Color difference judgment apparatus and method

• US 20040101195A1
• Filed: 11/12/2003
• Published: 05/27/2004
• Est. Priority Date: 11/27/2002
• Status: Active Grant

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Inventors: Akaishi, Takahisa
• Granted Patent: US 7,447,350 B2
• Time in Patent Office: Days
• US Class Current: 382/162
• CPC Class Codes: G06T 2207/10008 from scanner, fax or copier
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 5/77 Retouching; Inpainting; Scratch removal
• CPC Class Codes: H04N 1/58 Edge or detail enhancement; Noise or error suppression, e.g. colour misregistration correction H04N1/62 takes precedence
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

The present invention is related to a color difference judgment apparatus for judging a color difference between a first color and a second color in RGB color space. The method is comprising: distance calculating means for calculating distance between the first and second colors in RGB color space; threshold-value candidate storage means for storing a plurality of threshold-value candidates; threshold-value selecting means for selecting a threshold value from among the plurality of threshold-value candidates that have been stored in the threshold-value candidate storage means; and comparing decision means for comparing the distance calculated by the distance calculating means and the threshold value selected by the threshold-value selecting means and deciding that the color difference between the first and second colors lies within an allowable range if the distance is less than the threshold value.

Claims

1. A color difference judgment apparatus for judging a color difference between a first color and a second color in RGB color space, comprising:

distance calculating means for calculating distance between the first and second colors in RGB color space;

threshold-value candidate storage means for storing a plurality of threshold-value candidates;

threshold-value selecting means for selecting a threshold value from among the plurality of threshold-value candidates that have been stored in said threshold-value candidate storage means; and

comparing decision means for comparing the distance calculated by said distance calculating means and the threshold value selected by said threshold-value selecting means and deciding that the color difference between the first and second colors lies within an allowable range if said distance is equal to or less than the threshold value.

2. The apparatus according to claim 1, wherein if RGB values of the first color are (R1, G1, B1) and RGB values of the second color are (R2, G2, B2), said distance calculating means calculates a value, which is obtained by (R1−

R2)²+(G1−

G2)²+(B1−

B2)², as said distance.

3. The apparatus according to claim 1, wherein if RGB values of the first color are (R1, G1, B1) and RGB values of the second color are (R2, G2, B2), said distance calculating means calculates a value, which is obtained by [(R1−

R2)²+(G1−

G2)²+(B1−

B2)²]^1/2, as said distance.

4. The apparatus according to claim 1, wherein said threshold-value selecting means selects a threshold value of a partial space to which the first color or the second color belongs from the plurality of threshold-value candidates that have been stored in said threshold-value selecting means.

5. The apparatus according to claim 1, wherein said threshold-value candidate storage means stores the threshold-value candidates as a three-dimensional look-up table;

and

said threshold-value selecting means selects the threshold value uniquely upon referring to the three-dimensional look-up table based upon RGB values of whichever of the colors serves as a reference.

6. The apparatus according to claim 5, wherein said threshold-value candidate storage means stores a threshold-value candidate for each of N³-number of partial spaces obtained by dividing each of R, G, B axes of RGB color space into N equal segments.

7. The apparatus according to claim 6, wherein the value of N is a power of 2 and high-speed calculation is made possible by using a bit shift to calculate an address of the three-dimensional look-up table.

8. The apparatus according to claim 1, wherein said distance calculating means calculates a distance in RGB color space of the same extent as a color difference, which is based upon a CIE1994 color difference formula, between the first and second colors in CIELab color space.

9. The apparatus according to claim 1, wherein said distance calculating means calculates a distance in RGB color space of the same extent as a color difference between the first and second colors in CIELab color space.

10. A color difference judgment method for judging a color difference between a first color and a second color in RGB color space, comprising the steps of:

calculating distance between the first and second colors in RGB color space;

selecting a threshold value from among a plurality of threshold-value candidates;

comparing the calculated distance and the threshold value selected; and

deciding that the color difference between the first and second colors lies within an allowable range if the distance is equal to or less than the threshold value.

11. A program for causing a computer to judge a color difference between a first color and a second color in RGB color space, comprising:

a procedure for calculating distance between the first and second colors in RGB color space;

a procedure for selecting a threshold value from among a plurality of threshold-value candidates;

a procedure for comparing the calculated distance and the threshold value selected; and

a procedure for deciding that the color difference between the first and second colors lies within an allowable range if said distance is equal to or less than the threshold value.

12. A computer-readable recording medium storing the program set forth in claim 11.

13. A color difference judgment method for judging a color difference between a first color and a second color in a color space that is not a uniform color space, comprising the steps of:

calculating distance between the first and second colors in the color space that is not a uniform color space;

selecting a prescribed threshold value from a database in which a threshold value, which indicates an allowable range regarded as an approximately equal color difference in uniform color space, has been stored for every coordinate of a prescribed interval in the color space that is not the uniform color space;

comparing the calculated distance and the threshold value selected; and

deciding that the color difference between the first and second colors lies within an allowable range if said distance is equal to or less than the threshold value.