Color reproduction system

• US 4,500,919 A
• Filed: 05/04/1982
• Issued: 02/19/1985
• Est. Priority Date: 05/04/1982
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Schreiber, William F.
• Primary Examiner(s): Masinick, Michael A.
• Application Number:
  US06/374,804
• Time in Patent Office: 1,022 Days
• Field of Search: 358/75-80
• US Class Current: 358/518
• CPC Class Codes: H04N 1/6005 with luminance or chrominance signals, e.g. LC1C2, HSL or YUV
• CPC Class Codes: H04N 1/6011 with simulation on a subsidiary picture reproducer H04N1/622 takes precedence; matching two or more picture reproducers H04N1/6052
• CPC Class Codes: H04N 1/6075 Corrections to the hue

Abstract

A system for reproducing a color original in a medium using a selected multiplicity of reproduction colorants, comprising: a scanner for producing from said color original a train of appearance signals dependent on at least three color values of said original; aesthetic correction circuitry for introducing aesthetically desired alteration into said appearance signals to produce modified appearance signals; and colorant selection mechanism for receiving said modified appearance signals and for selecting corresponding reproduction signals representing values of said reproduction colorants to produce in said medium a color match of said modified appearance signals.

Claims

1. A system for reproducing a color original in a medium using a selected multiplicity of reproduction colorants, the system comprising in serial order:

a. a scanner for producing from said color original a set of three tristimulus appearance signals dependent on the colors in said original;

b. display means connected to the scanner for receiving the appearance signals and aesthetic correction circuitry for interactively introducing aesthetically desired alterations into said appearance signals to produce modified appearance signals; and

c. colorant selection mechanism for receiving said modified appearance signals and for selecting corresponding reproduction signals representing values of said reproduction colorants to produce in said medium a colorimetrically-matched reproduction.

2. The system of claim 1 wherein said appearance signals are the RGB tristimulus values of said original.

3. The system of claim 1 further comprising transformation circuitry in said colorant selection means for transforming said modified appearance signals into corresponding color densities for use in selecting said reproduction signals.

4. The system of claim 1 wherein the aesthetic correction circuitry comprises:

a. storage means for storing the appearance signals from the scanner;

b. modifying means independent of the storage means for modifying the appearance signals from the scanner;

c. translation means connected to the modifying means and the storage means for translating the stored appearance signals into aesthetically modified signals upon instruction from the modifying means, and

wherein the display means comprises a television display device connected to the modifying means for displaying the interactively modified appearance signals, the display means having three phosphor light sources, the intensities of which are directly proportional to the appearance signals as modified.

5. The system of claim 4 wherein the aesthetic correction circuitry further comprises an excessive gamut alarm for indicating when a modified appearance signal can not be reproduced by the reproduction colorants.

6. The system of claim 4 where in the translation means further comprises a first color balance means for altering the color values of the appearance signals independently to simulate a different exposure of the original.

7. The system of claim 6 wherein the color balance means further comprises a plurality of tone scale memories which incorporate film transfer characteristics for the separate color values.

8. The system of claim 4 wherein the translation means further comprises a gradation control means for altering the color values of the appearance signals independently to simulate a different contrast and brightness in the original.

9. The system of claim 8 wherein the gradation control means further comprises a plurality of tone scale memories which incorporate brightness and contrast characteristics for the separate color values.

10. The system of claim 4 wherein the translation means further comprises:

a. translator means for translating color values of appearance signals into luminance-chrominance values; and

b. second color balance means for altering the chrominance values to adjust independently highlights, midtones and shadows in the original without affecting the gradation.

11. The system of claim 4 wherein the translation means further comprises:

a. translator means for translating color values of appearance signals into luminance-chrominance values;

b. polar conversion means for converting cartesian values of chromiance to polar values of hue and saturation;

c. first color correction means for interactively modifying the luminance, hue, and saturation of individual image samples independently and selectively in a multiplicity of hue bands; and

d. cartesian conversion means for reconverting the modified polar hue and saturation values into cartesian chrominance values.

12. The system of claim 11 wherein the translation means further comprises a second color correction means for interactively modifying the luminance, hue, and saturation of individual image samples selectively within a chromatic neighborhood of adjustable hue and saturation ranges.

13. The system of claim 6, 7, 8, 9, 10, 11, or 12 wherein the system is divided into a first part comprising a high speed data path in which digital video signals are modified by interaction with the contents of a plurality of random access memories addressed by said digital video signals, and a second part in which the contents of said memories are calculated in response to operator-initiated control settings, at a rate fast enough to give an essentially continuous visual display, but at a rate slower than the data rate in the said first part.

14. The system of claim 1 wherein the colorant selection means further comprises:

a. first processing means to convert the appearance signals into first signals representative of the densities of idealized inks which each absorb light more or less in only one color band;

b. second processing means to convert said first signals into second signals representative of densities of idealized inks, each of which absorbs light in all color bands in approximately the same degree as the real inks to be used for reproduction; and

c. third processing means comprising a lookup table for converting said second signals into colorant values so that the combination of colorants forms a colorimetric match with said appearance signals.

15. The system of claim 14 wherein the system further comprises under-color removal circuitry comprising:

a. fourth processing means to calculate black colorant values as a function of the appearance signals; and

b. fifth processing means to calculate signals representative of reduced idealized ink densities, each absorbing in only one color band, which combination of reduced inks together with a selected amount of black, form a color which approximately matches that of the combination of idealized inks before reduction.

16. The system of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14 or 15 wherein the medium is a gravure printing system.

17. The system of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14 or 15 wherein the medium is a letterpress printing system.

18. The system of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14 or 15 wherein the medium is an offset printing system.

19. In a color reproduction system in which a lookup table is used to calculate the desired amount of colorant, the method of finding the entries of said table comprising the following steps:

a. printing a multiplicity of combination of amounts of colorants spanning the full range of such physically realizable combinations;

b. measuring the appearance values of all the combinations using a colorimetric and calculating the corresponding lookup table inputs;

c. choosing a trial set of entries of said table on the basis of the results of steps (a) and (b);

d. printing color patches corresponding to each of said entries, measuring their appearance values with a colorimeter, and calculating the corresponding lookup table inputs;

e. determining the lookup table outputs corresponding to the said inputs;

f. printing the colorants of step (e), measuring the corresponding appearance values, and calculating the corresponding lookup table inputs;

g. on the basis of the results of steps (d) through (f), adjusting the lookup table entries in such a direction as to reduce the discrepancy between successive sets of lookup table inputs; and

h. repeating the process until the discrepancy between results on successive iterations is acceptably small.

20. A method for reproducing a color original using a selected multiplicity of reproduction colorants, the method comprising:

(a) scanning said original to produce a set of three tristimulus appearance signals dependent on the colors in said original;

(b) surrounding and illuminating the original with a reference white border;

(c) displaying said appearance signals as an image on a television display device having three phosphor light sources, the intensities of which are directly proportional to the appearance signals, the display device also comprising an illuminated reference white element;

(d) modifying the appearance signals until the displayed image is a colorimetric match of the reference white illuminated original;

(e) translating the modified appearance signals into colorant value signals;

(f) selecting the amounts of particular colorants in response to the colorant value signals; and

(g) using said colorants to form a reproduction.

21. In a color reproduction system in which a lookup table is used to calculate the desired amount of colorant, the method of finding the entries of said table comprising the following steps:

a. printing a plurality of colorant combinations with varying amounts of colorants spanning the full range of physically realizable combinations;

b. measuring the appearance values of each of said combinations using a colorimeter and calculating a corresponding lookup table input value;

c. choosing a set of initial entries for said table on the basis of the results of steps (a) and (b);

d. determining the lookup table outputs corresponding to the said inputs; and

e. filling said table with additional input and output values by interpolating between said input and output values of said initial entries.