Method of and apparatus for establishing a reference point on a gradation correction curve
First Claim
1. A method of establishing a reference point on a gradation correction curve employed in gradation correction of image data, said method comprising the steps of:
- (a) preparing image data representing density values of a plurality of basic colors with respect to each pixel in an original;
(b) averaging said density values of respective basic colors with respect to each pixel to obtain an average density DM of each pixel;
(c) generating a histogram representing average density distribution in the form of a relation between said average density DM and a number of pixels Np ;
(d) generating a cumulative histogram representing said average density distribution in the form of a relation between said average density DM and a cumulative number of pixels RN;
(e) finding a reference average density DMrf corresponding to a prescribed value of a reference cumulative number of pixels RNrf on said cumulative histogram;
(f) selecting one of two terminal density values DMn at which said cumulative histogram saturates and which is closer to said reference average density;
(g) finding a set of pixels having said average density in a range essentially between said reference average density DMrf and a selected terminal density value DMm ;
(h) averaging said density values of said plurality of basic colors for said set of pixels with respect to each of said plurality of basic colors, to thereby obtain reference color density values Drf for respective basic colors; and
(i) establishing said reference point on the basis of said reference color density values Drf.
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Abstract
A highlight point (HLR, HLG, HLB) and/or a shadow point (SDR, SDG, SDB) on a gradation correction curve are established in consideration of decoloration and color fog of an original. Density values of respective primary colors are obtained for each pixel on an original. Average density is computed for each pixel and a cumulative histogram of average density distribution is generated. Temporary reference density values corresponding to prescribed cumulative frequencies are found on the cumulative histogram. Reference color density values (DRH, DGH, DBH) are computed by averaging the density of each color for the pixels having average density between the temporary reference density and terminal density on the cumulative histogram. The highlight and shadow points are specified with one of the reference color density values.
64 Citations
15 Claims
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1. A method of establishing a reference point on a gradation correction curve employed in gradation correction of image data, said method comprising the steps of:
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(a) preparing image data representing density values of a plurality of basic colors with respect to each pixel in an original; (b) averaging said density values of respective basic colors with respect to each pixel to obtain an average density DM of each pixel; (c) generating a histogram representing average density distribution in the form of a relation between said average density DM and a number of pixels Np ; (d) generating a cumulative histogram representing said average density distribution in the form of a relation between said average density DM and a cumulative number of pixels RN; (e) finding a reference average density DMrf corresponding to a prescribed value of a reference cumulative number of pixels RNrf on said cumulative histogram; (f) selecting one of two terminal density values DMn at which said cumulative histogram saturates and which is closer to said reference average density; (g) finding a set of pixels having said average density in a range essentially between said reference average density DMrf and a selected terminal density value DMm ; (h) averaging said density values of said plurality of basic colors for said set of pixels with respect to each of said plurality of basic colors, to thereby obtain reference color density values Drf for respective basic colors; and (i) establishing said reference point on the basis of said reference color density values Drf. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. An apparatus for establishing a reference point on a gradation correction curve employed in gradation correction of image data, said apparatus comprising:
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(a) means for preparing image data representing density values of a plurality of basic colors with respect to each pixel in an original; (b) means for averaging said density values of respective basic colors with respect to each pixel to obtain an average density DM of each pixel; (c) means for generating a histogram representing average density distribution in the form of a relation between said average density DM and a number of pixels Np; (d) means for generating a cumulative histogram representing said average density in the form of a relation between said average density DM and a cumulative number of pixels RN; (e) means for finding a reference average density DMrf corresponding to a prescribed value of a reference cumulative number of pixels RNrf on said cumulative histogram; (f) means for selecting one of two terminal density values DMm at which said cumulative histogram saturates and which is closer to said reference average density; (g) means for finding a set of pixels having said average density in a range essentially between said reference average density DMrf and a selected terminal density value DMm ; (h) means for averaging said density values of said plurality of basic colors for said set of pixels with respect to each of said plurality of basic colors to thereby obtain reference color density values Drf for respective basic colors; and (i) means for establishing said reference point on the basis of said reference color density values Drf. - View Dependent Claims (10)
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- 11. An apparatus in accordance with claim wherein said gradation correction curve is generated for each of said plurality of basic colors.
Specification