IL96829A - Apparatus and method for color printer domain transformation - Google Patents

Description

APPARATUS AND METHOD FOR COLOR PRI NTER DOMAIN TRANSFORMATION SCITEX CORPORATION LTD.

C: 1 1584 1-9004 11584lut.fil 27dec90 FIELD OF THE INVENTION The present invention relates to techniques for tone and color reproduction control in graphic arts.

BACKGROUND OF THE INVENTION Generally speaking, tone and color reproduction control in high quality graphic arts reproduction is still Tar from a science. This is particularly evident when a given acceptable result, already realized using given reproduction apparatus, is sought to be realized using other apparatus or using the same apparatus but with a different setting, such as a GCR setting relative to a normal "key black" setting. In such cases, a high degree of expertise, combined with time, effort, expense and patience is required to calibrate the additional apparatus. The results are not always satisfactory.

Unidimensional calibrations in graphic arts, in which a plurality of calibrations are carried out, each being a function of only one color, are known. State of the art techniques include gray balance correction and plotter output calibration techniques. Another example of unidimensional calibration is the automatic TCR (tone and color reproduction) correction process disclosed in published European Application 84307997.1 of Xerox Corporation (Publication number 0144188 A2) .

The disadvantage of unidimensional calibrations is that they are only accurate in certain portions of the color space, since a full determination of color is multidimensional, typically having three or four components. For example, the teaching of the above-mentioned published European Application 8437997-1 s relatively inaccurate except in the area of a particular machine's primary color coordinate axes. Gray balance techniques are relatively inaccurate except for a relatively small volume of the color space, comprising gray colors only. Also, the apparatus disclosed in the above-cited published European Application 8437997 · ! can be calibrated only by its own output.

Methods of computing a multidimensional function to fit a given set of vectors are known. Interpolative methods may be used if the data is suitably distributed. However the desired conditions regarding the distribution do not always hold in color processing applications, because the data is often not produced directly but rather is the end result of certain procedures (such as scanning, printing, etc.) which are performed on initial preselected data.

An article by Stone et al (Stone, M. C. ; Cowan, W. B. and Beatty, J. C, "Color Gamut Mapping and the P.rinting of Digital Color Images", ACM Transactions on Graphics , 7 ( 4 ) , Oct. 1988, 249-292) discloses use of a colorimeter in mapping a color gamut and printing digital color images, using human aesthetic judgment as a criterion. Human aesthetic judgment is not always the most relevant nor the most efficient criterion for calibrating a color processing device, particularly when it is desired to use another color processing device as a calibration reference. The use of a colorimeter as disclosed by Stone et al is typically cumbersome and time-consuming.

United States Patent 4,500,919 to Schreiber describes a system for producing color reproductions of an image in which an operator may interactively manipulate a display of the scanned image in order to introduce aesthetic, psychophysical^ referenced corrections therein. Schreiber teaches that it is desirable for such a system to provide automatic compensation for the effects of ink and paper while allowing the operator to input aesthetic alterations. " 'United States Patent 4,719,95^ to Fujita et al . describes a method and apparatus for creating a color conversion table between scanned colors of a color chart, typically in the Red-Green-Blue (RGB) color coordinate system, and printable colors, typically in the Cyan-Magenta-Yellow-Black (CMYK) color coordinate system, and for using the color conversion table to reproduce a selected measured color in a color specimen. If the selected measured color does not coincide with a value in the color conversion table, an interpolation step is performed.

The method of Fujita et al also includes a correction step when reproduction is performed under different printing conditions. The correction step compensates for the difference between the two printing conditions.

SUMMARY OF THE INVENTION The following terms as used in the present specification and claims should be construed in the following manner : Analog represe tation of a colored image : A representation of a colored image which is analogous or similar to the colored image.

Digital representation of a colored image : Any representation of a colored image which is expressed in discrete symbols, such as numerical symbols. A common digital representation of a colored image is a digital file comprising a plurality of numerical values corresponding to a plurality of pixels into which the colored image has been divided, each such numerical value representing some aspect pertaining to the colored appearance of the corresponding pixel.

Domain of a color printing device : The totality of color values which can be input by the color printing device.

Printing machine/device/system; output apparatus , recording apparatus etc . : Any apparatus which inputs a digital representation of a colored image and converts it into an analog representation thereof. For example: conventional printing systems, such as offset and gravure, or other printing apparatus employing inks, conventional or direct digital proofing machines, plotters or color recorders which expose photographic materials, electrostatic printing systems employing powder colorants, color monitors, and color CRT displays.

There is provided, in accordance with the present invention, apparatus for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device and a technique for producing same. The invention is described herein as an apparatus, it being understood that the invention includes a method for performing the operations of the apparatus.

The apparatus includes apparatus for providing a first transformation from a first digital representation of a colored image to a second digital representation thereof and a second transformation from -a third digital representation of a colored image to a fourth digital representation thereof and apparatus for comparing the first transformation with the second transformation. The second transformation corresponding to the second color printing device, the first transformation corresponding to the first color printing device and the second and fourth digital representations being defined within a single color space.

Additionally, in accordance with the present) invention, the apparatus includes apparatus for storing the output of the apparatus for comparing.

Further, in accordance with the present invention, the output of the apparatus for comparing defines a first plurality of matchings between a first plurality of elements of the domain of the second color printing device and a corresponding first plurality of elements of the domain of the first color printing device and wherein the apparatus for storing include apparatus for providing a second plurality of matchings between the domain of the first color printing device and the domain of the second color printing device, the second plurality exceeding the first plurality in number.

Still further, in accordance with the present invention, the apparatus for storing also includes apparatus for modifying at least some of the corresponding first plurality of elements of the domain of the first color printing device operative prior to the operation of the apparatus for" providing a second plurality of matchings.

Moreover, in accordance with the present invention, the apparatus for providing includes apparatus for interpolating between individual ones of the first plurality of matchings, thereby to provide individual ones from among the second plurality of matchings. The apparatus for interpolating includes apparatus for carrying out non-linear interpolation between individual ones of the first plurality of matchings. The apparatus for carrying out non-linear interpolation includes apparatus for fitting a tensor of splines to individual ones of the first plurality of elements of the first domain.

Additionally, in accordance with the present invention, the apparatus of the present invention additionally includes apparatus for employing the stored results to control operation of the first color printing device.

Furthermore, in accordance with the present invention, the apparatus for comparing includes apparatus for searching among the elements of the second digital representation for a plurality of close elements whose values are close to an element of the fourth digital representation, apparatus for forming a multiplicity of subsets, each individual one of the subsets containing some of the plurality of close elements, and apparatus for selecting individual ones from among the multiplicity of subsets. The apparatus for comparing also includes for each individual selected subset, first apparatus for combining the elements of the first digital representation -corresponding to the members of the individual selected subset, thereby to define a combination value for the individual selected subset and second apparatus for combining the combination values of each of the individual selected subsets, thereby to provide an element within the first digital representation corresponding to the element within the fourth representation.

Additionally, in accordance with the present invention, the first apparatus for combining includes apparatus for assigning a weight to each member of each individual selected subset, the weights being characterized in that, for each individual selected subset, the element of the fourth digital representation is the center of mass of the weighted (members of that subset and apparatus for computing a weighted sum of the ■members of the individual selected subset. The second apparatus for combining includes apparatus for assigning a weight to each individual selected subset, the weights reflecting the arrangement of the members of the individual selected subset relative to the element of the fourth digital representation and apparatus for computing a weighted sum of the members of the individual selected subset.

Moreover, in accordance with the present invention, the apparatus for selecting includes apparatus for inspecting the arrangements of the members of the multiplicity of subsets relative to the element of the fourth digital representation and apparatus for selecting individual ones from among the multiplicity of subsets in accordance with the result of the step of inspecting.

Still further, in accordance with the present invention, 'the apparatus includes apparatus for producing a new first transformation using the stored output and apparatus for repeatedly operating the apparatus for comparing and for storing thereby to produce a new transformation between an element of the domain of the first color printing device to an element of the domain of the second color printing device. The apparatus for employing utilize a color converter for converting the domain of the first color printing device to the domain of the second color printing device.

There is also provided, in accordance with the present invention, apparatus for quantifying the appearance of an analog representation of a location of a colored image including apparatus for providing an n-dimensional representation of the location, wherein n is at least 4.

Additionally, in accordance with the present invention, the analog representation of the location is provided by printing a digital representation of the location and wherein the apparatus for providing includes apparatus for providing a 3 dimensional representation of the color of the location and apparatus for computing at least a fourth value as a function of the digital representation of the location.

Alternatively, the apparatus for computing may be replaced by apparatus for measuring the fourth value directly from the analog representation.

Furthermore, in accordance with the present invention, the three dimensional representation is a colorimetric representation which can be CIE XYZ or CIE L*a*b*.

There is additionally provided, in accordance with the present invention, a system for converting between a digital and an analog representation of an image comprising a translation system for translating a reading and writing point across a substrate, a color proofer comprising the writing point for writing, from the digital representation, the analog representation onto the substrate and a color reading system comprising the reading point for reading the digital representation from the analog representation.

Additionally, in accordance with the present invention, the translation system comprises a drum for rotating the substrate and a translating carriage for translating along the drum.

Finally, in accordance with the present invention, the system includes apparatus for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device such as described hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a block diagram illustration of a technique for transforming a domain of a first color printing device to a domain of a second color printing device, operative in accordance with a preferred embodiment of the present invention; Fig. 2 is a flow chart illustration of a CMY-CMY conversion technique useful in the technique of Fig. 1; Fig. 3 is a flow chart illustration of an interpolation technique useful in the embodiment of Fig. 1; Fig. k is a block diagram illustration of the context for which the technique of Figs. 1 is utilized; and Fig. 5 is a block diagram illustration of a system for reading and writing an image constructed and operative in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig. k which illustrates the context in which the color transformation of the present invention is sought to be operated.

A digital representation 1G of an image, typically in the CMY color coordinate system but alternatively, in any N-dimensional color coordinate system, is sought to be produced by at least two color reproduction systems, typically a proofer 12 and a printer 14, such that measurable digital representations 20 and 2-2 of analog representations 16 and 18, respectively, are generally identical. - Measurable digital representations 2Q and 22 are typically produced by colorimeters or other color measuring system, and are typically in a colorimetric color coordinate system, such as XYZ or L*a*b*, or any other measurable N-dimensional color coordinate system. Typically, the three dimensions are the CIE defined coordinate system, such as XYZ or L*a*b*, and the remaining dimensions are any which may be desired to be measured, such as non-colorimetric effects seen by a human observer. One such dimension P might be the shininess of each color, or P might be derived from CMYK values using a formula such as : P = K - (C+M+Y)/3 (1) Formula 1 gives an indication of the amount of black used to produce a given color.

The drawings in this application indicate that the digital representations are in the CMY and XYZ color coordinate systems. It will be appreciated that the present invention is operative for N-dimensional transformations and that the notation CMY and XYZ is by way of explanation only.

It is known in the art that the two reproduction systems, when input an identical digital representation 10, will generally produce somewhat different analog representations 16 and 18. Since it is generally desired to match the output of one color reproduction system, defined here as the proofer 12, to the output of the other reproduction system, defined h¾re as the printer 14, a color transformation 24 between the CMY color coordinate system of the digital representation 10 to the CMY color coordinate system of the proofer 12 is necessary.

The color transformation 24 typically takes as input an N-dimensional transformation lookup table 42 relating the printable coordinate system of the input digital representation 10 with the printable coordinate system of the proofer 12.

A method to produce the transformation lookup table 42 is the subject of the present invention.

Reference is now made to Fig. 1 which illustrates apparatus for performing the method of the present invention. An N-dimensional color conversion table 30 is built for the printer l4 which converts between a first input digital representation, typically in CMY, CMYK or any N-dimensional printable color coordinate system, and measurable digital representation 22 of the output analog representation 18, in a measurable color coordinate system, such as XYZ or XYZP . Typically but not necessarily, the number of input dimensions is equivalent to the number of output dimensions. Typically the first input digital representation comprises a "swatches" pattern to be used to calibrate the printer which contains a plurality of digital color values and covering much of the range of printable colors.

The conversion table 30 is built as follows: the first input digital representation is sent to printer 14 to be printed. The resultant analog representation 18 is measured, via colorimeters or other measurement devices, and the color value of each printed color is stored in conversion table 30 opposite the input digital value which produced it. The precise details of the production of table 30 are given in the manual for the TRANS/ color converter, manufactured by Scitex Corporation Ltd. of Herzliya, Israel, incorporated herein by reference and set forth in Annex A.

The first input digital representation typically is organized on a N-dimensional grid, where a grid is a set of N vectors, possibly of different lengths, each containing a monotonically increasing series of positions along one color axis'. A typical three-dimensional grid might be: Cyan: ( 0 , 10 , 20 , 30 , 40 , 50 , 60 , 70 , 80 , 90 , 100 ) , Magenta: (0, 20, 40, 80, 100) , Yellow: (0, 40,50. 100) The colors to be printed are those whose values are at the intersection points of the grid. For example, a color to be printed might be: (10,40,50) In accordance with the present invention, an N-dimensional conversion table 3 is built for proofer 12 which converts between a second input digital representation, typically different from the first input digital representation and typically in a CMY or CMYK or any N-dimensional printable color coordinate system, and a measurable digital representation of an output analog representation produced without the color transformation 24, in a measurable color coordinate system, such as XYZ. The conversion table 32 is built as described hereinabove for conversion table 30. The conversion table 32 is not necessarily built using the same grid as that of table 30. It can be built from any grid or from no grid. Its ¾ CMY values can be different from the CMY values used to produce conversion table 30 or they can be identical. Typically but not necessarily, the number of input dimensions is equivalent to the number of output dimensions.

Conversion tables 30 and 32 are input into a printer- to-proofer transformer 34, described in more detail hereinbelow with reference to Fig. 2, for transforming a plurality of printer CMY values to a plurality of proofer CMY values where the XYZ values of the proofer CMY values are generally close to the XYZ values of the printer CMY values. An XYZ value of a CMY value is defined as the XYZ measured from the color produced by the color reproduction system when the CMY value is input to it.

It will be appreciated that the output proofer CMY value does not necessarily appear in conversion table 32.

Printer- to-proofer transformer 3 can be utilized for producing a transformation table 36 of proofer CMY values for each of the printer CMY values of conversion table 30, or, alternatively as shown by a dotted arrow, for producing a proofer CMY value upon input of a printer CMY value. Transformation table 36 is on the same grid as conversion table 30.

Transformation table 36 can optionally be stored in a color converter 44, such as the TRANS/4 color converter, for converting a printer CMY value to a proofer CMY value in accordance with table 36. For any printer CMY value not found in table 36, a linear interpolation is performed on table 36 in order to produce the output proofer CMY value.

Alternatively, the transformation table 36 can be input to an optional editor 38 for editing table 36. Editor 38 can be any kind of editor or text processor and is operative to allow an operator to manually correct the table 36, if necessary.

For example, in a CMY color coordinate system using a 'percent {%) dot' scale indicating the percentage of each ink to be used to produce the color, white is denoted by (0,0,0). When the white of the printer 14 is darker than the white of proofer 12, transformer 34 typically produces a proofer CMY white value greater than (0,0,0). This typically produces undesired effects such as the existence of screen dots in the analog output of the proofer 12 where none exist in the analog output of the printer 14.

The above undesired effects can be eliminated by editing table 36, via editor 38, to put a (0,0,0) proofer CMY value for a (0,0,0) printer CMY value. However, it will be appreciated that this produces an incorrect colorimetric (XYZ) value for the white produced by proofer 12.

The output of optional editor 38 , an edited table 36 , or, in the absence of editor 38 . a non-edited table 36 , is input to a non-linear interpolator 40 for non-linearly interpolating table 36 thereby to produce transformation table 42 which is denser than transformation table 36 . The operation of interpolator 40 is described in more detail hereinbelow with reference to Fig. 3 · Transformation table 42 is an N-dimensional CMY-CMY lookup table which is then stored in color converter 44 . For any printer C Y value not found in table 36 , a linear interpolation is performed on table 42 in order to produce the output proofer CMY value. It will be appreciated that the linearly interpolated value produced from table 42 is generally more accurate than the linearly interpolated value produced from table 36 since table 42 is denser than table 36 .

It will be appreciated that, alternatively, color converter 44 can perform a non-linear interpolation from table 36 . The present invention does not illustrate this alternative since cost and speed considerations using current computer technology indicate that the above method is presently more desirable .

Transformation table 42 can be used, as is, or it can be made more accurate as follows: 1 ) Transformation table 42 is used as shown to Fig. 4 in color transformation 24 and the colorimetric values of the output analog representation 16 are measured. 2 ) A new conversion table is produced which is then concatenated to conversion table 32 to produce a new version of conversion table 32 . 3 ) The method of Figs. 1 - 3 is repeated, using the new version of conversion table 32 .

Steps 1 - 3 can be repeated any number of times to produce an accurate transformation table 42 .

Reference is now made to Fig. 2 which details, in flow chart format, the operation of printer- to-proofer transformer 34 .

For each entry in the printer conversion tajale 30 , the operation involves searching the proofer conversion table 32 for entries whose XYZ value is near, by some definition, to the printer XYZ value. 'The results are stored in a table of close values. This is shown in steps 50 - 64 of Fig. 2 .

Specifically, step 54 requires the initialization of the table of close values. This may take the form of defining a closeness threshold less than which indicates closeness, or it may take the form of a maximum number of close values allowed.

In the second case, as the proofer table 3 is searched, the closest values found, and their distances, are stored. If a closer one is found, the furthest of 'the stored values is released and the closer one kept. Thus, in step 54 , the distances for the initial close values are set to high numbers .

In step 8 , the distance between the printer XYZ value and the present proofer XYZ value is calculated, typically as the Euclidean distance.. Other suitable distance measures can alternatively be used.

Typically, combinations of the close proofer values found in step 62 are then utilized for the next set of calculations. Combinations must minimally be of one more than the dimension size of the input digital representations. Thus, if the digital representations are in CMY, then there will minimally be four proofer values in each combination. The number of elements in the combination can be predetermined by the operator .

The distance between the printer XYZ value and the vector combination of proofer XYZ values is calculated as a combination of two elements. The first is the sum of the distances between the individual proofer XYZ values and the printer XYZ value.

The second is an "insideness" measure definin whether or not the printer XYZ value falls within the constellation of proofer XYZ values and can be calculated in a number of ways. For example, a set of linear equations can be solved where equations 2 - 5 below are an exemplary set for use with a combination containing four proofer points: + + + Px ■ = Fxlml Fx2m2 (2) Fx3m3 Fx4m4 + + py ' Fy2m2 Fy3m3 + Fy4mzj (3) Pz 's Fzlml + + + Fz2m2 (4) Fz3m3 Fz HmH 1.0 - ml + + + m^ (5) where the m^ are unknown values, indicates the printer X, Y or Z values and Fj indicates the proofer X, Y or Z values.

If the printer XYZ value falls inside the proofer XYZ values, indicated by all positive m^, the insideness measure is given a small positive value. Otherwise, the insideness measure is defined as the absolute value of the sum of the negative m^.

The distance between the printer XYZ value and the proofer XYZ values is defined as the combination of the insideness measure and the distance sum, where the combination is typically by multiplication but can be by any other suitable operation .

For all proofer XYZ combinations close to the printer XYZ, the steps 72 - 76 are performed.

Weights are calculated such that the weighted vector sum of the proofer XYZ combination is the printer XYZ. In other words, the printer XYZ is at the 'center of mass' of the proofer XYZ combination. This is calculated in step 72 and involves the solution of a linear set of equations, such as equations 2 - 5 · A preliminary proofer CMY value is then calculated in step i . Specifically, a weighted vector addition of the proofer CMY values of this combination, found in table 32 , is performed using the weights calculated in step 72 .

In step 76 , the preliminary proofer CMY value of step 74 is assigned a weight which is a function of the distance of the combination to the printer XYZ value as calculated in step 68 . Typically, the weight assigned is the distance between the combination and the printer XYZ value divided by the sum of these distances over all combinations close to the printer XYZ value.

The output proofer CMY value which produces a generally identical XYZ value as the printer XYZ value is calculated in step 80 as the weighted sum of all the preliminary proofer CMY values where the weights are those assigned in step 76 .

It will be appreciated that alternatively, it is possible to select only the closest proofer combination, calculate its CMY value and use that value as the output proofer CMY value.

Table 36 is a lookup table with the printer CMY value vs. the output proofer CMY value calculated in step 80.

Reference is now made to Fig. 3 which details the nonlinear interpolation method of interpolator 4θ.

A non-linear function, such as a tensor of splines, which defines the relationship between printer CMY values and proofer CMY values is fit to optionally edited conversion table 36 in step 94. Tensors of splines are described in detail in chapter 17 of A Practical Guide to Splines , by Carl De Boor, Vol. 27 of the Applied Mathematical Sciences series published by Springer Verlag, New York, 1978, which is incorporated herein by reference. A further reference is An Introduction to Splines for use in Computer Graphics and Geometric Modeling , by Richard H. Bartels et al . , published by Morgan Kaufmann Publishers, Inc., Los Altos, California, 1987.

Once the number and placement of printer CMY values in transformation table 42 are defined, typically via the operator and typically on a grid used by color converter 44, the nonlinear function is used to calculate the proofer CMY value for each printer CMY value in table 42. In this manner, table 42 is produced.

Object code for implementing the operations of transformer 34 and interpolator 40, shown in the flow charts of Figs. 3 and 4, are set forth in Annexes B and D, respectively. Exemplary source code indicating how to interface with the object codes of Annexes B and D is set forth in Annex C along with instructions as to how to use the source code.

Reference is now made to Fig. 5 which illustrates a system for reading and writing an image. The system is capable of producing the conversion tables 36 and/or 42.

The system comprises a color proofer 100, for writing an analog representation of an image from a digital representation and a color reading system 102, ^ such as a colorimeter or a spectrophotometer, for creating a digital representation of an image from an analog representation. The color reading system 102 is typically directly connected to the color reproduction system 100. A digital data processor 130 controls the operations of both the color proofer 100 and the color reading system 102.

As is known in the art, color proofer 100 typically comprises a translation system comprising a drum 104 for rotating a substrate 105i such as a piece of paper, upon which will be printed the analog representation of the image, and a translating carriage 107 which moves in one direction as the' drum 104 rotates. Onto the translating carriage 107 are attached nozzles 106 for receiving inks in accordance with the digital representation of the image and for effecting the printing of the analog representation of the image onto the substrate 105· A controller 108 simultaneously controls nozzles 106 and the movement of drum 104 and translating carriage 107 in accordance with the digital representation of the image received from data processor 130. Controller 108 also reports information regarding the status of proofer 100 to processor 130.

Any suitable color proofer 100 can be used. An example of which is the Iris color proofer, model 3024, manufactured by Iris Graphics Inc. of Massachusetts, USA.

As is known in the art, color reading system 102 typically comprises a reading head 120, such as a spectrophotometry head, for reading the colors of an analog representation and for producing from them a digital representation of the analog representation and a color reading processor 122 for controlling the operation of the reading head 120 and for receiving its output. Reading head 120 typically comprises a light source and a light detector coupled via an optical system. The elements of the reading head 120 are not shown since they are known in the art. Processor 122 is digitally connected to processor 130 from which it receives instructions and to whom it provides data.

Any suitable color reading system 102 may be used. An example color reading system is the Gretag SPM-700 manufactured by Gretag Ltd. of Regensdorf, Switzerland.

In accordance with the present invention, the reading head 120 is fixed to the translating carriage 107 of the color proofer 100. This physical connection 103 is noted in Fig. 5 by a curved line. The analog representation to be read is placed on drum 10k and reading is effected during the simultaneous rotation of the drum 104 and translation of carriage 107· Preferably, during the reading operation, nozzle 106 is not activated and thus, no writing is performed.

It will be appreciated that the placement of the reading head 120 onto the translating carriage 107 enables reading and writing to be performed in one machine.

Data processor 130 typically comprises a processor 13 for receiving data from color reading system 102 and for transmitting instructions to controller 108 and a processing unit 134 for controlling processor 132. Processing unit 134 is operative to match the output of a second color reproduction system to the output of color proofer 100 as described hereinabove with reference to Figs. 1 - 4 and to this end, it controls which operation, the reading or the writing operation, will occur at a given moment.

Processor 13 is typically the processor provided with the color proofer 100, such as the processor provided with the Iris color proofer model 3024 and processing unit 13 is typically a workstation, such as the Whisper workstation manufactured by Scitex Corporation Ltd. of Herzliya, Israel.

The operations described hereinabove with reference to Figs: 1 - 4 are performed on the system of Fig. 5 as follows, where similar reference numerals are used to refer ,to similar, elements: 1) Analog representation 18 from the second color reproduction system (i.e. printer 14) is placed on drum 104 and its color values are read by reading head 120, thereby producing digital representation 22 which is stored in processing unit 134. 2) Analog representation l6 is produced by color proofer 100 using digital representation 10, the representation used to produce analog representation 18. 3) Analog representation 16 remains on drum 10k and its color values are read by reading head 120, thereby producing digital representation 20. k) Processing unit 13^ first produces color conversion tables 30 &nd 32 from representations 16, 18, 20 and 22 and from them produces the transformation table k2.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather"*, the scope of the present invention is defined only by the claims that follow : 11584ame.cla 1-103-3 6.4.94 96829/2

Claims (49)

1. A method for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device, the method comprising the steps of: providing a first transformation from a first digital representation of a colored image to a second digital representation thereof and a second transformation from a third digital representation of a colored image to a fourth digital representation thereof, said second transformation corresponding to said second color printing device, said first transformation corresponding to said first color printing device and said second and fourth digital representations being defined within a single color space; and comparing said first transformation with said second transformation .

2. A method according to claim 1 and also comprising the step of storing the results of said comparing step.

3. A method according to claim 2 wherein the results of said comparing step define a first plurality of matchings between a first plurality of elements of the domain of the second color printing device and a corresponding first plurality of elements of the domain of the first color printing device and wherein 96829/2 said step of storing comprises the step of providing a second plurality of matchings between the domain of the first color printing device and the domain of the second color printing device, said second plurality exceeding said first plurality in number.

4. A method according to claim 3 and wherein said storing step also comprises, prior to said step of providing a second plurality of matchings, the step of modifying at least some of said corresponding first plurality of elements of the domain of the first color printing device.

5. A method according to claim 3 and wherein said step of providing comprises the step of interpolating between individual ones of said first plurality of matchings, thereby to provide individual ones from among said second plurality of matchings.

6. A method according to claim 5 and wherein said step of interpolating comprises the step of . carrying out non-linear interpolation between individual ones of said first plurality of matchings .

7. A method according to claim 6 and wherein said step of carrying out non-linear interpolation comprises the step of fitting a tensor of splines to individual ones of said first plurality of elements of said first domain.

8. A method according to claim 2 and also comprising the step of employing said stored results to control operation of said first color printing device.

9. A method according to claim 1 and wherein said step of comparing comprises the steps of: searching among the elements of said second digital representation for a plurality of close elements whose values are close to an element of said fourth digital representation; forming a multiplicity of subsets, each individual one of said subsets containing some of said plurality of close elements; selecting individual ones from among said multiplicity of subsets; for each individual selected subset, combining the elements of said first digital representation corresponding to the members of the individual selected subset, thereby to define a combination value for said individual selected subset; and combining said combination values of each of said individual selected subsets, thereby to provide an element within said first digital representation corresponding to said element within said fourth representation.

10. A method according to claim 9 and wherein said first step of combining comprises the steps of: assigning a weight to each member of each individual selected subset, said weights being characterized in that, for each individual selected subset, said element of said fourth digital representation is the center of mass of the weighted members of that subset; and computing a weighted sum of said members of said individual selected subset.

11. A method according to claim 9 and wherein said second step of combining comprises the steps of: assigning a weight to each individual selected subset, said weights reflecting the arrangement of the members of said individual selected subset relative to said element of said fourth digital representation; computing a weighted sum of said members of said individual selected subset.

12. A method according to claim 11 and wherein said step of selecting comprises the steps of: inspecting the arrangements of the members of said multiplicity of subsets relative to said element of said fourth digital representation; and selecting individual ones from among said multiplicity of subsets in accordance with the result of said step of inspecting .

13. A method for quantifying the appearance of an analog representation of a location of a colored image comprising the step of: providing an n-dimensional representation of said location, wherein n is at least 4.

14. A method according to claim 13 wherein said analog representation of said location was provided by printing a digital representation of said location and wherein said step of providing comprises the steps of: providing a 3 dimensional representation of the color of said location; computing at least a fourth value as a function of said digital representation of said location.

15. A method according to claim 14 and wherein said three dimensional representation is a colorimetric representation.

16. A method according to claim 15 and wherein said colorimetric representation is CIE XYZ .

17. A method according to claim 15 and wherein said * * * colorimetric representation is CIE L a b .

18. A method according to claim 2 and including the step of producing a new first transformation using said stored results and repeating said steps of comparing and storing thereby to produce a new transformation between an element of said domain of said first color printing device to an element of 29 96829/2 said domain of said second color printing device.

19. A method according to claim 18 and wherein said step of employing utilizes a color converter for converting said domain of said first color printing device to said domain of said second color printing device.

20. Apparatus for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device comprising; means for providing a first transformation from a first digital representation of a colored image to a second digital representation thereof and a second transformation from a third digital representation of a colored image to a fourth digital representation thereof, said second transformation corresponding to said second color printing device, said first transformation corresponding to said first color printing device and said second and fourth digital representations being defined within a single color space; and means for comparing said first transformation with said second transformation.

21. Apparatus according to claim 20 and also comprising means for storing the output of said means for comparing.

22. Apparatus according to claim 20 wherein the output of the means for comparing define a first plurality of matchings 96829/2 between a first plurality of elements of the domain of the second color printing device and a corresponding first plurality of elements of the domain of the first color printing device and wherein the means for storing comprise means for providing a second plurality of matchings between the domain of the first color printing device and the domain of the second color printing device, said second plurality exceeding said first plurality in number.

23. Apparatus according to claim 22 and wherein said means for storing also comprise means for modifying at least some of said corresponding first plurality of elements of the domain of the first color printing device operative prior to the operation of said means for providing a second plurality of matchings,

24. Apparatus according to claim 22 and wherein said means for providing comprise means for interpolating between individual ones of said first plurality of matchings, thereby to provide individual ones from among said second plurality of matchings.

25. Apparatus according to claim 24 and wherein said means for interpolating comprise means for carrying out non-linear interpolation between individual ones of said first plurality of matchings.

26. Apparatus according to claim 25 and wherein said means 96829/2 for carrying out non-linear interpolation comprise means for fitting a tensor of splines to individual ones of said first plurality of elements of said first domain.

27. Apparatus according to claim 21 and also comprising means for employing said stored results to control operation of said first color printing device.

28. Apparatus according to claim 20 and wherein said means for comparing comprise: means for searching among the elements of said second digital representation for a plurality of close elements whose values are close to an element of said fourth digital representation; means for forming a multiplicity of subsets, each individual one of said subsets containing some of said plurality of close elements; means for selecting individual ones from among said multiplicity of subsets; for . each individual selected subset, first means for combining the elements of said first digital representation corresponding to the members of the individual selected subset, thereby to define a combination value for said individual selected subset; and second means for combining said combination values of each of said individual selected subsets, thereby to provide an element within said first digital representation corresponding to 96829/2 said element within said fourth representation.

29. Apparatus according to claim 28 and wherein said first means for combining comprise; means for assigning a weight to each member of each individual selected subset, said weights being characterized in that, for each individual selected subset, said element of said fourth digital representation is the center of mass of the weighted members of that subset; and means for computing a weighted sum of said members of said individual selected subset.

30. Apparatus according to claim 28 and wherein said second means for combining comprise; means for assigning a weight to each individual selected subset, said weights reflecting the arrangement of the members of said individual selected subset relative to said element of said fourth digital representation; and means for computing a weighted sum of said members of said individual selected subset.

31. Apparatus according to claim 30 and wherein said means for selecting comprise; means for inspecting the arrangements of the members of said multiplicity of subsets relative to said element of said fourth digital representation; and 96829/2 . means for selecting individual ones from among said multiplicity of subsets in accordance with the result of said step of inspecting.

32. Apparatus for quantifying the appearance of an analog representation of a location of a colored image comprising; means for providing an n-dimensional representation of said location, wherein n is at least 4.

33. Apparatus according to claim 32 wherein said analog representation of said location is provided by printing a digital representation of said location and wherein said means for providing comprise: means for providing a 3 dimensional representation of the color of said location; and means for computing at least a fourth value as a function of said digital representation of said location.

34. Apparatus according to claim 33 and wherein said three dimensional representation is a colorimetric representation.

35. Apparatus according to claim 33 and wherein said colorimetric representation is CIE XYZ.

36. Apparatus according to claim 33 and wherein said colorimetric representation is CIE L*a*b*. 96829/2

37. Apparatus according to claim 21 and including means for producing a new first transformation using said stored output and means for repeatedly operating said means for comparing and for storing thereby to produce a new transformation between an element of said domain of said first color printing device to an element of said domain of said second color printing device.

38. Apparatus according to claim 27 and wherein said means for employing utilize a color converter for converting said domain of said first color printing device to said domain of said second color printing device.

39. A system for converting between a digital and an analog representation of an image comprising: a translation system for translating a reading and writing point across a substrate; a color proofer comprising said writing point for writing, from said digital representation, said analog representation onto said substrate; and a color reading system comprising said reading point for reading said digital representation from said analog representation.

40. A system according to claim 39 and wherein said translation system comprises a drum for rotating said substrate and a translating carriage for translating along said drum. 96829/2

41. A system according to claim 39 and also including means for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device.

42. A system according to claim 41 and wherein said means for transforming comprise; means for providing a first transformation from a first digital representation of a colored image to a second digital representation thereof and a second transformation from a third digital representation of a colored image to a fourth digital representation thereof; and means for comparing said first transformation with said second transformation; said second transformation corresponding to said second color printing device, said first transformation corresponding to said first color printing device and said second and fourth digital representations being defined within a single color space.

43. A method according to claim 42 and wherein said means for comparing comprise: means for searching among the elements of said second digital representation for a plurality of close elements whose values are close to an element of said fourth digital representation; means for forming a multiplicity of subsets, each 96829/3 individual one of said subsets containing some of said plurality of close elements; means for selecting individual ones from among said multiplicity of subsets; for each individual selected subset, means for combining the elements of said first digital representation corresponding to the members of the individual selected subset, thereby to define a combination value for said individual selected subset; and means for combining said combination values of each of said individual selected subsets, thereby to provide an element within said first digital representation corresponding to said element within said fourth representation.

44. A method according to any of claims 1 - 19 substantially as described hereinabove.

45. A method according to any of claims 1 - 19 substantially as illustrated in any of the drawings.

46. Apparatus according to any of claims 20 - 38 substantially as described hereinabove.

47. Apparatus according to any of claims 20 - 38 substantially as illustrated in any of the drawings.

48. A system according to any of claims 39 - 43 substantially as described hereinabove. 96829/3

49. A system according to any of claims 39 - 43 substantially as illustrated in any of the drawings. Law Offices of A. Tally Eitan - Zeev Pearl & Co. P-724-IL