signal M.
sub.
da ; (j) generating difference signals M.
sub.
Dx =(M.
sub.
dx -M.
sub.
da) between the accumulated average signals M.
sub.
dx and the gray average signal M.
sub.
da ; (k) modifying a predetermined reference signal-conversion characteristic C as a function of the difference signals M