Color printing system usable for reproduction of computer-generated images

I claim:

1. A method for controlling production of black and cyan, magenta and yellow chromatic colors in reproduction of images, comprising the steps of supplying original image digital data in the form of groups of input data each of which defines R, G and B values variable within a certain range and respectively corresponding to intensity values of red, green and blue component colors of an individual pixel of an original image, and digitally processing said groups of input data to produce from each group of input data a corresponding group of output digital data for control of reproduction of an individual pixel of said original image in at least three output colors, said processing of said groups of input data including the steps of determining from said R, G and B values for each original image pixel a grey factor corresponding to whichever of said R, G and B values represents the least effective intensity, determining an output black value as a function of said grey factor, producing cyan, magneta and yellow color values which are such as to obtain a color reproduction of each original image pixel having a color other than grey, and producing cyan, magenta and yellow output chromatic values from said color values for use with said output black value in reproduction of each said original image pixel, said producing of said cyan, magenta and yellow color values including the steps of determining from said R, G and B values a saturation factor corresponding to whichever of said R, G and B values represents the greatest intensity, and thereafter determining each of said cyan, magenta and yellow output color values as a direct function of calibration values and said grey factor, said saturation factor and said R, G and B values for each original image pixel to produce a tone correction for each pixel.

2. A method as defined in claim 1, wherein said producing of said output color values includes the steps of determining a cyan output value primarily as an inverse function of said R value and additionally as a function of cyan correction terms including a first tone correction term developed from a comparison of said R value with said grey and saturation factors, determining a magenta output value primarily as an inverse function of said G value and additionally as a function of magenta correction terms including a second tone correction term developed from a comparison of said G value with said grey and saturation factors, and determining a yellow output value primarily as an inverse function of said B value and additionally as a function of yellow correction terms including a third tone correction term developed from a comparison of said R value with said grey and saturation factors.

3. A method as defined in claim 2, wherein look-up tables are used in developing said first, second and third tone correction terms.

4. A method as defined in claim 2, wherein said cyan correction terms include a cyan color correction term developed as a function of a comparison of said R value with said G and B values and with said grey and saturation factors, wherein said magneta correction terms include a magenta color correction term developed as a function of a comparison of said G value with said R and B values and with grey saturation factors, and wherein said yellow correction terms include a yellow color correction term developed as a function of a comparison of said B value with said G and R values and with said grey and saturation factors.

5. Apparatus for controlling production of black and cyan, magenta and yellow chromatic colors in reproduction of images, comprising means for supplying original image digital data in the form of groups of input data each of which defines R, G and B values variable within a certain range and respectively corresponding to intensity values of red, green and blue component colors of an individual pixel of an original image, and processing means for digitally processing said groups of input data to produce from each group of input data a corresponding group of output digital data for control of reproduction of an individual pixel of said original image in at least three output colors, said processing means including means for determining from said R, G and B values for each original image pixel a grey factor corresponding to whichever of said R, G and B values represents the least effective intensity, means for determining an output black value as a function of said grey factor, color value producing means for producing cyan, magenta and yellow color values which are such as to obtain a color reproduction of each original image pixel having a color other than grey, and output means for producing from said cyan, magenta and yellow color values cyan, magenta and yellow output color values for use with said output black value in reproduction of each said original image pixel, said color value producing means comprising means for determining from said R, G and B values a saturation factor corresponding to whichever of said R, G and B values represents the greatest intensity, and means for producing each of said cyan, magenta and yellow color values as a direct function of calibration values, said grey factor, said saturation factor and said R, G and B values for each original image pixel to obtain a tone correction for each pixel.

6. Apparatus as defined in claim 5, wherein said output color producing means includes means for determining a cyan output value primarily as an inverse function of said R value and additionally as a function of cyan correction terms including a first tone correction term developed from a comparison of said R value with said grey and saturation factors, means for determining a magenta output value primarily as an inverse function of said G value and additionally as a function of magenta correction terms including a second tone correction term developed from a comparison of said G value with said grey and saturation factors, and means for determining a yellow output value primarily as an inverse function of said B value and additionally as a function of yellow correction terms including a third tone correction term developed from a comparison of said R value with said grey and saturation factors.

7. Apparatus as defined in claim 6, wherein said output color producing means includes look-up tables for use in developing said first, second and third tone correction terms.

8. Apparatus as defined in claim 6, wherein said cyan correction terms include a cyan color correction term developed as a function of a comparison of said R value with said G and B values and with said grey and saturation factors, wherein said magneta correction terms include a magenta color correction term developed as a function of a comparison of said G value with said R and B values and with said grey and saturation factors, and wherein said yellow correction terms include a yellow color correction term developed as a function of a comparison of said B value with said G and R values and with said grey and saturation factors.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a color printing system and more particularly to a system which is usable for accurate and reliable reproduction of images generated by computers and displayed on color monitors. The system of this invention produces prints which have an accurate visual match to original images produced on a color monitor screen. The system is easy to use and makes possible the use of calibration procedures which directly and quickly converge to an accurate calibration.

2. Background of the Prior Art

Various methods have heretofore been used or proposed for reproducing images of a type which are generated by an artist on a color monitor screen of a computer using "paint" programs and the like. One method which has been used involves the sending of a digital image to a color film recorder to produce a transparency which is then placed on a conventional color separation scanner for processing of color separation half-tone films for making lithographic printing plates. This type of operation is cumbersome and more importantly, does not reliably produce an accurate color reproduction.

There have also been a number of proposals for converting red, green and blue or "RGB" values to cyan, magenta, yellow and black or "CMYK" values, ostensibly appropriate for use with a computer system. However, the conversion methods as proposed in the prior art have been quite complicated and cumbersome and are such that it would appear to be very difficult to obtain reliable results in attempting to convert computer-generated or similar signals to signals appropriate for color print reproductions.

SUMMARY OF THE INVENTION

This invention was evolved with the general object of providing a system for quick and accurate conversion of data such as supplied to a computer monitor screen into data for control of printing operations, to obtain accurate reproduction of an original image.

In accordance with the invention, a system is provided in which chromatic output values are derived from the values of the component colors of a pixel of an original image such as produced on a screen of a color monitor. An important feature is in the generation of tone correction values which are used in producing the chromatic output values and which are such as to provide a grey balance throughout a full range of greys. Preferably, and in accordance with an important feature of the invention, the tone correction values are determined through look up tables.

The system is such that an accurate grey balance is obtainable from color inks, without a black ink, and through a range which is quite wide. This feature facilitates the achievement of an accurate reproductions of colors through wide ranges of colors. Preferably, black is added to provide optimum contrast and otherwise facilitate an accurate reproduction of the original image. In accordance with the invention, black is added as a function of a grey factor corresponding to the screen color which has the lowest effective intensity value.

After obtaining an accurate grey balance, color correction systems are usable to provide accurate reproduction of colors of an original image. Color correction values are developed which preferably include scale values which operate in a linear fashion to obtain a balance which is close to the optimum balance. In addition, the color correction values include selective values which are added to obtain the optimum balance and highly accurate reproduction of colors. The selective values operate with respect to the screen and ink colors and are also preferably operative with respect to selected other colors such as light red, purple and brown.

Important features relate to the use of a view box for viewing a reproduction in side-by-side relation to a monitor screen under standard lighting conditions.

Additional important features relate to calibration procedures which are readily followed and which provide for a rapid convergence to an accurate calibration of the system and highly reliable reproductions of original images.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a color print reproduction system usable in the practice of the invention;

FIG. 2 illustrates a physical arrangement of a computer, monitor and view box for use in the practice of the invention;

FIG. 3 is a flow diagram illustrating processing operations performed in accordance with the invention; and

FIG. 4 is a schematic flow diagram illustrating basic operating procedures which are used in conjunction with the invention, also showing calibration procedures of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In FIG. 1, reference numeral 10 generally designates a system usable in the practice of the invention. An original image of a desired form is produced on the screen of a color monitor 11 using a computer 12, in a arrangement such as shown in FIG. 2 and as hereinafter described. Data as to the stored image, in the form of a group of red, green and blue pixel data, are then stored on a suitable storage medium, such as a magnetic tape, using a storage unit 13. As indicated by dotted line 14, the tape or other storage medium may then be physically transferred to a storage unit 15 which is then operated to transfer the data from the storage medium to the memory of a second computer 16. Computer 16 processes the red, green and blue pixel data and develops output data for use in reproduction of the image, the computer 16 being supplied with calibration data which define the relationship of characteristics of the monitor 11 and known or assumed characteristics of inks to be used in a half-tone reproduction of the original image.

The output data developed by the computer 16 includes data defining the black component and at least three color components for control of half-tone reproduction of the image. For example, the output data may control cyan, magenta and yellow inks, in addition to black ink. However, the invention is also usable for control of printing processes which use additional inks such as light cyan and pink inks, for example, and is not limited to control of four ink processes.

The output data produced by the computer 16 may be stored by a storage unit 17 on a suitable medium which, as indicated by dotted line 18, may then physically be transferred to another storage unit 19. Storage unit 19 is operative to read out the output data a standard type of color processing system 20 which is coupled to a half-tone film recorder 21. Films produced by the recorder 21 are then used to produce one or more reproductions, using a press 22 which may be either a production press or a proof press.

As indicated by dotted line 23, a reproduction produced by the press 22 may then be physically transferred to a view box 24 for side-by-side comparison with the original image on the color monitor 11. Both the view box 24 and the color monitor 11 are preferably subjected to a standard lighting condition, as indicated by the broken line rectangle 26 in FIG. 1. If the reproduction image does not accurately correspond to the original image, procedures as hereafter described may be used to make changes in the calibration data which are supplied to the computer 16 and to obtain a more accurate reproduction. Once satisfactory reproductions are obtained and the reliability of the calibration data is established, reproductions of other original images may be generated.

The system is also arranged for storage of calibration data in a file along with an identification of the particular inks which are used, filed calibration data being thereafter retrievable for use by the computer 15 whenever the same inks are subsequently used. The filed calibration data may also be retrieved to provide initial calibration data when ink combinations are used which have not been used previously but which are similar to those identified in the file.

FIG. 2 shows how the monitor 11, computer 12, storage unit 13 and view box 24 may be physically arranged on a table 28 and in a room or enclosure 30 which includes side walls 31 and 32, a rear wall 33. Indirect lighting, not shown, provides the aforementioned standard lighting condition 26. The surfaces of the walls 31-33 may preferably be of a neutral color with flat, non-reflective characteristics and the indirect lighting is so used as to avoid reflections from the surfaces of a screen 11a of the color monitor as well as from the view box 24 and a reproduction disposed therewithin. The lighting should be quite dim but sufficient to avoid excessive contrast between the screens 11a and the reproduction and the background wall surfaces.

The view box 24 preferably has standard 5000K lamps which are variable in intensity using an intensity control knob 24a. Such lamps may for example be fluorescent lamps hidden behind wall portions which define four peripheral edges of an open rectangular front window of the box 24, the reproduction being disposed against a rear wall 24b of the box 24, to be uniformly illuminated by the lamps in generally side-by-side relationship to the original image on the monitor screen 11a.

As is also shown in FIG. 2, the computer 12 is connected to a keyboard 35 and also to a status monitor 36. Computer 12 preferably includes disc drives 37 and 38 for receiving transportable discs and it may preferably also include a hard disc drive. In addition, the computer 12 may be connected to a graphics tablet 40 and associated puck 41, usable by an artist to develop a desired image on the screen 11a of color monitor 11, using a standard type of "paint" software. A scanner, not shown, may also be provided for scanning existing graphics and producing a corresponding image on the screen 11a for editing by the artist as desired. A computer graphics arrangement of this type is very flexible and allows an artist to make changes easily and rapidly produce original images of a form desirable for reproduction.

In the practice of the invention, calibration programs may be loaded into the computer 12 for generation or editing of calibration data files which may be loaded and saved from and to a hard disc of computer 12 and/or transportable discs placed in disc drives 37 or 38. During calibration, original test images may be developed on the monitor screen 11a, for comparison with reproductions of such original test images which have been produced with the system of the invention and which are placed in the view box 24. The system of the invention facilitates an orderly calibration procedure such that errors are readily and quickly detected and corrected to directly converge to an accurate calibration for any particular combination of inks used in the reproduction process. Then the calibration data file may be stored and retrieved for use whenever that particular combination of inks is subsequently used.

FIG. 3 is a flow diagram illustrating the digital processing operations performed by the computer 16 in the illustrated system 10. Initially, an original image file is loaded into the memory of the computer 16 from the storage unit 15 and is organized into sequential groups of pixel data, in an order corresponding to that required by the color processing system 20. Each group of pixel data contains sequential R, G and B groups of data which are 8 bit bytes in the system as described herein, each having a decimal value of from 0 to 255 and each defining the effective intensity of the corresponding component color of the pixel. It will be understood that the system is not limited to use of data in the form of 8 bit bytes and groups of data with a greater or lesser number of bits may be used.

Such numeric values of the pixel data bytes are referred to herein as the R, G and B values or collectively as the RGB values. In the system as described herein and as is conventional, they have a complementary or inverse relationship to actual intensity, i.e., a zero value corresponds to a maximum actual intensity of the component color and a value of 255 corresponds to a minimum actual screen intensity of the component color. Maximum values for all three RGB values correspond to a black pixel and minimum values for all three RGB values correspond to a white pixel. If two of the RGB values are maximum values and the third is zero, the corresponding pixel is of one color at maximum saturation.

After loading of group of pixel data and testing for an end-of-file marker, grey factor and saturation factor determinations are made. The grey factor corresponds to whichever of the red, green or blue component colors of a pixel has the lowest intensity, i.e., to whichever of the RGB values is the highest. The saturation factor corresponds to whichever of the red, green or blue component colors has the highest intensity, i.e. to whichever of the RGB values is the lowest.

In the operation as shown, the next step is to determine the black output value which is a function of the difference between a fixed value and the product of the grey factor and a black calibration value. For example, a certain shade of grey, or an approximation thereof, may be produced on the screen 11a when all three RGB values are 200 and a darker shade of grey, or an approximation thereof, may be produced when all three RGB values are reduced from 200 to 100. The grey factor in this example is reduced from 200 to 100 and the black output value is increased since it is a function of the difference between a fixed value and the grey factor, the result being the use of an increased percentage of black ink for the pixel of the reproduction. Preferably, a black value is added as necessary or desirable for contrast, but the system is such that an accurate grey balance is achievable over a wide range from the chromatic output values alone.

For control of color components in the system of the invention, chromatic output values are developed, such values being used for control of cyan, magenta and yellow inks and being designated as C, M and Y values or collectively as CMY values. Each of such chromatic output values is a result of a summation of a plurality of color values which are derived from the RGB values under control of calibration values.

The operation of the system is advantageously based in part upon an assumed approximate relationship of the CMY ink colors to the RGB screen colors. The assumed relationship is that cyan is the negative or inverse of red, that magenta is the negative or inverse of green and that yellow the negative or inverse of blue. In determining each of the CMY values, o