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Claims  |
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What is claimed is:
1. A system for controlling color reproduction of input color image data comprising:
a network having nodes in which at least two of said nodes each comprise at least one rendering device and network interfacing means for enabling the node to communicate with one or more other said nodes in said network;
means for distributing said input color image data from said one of said nodes to other said nodes;
means for providing a data structure in said network;
means for providing color calibration data at each said node characterizing output colors of the rendering device of the node;
means for producing for each said node, responsive to the color calibration data of the rendering device of the node, information for transforming the input color image data into output color image data at the rendering device of the node;
means for storing said information in said data structure;
means for transforming for each said node said input color image data into output color image data for the rendering device of the node responsive to said information in said data structure; and
means for rendering at the rendering device of each said node a color reproduction responsive to said output color image data, wherein colors displayed in said reproduction at the rendering device of each said node appear substantially the same
within output colors attainable by the rendering devices.
2. The system according to claim 1 further comprising:
means for verifying at each said node that said information for the rendering device of the node properly transformed the input color image data into the output color image data; and
means for revising said information stored in the data structure at the node responsive to results of said verifying means.
3. The system according to claim 2 wherein each said node further comprises a computer, coupled to the rendering device of the node, which controls said rendering device and communicates with other said nodes in the network, wherein said
producing means, said transforming means, said rendering means, said verifying means and said revising means are operative by said computer at each said node.
4. The system according to claim 1 wherein the rendering device of at least one of said nodes is a high volume printing press, and said rendering means at said one of said nodes further comprises means for controlling said high volume printing
press to render said color reproduction responsive to said output color image data.
5. The system according to claim 1 wherein said transforming means further comprising means for transforming at one of said nodes said input color image data into output color image data to represent the rendering device at another said node
responsive to said information in said data structure; and
said rendering means further comprises means for rendering at said rendering device of said one of said nodes a color reproduction responsive to output color image data representing the reproduction at the rendering device at said another node.
6. The system according to claim 1 wherein said producing means for each said node further comprises:
means for building a forward model responsive to said color calibration data of the rendering device of the node defining the relationship of input colors to output colors attainable by the rendering devices in the network;
means for preparing a forward model table based on said forward model of the input colors to combinations of the output colors attainable by the rendering devices in the network;
means for preparing gamut descriptor data defining the output colors of the rendering device of the node;
means for inverting the forward model table to provide a prototype transformation table defining the output colors of the rendering device of the node based on combinations of the input colors;
means for revising the prototype transformation table to include those output colors in said gamut descriptor data;
means for converting the output colors of the prototype transformation table responsive to first color preference data;
means for building an output color transform table responsive to second color preference data; and
means for combining the output color transform table and the converted prototype transformation table to provide a rendering table defining the output colors of the rendering device of the node based on combinations of the input colors, whereby
said information for transforming the input color image data into output color image data for the rendering device of the node comprises at least said rendering table.
7. The system according to claim 6 wherein said information for the rendering device of one of said nodes comprises said color calibration data, said forward model, said forward model table, said gamut descriptor data, said prototype
transformation table, said converted prototype transformation table, and said output color transform table.
8. The system according to claim 6 wherein said second color preference data comprises gamut configuration data for the rendering device of the node defining a relationship between output colors in the gamut descriptor data and output colors in
the prototype transformation table absent in the gamut descriptor data, and a neutral output color function.
9. The system according to claim 6 wherein said first color preference data comprises black color data which defines appearance of a black one of said output colors and a relationship of other said output colors to said black output color.
10. The system according to claim 9 wherein said black color data comprises a percentage of UCR, a maximum black color; and GCR data.
11. The system according to claim 1 wherein each said node further comprises a database, and said system further comprise means for storing a summary of said color calibration data for said rendering device in said database at said node
associated with the rendering device.
12. The system according to claim 1 further comprising for one of said nodes:
means for generating a filter table responsive to said information of the node having indicators for possible output colors for combination of the input colors; and
means for storing said filter table in said data structure.
13. The system according to claim 1 further comprising for one of said nodes:
means for generating a filter table responsive to said information of the node having indicators for possible output colors for combination of the input colors;
means for storing said filter table in said data structure;
means for applying said filter table to the rendering table of the rendering device of another said node to provide a filtered rendering table;
means for transforming said input color image data into filtered output color image data for the rendering device of said another said node responsive to said filtered rendering table; and
means for rendering at the rendering device of said another said node a color reproduction responsive to said filtered output color image data.
14. The system according to claim 1 wherein the rendering device of one of said nodes has more than four output colors.
15. The system according to claim 1 wherein the rendering device of one of said nodes has one of three and four output colors.
16. The system according to claim 2 wherein said verifying means at each said node further comprising:
means for rendering verification forms by the rendering device of the node;
means for measuring colors of the verification forms;
means for comparing statistically the measured colors with reference data for said verification forms to provide color error data at the node; and
said revising means further comprises means for revising said information of said data structure at the node responsive to said color error data.
17. The system according to claim 1 wherein said information producing means further comprises means for producing for each said node said information utilizing one or more neural networks.
18. The system according to claim 1 further comprising:
means for providing linearization information characterizing correction of non-linearity in rendering output colors for the rendering device of the node; and
means for revising said information for transforming said input color image data into output color image data responsive to said linearization information.
19. The system according to claim 1 further comprising:
means for revising said information for transforming said input color image data into output color image data to provide rendering of greater than four output colors.
20. The system according to claim 1 further comprising means for conferencing between said nodes to allow users at said nodes to negotiate characteristics of colors displayed in said reproduction at said nodes.
21. The system according to claim 1 wherein said data structure comprises components shared by the nodes and other components present only at each said node, and said storing means stores said information in said data structure in different ones
of said shared and said other components.
22. The system according to claim 1 wherein said information for transforming the input color image data into output color image data at the rendering device of the node is in accordance with human perception of color at the rendering device.
23. The system according to claim 1 wherein the rendering device at one or more said nodes is a color display.
24. The system according to claim 1 wherein said transforming means operates responsive to one of said information provided from said information producing means and information representing a standard transformation profile.
25. The system according to claim 6 wherein said forward model represents a mapping of the output colors of the rendering device in color coordinates independent of the rendering device.
26. The system according to claim 11 wherein said means for storing color calibration data stores a plurality of said summary of said color calibration data from different operations of said means for providing color calibration data at each
said node, and said system further comprises means for comparing said stored color calibration data from said different operations of said means for providing color calibration data to verify the integrity of said calibration data.
27. The system according to claim 12 further comprising for one of said nodes:
means for overlaying a graphical representation of said filter table onto said reproduction rendered at said rendering device of the node.
28. The system according to claim 1 wherein at least one of the rendering devices is a color display and said calibration data for said color display accounts for the influence of ambient illumination diffusely reflected from the color display.
29. The system according to claim 1 wherein said network represents one of a telecommunications network and local area network.
30. The system according to claim 1 wherein said network represents one of a wide area network, and Internet.
31. The system according to claim 1 wherein at least one of said network interfacing means at said nodes comprises one of a modem, satellite link, leased communication line, ISDN, SMDS, ATM, TCP/IP, and token ring.
32. The system according to claim 1 wherein at least two of said nodes are remote from each other.
33. The method according to claim 1 wherein at least two of said nodes are remote from each other.
34. A method for controlling color reproduction at a plurality of nodes in a network in which at least two of said nodes each have at least one rendering device, said method comprising the steps of:
providing a data structure in said network;
providing at each said node common input color image data in which each of said nodes are capable of communicating with one or more other ones of said nodes;
providing color calibration data at each said node characterizing output colors of the rendering device of the node;
producing for each said node, responsive to the color calibration data of the rendering device of the node, information for transforming the input color image data into output color image data at the rendering device of the node;
storing said information in said data structure;
transforming for each said node said input color image data into output color image data for the rendering device of the node responsive to said information in said data structure; and
rendering at the rendering device of each said node a color reproduction responsive to said output color image data, wherein colors displayed in said reproduction at the rendering device of each said node appear substantially the same within
output colors attainable by the rendering devices.
35. The method according to claim 34 further comprising the steps of:
verifying at each said node that said information for the rendering device of the node properly transformed the input color image data into the output color image data; and
revising said information stored in the data structure at the node responsive to results of said verifying step.
36. The method according to claim 34 further comprising the step of:
proofing said rendered color reproduction at least one said node in the network.
37. The method according to claim 34 wherein the rendering device of at least one of said nodes is a high volume printing press, and said rendering step at said one of said nodes further comprises controlling said high volume printing press to
render said color reproduction responsive to said output color image data.
38. The method according to claim 34 wherein said transforming step further comprising the step of transforming at one of said nodes said input color image data into output color image data to represent the rendering device at another said node
responsive to said information in said data structure; and
said rendering step further comprises the step of rendering at said rendering device of said one of said nodes a color reproduction responsive to output color image data representing the reproduction at the rendering device at said another node.
39. The method according to claim 34 wherein said producing step for each said node further comprising the steps of:
building a forward model responsive to said color calibration data of the rendering device of the node defining the relationship of input colors of said input color image data to output colors attainable by the rendering devices in the network;
preparing a forward model table based on said forward model of the input colors to combinations of the output colors attainable by the rendering devices in the network;
preparing gamut descriptor data defining the output colors of the rendering device of the node;
inverting the forward model table to provide a prototype transformation table defining the output colors of the rendering device of the node based on combinations of the input colors;
revising the prototype transformation table to include those output colors in said gamut descriptor data;
converting the output colors of the prototype transformation table responsive to first color preference data;
building an output color transform table responsive to second color preference data; and
combining the output color transform table and the converted prototype transformation table to provide a rendering table defining the output colors of the rendering device of the node based on combinations of the input colors, whereby said
information for transforming the input color image data into output color image data for the rendering device of the node comprises at least said rendering table.
40. The method according to claim 39 wherein said information for the rendering device of one of said nodes comprises said color calibration data, said forward model, said forward model table, said gamut descriptor data, said prototype
transformation table, said converted prototype transformation table, and said output color transform table.
41. The method according to claim 39 wherein said second color preference data comprises gamut configuration data for the rendering device of the node defining a relationship between output colors in the gamut descriptor data and output colors
in the prototype transformation table absent in the gamut descriptor data, and a neutral output color function.
42. The method according to claim 39 wherein said first color preference data comprises black color data which defines appearance of a black one of said output colors and a relationship of other said output colors to said black output color.
43. The method according to claim 39 wherein said black color data comprises a percentage of UCR, a maximum black color; and GCR data.
44. The method according to claim 34 further comprising storing a summary of said color calibration data for said rendering device in a database at said node associated with the rendering device.
45. The method according to claim 39 further comprising for one of said nodes:
generating a filter table responsive to the converted prototype transformation table of the rendering table of the node having indicators for possible output colors for combinations of the input colors;
storing said filter table in said data structure; and
overlaying a graphical representation of said filter table onto said reproduction rendered at said rendering devices of the node.
46. The method according to claim 39 further comprising at one of said nodes the steps of:
generating a filter table responsive to the converted prototype transformation table of the rendering device of the node having indicators for possible output colors for combination of the input colors;
storing said filter table in said data structure;
applying said filter table to the rendering table of the rendering device of another said node to provide a filtered rendering table;
transforming said input color image data into filtered output color image data for the rendering device of said another said node responsive to said filtered rendering table; and
rendering at the rendering device of said another said node a color reproduction responsive to said filtered output color image data.
47. The method according to claim 34 wherein the rendering device of one of said nodes has more than four output colors.
48. The method according to claim 34 wherein the rendering device of one of said nodes has one of three and four output colors.
49. The method according to claim 35 wherein said step of verifying at each said node further comprising the steps of:
rendering verification forms by the rendering device of the node;
measuring colors of the verification forms; and
comparing statistically the measured colors with reference data for said verification forms to provide color error data at the node; and
said revising step further comprises revising said information of said data structure at the node responsive to said color error data.
50. The method according to claim 34 further comprising:
providing linearization information characterizing correction of non-linearity in rendering output colors for the rendering device of the node; and
revising said information for transforming said input color image data into output color image data responsive to said linearization function.
51. The method according to claim 34 further comprising:
revising said information for transforming said input color image data into output color image data to provide rendering of greater than four output colors.
52. The method according to claim 34 further comprising conferencing between said nodes to allow users at said nodes to negotiate characteristics of color displayed in said reproduction at said nodes.
53. The method according to claim 34 wherein said data structure comprises components shared by the nodes and other components present only at each said node, and said storing step stores said information in said data structure in different ones
of said shared and said other components.
54. The method according to claim 34 wherein said information for transforming the input color image data into output color image data at the rendering device of the node is in accordance with human perception of color at the rendering device.
55. The method according to claim 34 wherein the rendering device at one or more said nodes is a color display.
56. The method according to claim 34 wherein said transforming step operates in accordance with one of said information provided from said information producing step and information representing a standard transformation profile.
57. The method according to claim 39 wherein said forward model represents a mapping of the output colors of the rendering device in color coordinates independent of the rendering device.
58. The method according to claim 44 wherein said step of storing color calibration data stores a plurality of said summary of said color calibration data from different operations of said step for providing color calibration data at each said
node, and said method further comprises the step of comparing said stored color calibration data from said different operations of said step for providing color calibration data to verify the integrity of said calibration data.
59. The method according to claim 34 wherein said network represents one of a telecommunications network and local area network.
60. The method according to claim 34 wherein said network represents one of a wide area network, and Internet.
61. The method according to claim 34 wherein each of said nodes is capable of communication through a network interface with one or more other ones of said nodes.
62. The method according to claim 61 wherein at least one of said network interfacing means at said nodes comprises one of a modem, satellite link, leased communication line, ISDN, SMDS, ATM, TCP/IP, and token ring.
63. A color reproduction apparatus for rendering a page with color uniform with a remote device rendering the same said page comprising:
means for communicating with said remote rendering device;
means for receiving input color image data corresponding to said page via said communicating means;
means for producing a data structure with components shared by said apparatus and said remote device and components local to said apparatus, said data structure having information for transforming the input color image data into output color
image data which will provide uniform appearance of color of said page at both said apparatus and said remote device, said communicating means being operative by said producing means to provide said shared data structure;
means for transforming said input color image data into output color image data responsive to said information in said data structure;
means for rendering said page at said apparatus responsive to said output color image data;
means for verifying that the information in said data structure properly transformed said input color image data into said output color image data; and
means for revising the information in the data structure responsive to said verifying means; and
means for producing color calibration data characterizing output colors of the apparatus which is operative by said means for producing a data structure and said means for verifying.
64. A method of virtual proofing at a plurality of rendering devices configured into a network, each said rendering device having a calibration transform to render color images, said method comprising the steps of:
transferring color image data from one of said plurality of rendering devices to others of said rendering devices in said network;
calibrating color measure instruments associated with each said rendering device;
rendering a known color image at each said rendering device;
measuring the rendered image at each said rendering device with the color measuring instrument to provide color data;
comparing the measured color data with color data of the known color image to provide color-error data;
evaluating the color-error data responsive to tolerance levels to indicate when the calibration of each said rendering device is one of within said tolerance levels and outside said tolerance levels;
producing at one of said plurality of rendering devices when said calibration of the rendering device is outside said tolerance levels another calibration transform;
building a correction transform based upon said color-error data when said calibration of one of said rendering devices is within said tolerance levels;
revising the calibration transform of said rendering device with said correction transform; and
rendering said image data at each rendering device responsive to one of said revised calibration transform and said another calibration transform.
65. A system for controlling color reproduction of input color image data at a plurality of rendering devices comprising:
a plurality of rendering devices;
a plurality of computers, each being coupled to at least one of said rendering devices, which share information for transforming at the rendering devices said input color image data into output color image data, wherein said shared information
includes at least data in device-independent color units; and
each of said computers operate to enable their coupled rendering device to produce a rendering of the image data in accordance with said shared information at the computer, wherein colors in the rendering produced by the rendering devices appear
substantially the same within output colors attainable by each of the rendering devices.
66. The system according to claim 65 wherein each of said computers further comprise:
means for communicating with each other on a network to share at least part of said information.
67. The system according to claim 65 wherein said data in device-independent color units is in accordance with human perception of color.
68. The system according to claim 65 wherein said information is in CIE Standard Observer coordinates.
69. The system according to claim 65 further comprising:
a color measuring instrument coupled to each of said computers to provide calibration data characterizing the color produced by the rendering device coupled to the computer; and
means for each of said computers for producing said information for transforming at the rendering device coupled to the computer said input color image data into output color image data in accordance with said calibration data.
70. The system according to claim 65 wherein at least one of said rendering devices is a proofing device.
71. The system according to claim 70 wherein said proofing device represents one of a color display and a hardcopy output device.
72. A system for controlling color reproduction of input color image data at a plurality of rendering devices comprising:
a plurality of rendering devices;
a plurality of computers, each being coupled to at least one of said rendering devices, which are capable of sharing information for transforming at the rendering devices said input color image data into output color image data, wherein at least
one of said rendering devices is a proofing device and said proofing device represents one of a color display and a hardcopy output device; and
each of said computers operate to enable their coupled rendering device to produce a rendering of the image data in accordance with at least said shared information at the computer, wherein colors in the rendering produced by the rendering
devices appear substantially the same within output colors attainable by each of the rendering devices.
73. A system for controlling color reproduction at multiple sites in which each of said sites has at least one rendering device, said system comprising:
a computer system having a user interface at at least one of said sites to enable a user at the site to select a plurality of sites and to connect said plurality of sites in a network in which said connected sites each have means for enabling the
site to communicate with one or more other said connected sites in said network; and
means for enabling the production of information for transforming input color image data into output color image data for the rendering devices at at least two of said connected sites such that colors produced by the rendering devices appear
substantially the same within output colors attainable by the rendering devices. |
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Description |
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FIELD OF THE INVENTION
This invention relates to a system (method and apparatus) for distributing and controlling color reproduction at multiple sites, and particularly to, a system for distributing and controlling the color output of rendering devices, such as
proofing devices and presses, at multiple sites or nodes of a network to provide a uniform appearance of color within the output colors attainable at each rendering device. The system is controlled by computers at each node and utilizes a data
structure, referred to herein as a Virtual Proof, to store and distribute color transformation information in the network. Color image data representing one or more pages or page constituents can be distributed to the nodes separately from the Virtual
Proof.
BACKGROUND OF THE INVENTION
In the printing and publishing industry, the increasing modularity of manufacturing operations is enabling customization of products. At the same time, pressures to reduce inventories and to keep them fresh are driving a trend toward
just-in-time production and stocking. Wherever the manufacturing can be decentralized and distributed geographically, just-in-time production is facilitated because producers are closer to consumers in space and time. There is an ecological dividend,
as well, in reduced demands on the transportation system. Overall product cost may decrease with shipping expense. At the same time, however, the challenge of maintaining uniform quality across a network of production sites increases. Minimizing
startup waste gains in importance as does compensating for uneven skill and experience of operators. Color is a key variable to control because it affects product appearance and perceived quality.
Today for example, a magazine with a national circulation of 5 million may be printed at 5 regional plants scattered across the nation. Distribution (transportation and postage) generally account for one third of the cost of the product while
transit time has a significant impact on product "freshness," i.e., the timeliness of the information delivered.
Production is as centralized as it is partly in order to maintain reasonably homogeneous quality. Nevertheless, printed color varies within a press run and from site to site because there have been only limited means of coordinating control of
product appearance among sites. The scope and significance of this problem is apparent when one considers how much commerce and economic activity are leveraged by advertising and that generally more than 60% of all printing is advertising-related.
Analogous problems also arise in other media, particularly now that digital video images can be edited in real time and broadcast directly.
The preceding paragraphs have spoken about parallel mass-production at multiple sites. Publishing is also distributed in the sense that the sequential steps of preparation for volume production occur at distinct sites, as illustrated in FIG. 1.
Oftentimes, the sites represent different business entities (for example, an advertising agency, a publisher, or an "engraver") which are geographically separated. Solid lines in FIG. 1 represent links connecting the sites in the production process.
Overlaid in FIG. 1 are dotted boundaries indicating a cluster of pre-publishing facilities which handle sequential phases of the process under Product Prototype 1, and printing facilities which may be involved in parallel Volume Production 2.
Currently prevalent volume printing technologies such as offset lithography, employ a printing "plate" which bears fixed information and is the tool or die of volume production. The tool is mounted on a press and numerous copies of the printed
product are stamped out. For technologies such as ink jet and electrophotography the information on the plate can be changed from one revolution of the press to the next. This technological development enables significant product customization and is
compatible with just-in-time production scenarios. It also enables process control in which the electronic data flowing to the device are modified to adapt to changes in the marking engine. However, the consistency (or repeatability) of these processes
makes them even more susceptible to regional variations in quality across the production sites than lithography and its relatives.
For all of the printing technologies mentioned, there is a common problem of uniform and accurate color reproduction. Analogous problems also exist in other media for distributing color graphic or image content, such as CDROM or the Internet.
Consider an advertiser in New York, physically removed from the five production sites mentioned above, or the more numerous sites that may be involved in the future. There is a keen interest in having the product portrayed in as faithful an accord with
the advertiser's artistic conceptions as possible, even when the ad is to appear in different publications printed on different substrates by different machinery or in the same publication disseminated through different media.
Today, the approval cycle, the means by which print buyer and printer reach contractual agreement about the acceptability of product, often proceeds as outlined in FIG. 2, in the publication segment of the industry. Phases or functions of
production are enclosed in ellipses 1a, 1b and 1c and key products of theses functions are enclosed by rectangles 3, 5, 6, 7, 8 and 9. The dashed line between creation 1a and prepress 1b shows the blurring of those functions in the development of
intermediate products, such as page constituents like lineart, images, text and comps. Prepress 1b on the way to film 5 may include rasterization, separation and screening 4. However, acceptance of computer-to-plate technology will blur the boundary
between prepress 1b and production 1c.
The long, heavy boundary line between press-proofing in low volume reproduction 1c and high volume production 2 represent the distinctness of the two functions; the former is carried out by engravers or commercial printers. Note that volume
production 2 may occur at multiple sites. Linkages in the approval process are shown by arcs 10a and 10b at the bottom of FIG. 2, where 10a is the traditional off-press proof and 10b is a press proof. The transactions in the approval process involve
one or more generations of static proofs which are prepared with limited regard for the capabilities of the final, volume-production devices. In other words, there is no feedback from production to earlier functions. The process results in idle time
for equipment and personnel and waste of consumables (paper, ink etc.) Furthermore, it usually does not give the print buyer any direct say about the appearance of the ultimate product unless the buyer travels to the printing plant, an expensive
proposition.
The workflow for commercial printing is slightly different from that described above, since press-proofs are seldom used and the print buyer or his agent often go to the printer's for approval. However, the essential lack of feedback is also
prevalent in the commercial environment as well.
It is clear that a common language of color could insure improved communication, control and quality throughout the sites of FIG. 1. The common language is a color space, typically based on the internationally accepted Standard Observer which
quantifies color in terms of what normal humans see, rather than in terms of specific samples or instances of color produced by particular equipment. The Standard Observer is the basis of device-independent, colorimetric methods of image reproduction
and is defined by the Commission Internationale de L'Eclairage in CIE Publication 15.2, 1986, Central Bureau of the CIE, Box 169, Vienna, Austria. Approximately uniform perceptual color spaces based upon the Standard Observer are also discussed in this
publication.
Color Space is defined as a three-dimensional, numerical scheme in which each and every humanly perceivable color has a unique coordinate. For example, CIELAB is a color space defined by the CIE in 1976 to simulate various aspects of human
visual performance. Color in the next section will refer to CIE color or what we see, while colorant will refer to particular physical agents, such as dyes, pigments, phosphors, and the like that are instrumental in producing sensations and perceptions
of color in a human at rendering devices, such as presses and video screens.
Prior Art
An early machine for converting color image data to colorant specifications for a 3 or 4-channel reflection reproduction process was described by Hardy and Wurzburg (Color correction in color printing, J. Opt. Soc. Amer. 38: 300-307, 1948.)
They described an electronic network provided with feedback to control convergence to the solution of an inverse model of colorant mixture and produce 4-colorant reproductions "indistinguishable" from 3-colorant reproductions made under like conditions.
The set point for the control was the color of the original. This work serves as a starting point for many subsequent developments in the art particularly as regards device independent color reproduction technologies and "color separation," i.e., the
preparation of printing plates for 3 or more colorants.
In U.S. Pat. No. 2,790,844, Neugebauer discloses a system to extend the Hardy-Wurzburg machine. It describes the capture and representation of color imagery in a colorimetric (or device independent) coordinate system. To enable an operator to
judge the effect of color corrections while he is making these color corrections, the system provides for a soft proof realized by projecting video images onto the type of paper stock to be used in the final reproduction with careful regard to making the
surround illumination and viewing conditions comparable to those prevailing when the final product is viewed. The objective of the soft proof was to simulate a hard copy proof or final print. This is in contrast to U.S. Pat. No. 4,500,919, issued to
Schreiber, which discloses a system to match the hard copy to the monitor image.
Concerning models of color formation by combination of colorants, Pobboravsky (A proposed engineering approach to color reproduction, TAGA Proceedings, 1962, pp. 127-165) first demonstrated the use of regression techniques ("curve fitting") to
define mathematical relationships between device independent color ("in the CIE sense") and amounts of colorant with accurate results. The mathematical relationships took the form of low order polynomials in several variables.
Schwartz et al. (Measurements of Gray Component Reduction in neutrals and saturated colors, TAGA Proceedings, 1985, pp. 16-27) described a strategy for inverting "forward" models (mathematical functions for converting colorant mixtures to
color.) The algorithm was similar to Hardy and Wurzburg's but implemented with digital computers; it consists of iteratively computing (or measuring) the color of a mixture of colorants, comparing the color to that desired and modifying the colorants in
directions dictated by the gradients of colorants with respect to color error until color error is satisfactorily small. Color error is computed in CIE uniform coordinates. The context of the work was an implementation of an aspect of the art known as
Gray Component Replacement (GCR.) Because normal human color perception is inherently 3-dimensional, the use of more than 3 colorants is likely to involve at least one colorant whose effects can be simulated by a mixture of two or more of the others
("primaries.") For example, various amounts of black ink can be matched by specific mixtures of the "primary" subtractive colorants cyan, magenta and yellow. The goal of Schwartz et al. was a method for finding calorimetrically equivalent
("indistinguishable" in Hardy and Wurzburg's words) 4-colorant solutions to the problem of printing a given color that used varying amounts of black. Additional colorants (more than 3) are used to expand the gamut; black enables achievement of densities
in reflection reproduction processes that are not otherwise available. A gamut is the subset of human perceivable colors that may be outputted by a rendering device. However, increased reliance on black through GCR has other important dividends: a)
there is an economic benefit to the printer and an environmental benefit at large in using less colored ink, b) use of more black affords better control of the process.
Boll reported work on separating color for more than four colorants (A color to colorant transformation for a seven ink process, SPIE Vol. 2170, pp. 108-118, 1994, The Society for Photo-Optical and Instrumentation Engineers, Bellingham, Wash.).
He describes the "Supergamut" for all seven colorants as a union of subgamuts formed by combinations of subsets of 4-at-a-time of the colorants. Because of the manner in which his subsets are modelled, the method severely limits flexibility in
performing GCR.
Descriptions of gamuts in colorimetric terms date at least to Neugebauer (The colorimetric effect of the selection of printing inks and photographic filters on the quality of multicolor reproductions, TAGA Proceedings, 1956, pp. 15-28.) The
first descriptions in the coordinates of one of the CIE's uniform color spaces are due to Gordon et al. (On the rendition of unprintable colors, TAGA Proceedings, 1987, pp. 186-195.) who extended the work to the first analysis of explicit gamut
operators--i.e., functions which map colors from an input gamut to correspondents in an output gamut.
A detailed review of requirements of and strategies for color systems calibration and control was published by Holub, et al. (Color systems calibration for Graphic Arts, Parts I and II, Input and output devices, J. Imag. Technol., 14: 47-60,
1988.) These papers cover four areas: a) the application of color measurement instrumentation to the calibration of devices, b) requirements for calorimetrically accurate image capture (imaging colorimetry,) c) development of rendering transformations
for 4-colorant devices and d) requirements for soft proofing.
Concerning the first area (a), U.S. Pat. No. 5,272,518, issued to Vincent, discloses a portable spectral calorimeter for performing system-wide calibrations. The main departure from the work of Holub et al., just cited, is in the specification
of a relatively low cost design based on a linearly variable spectral filter interposed between the object of measurement and a linear sensor array. Vincent also mentions applicability to insuring consistent color across a network, but does not discuss
how distributed calibration would be implemented. There is no provision for self-checking of calibration by Vincent's instrument nor provision for verification of calibration in its application.
U.S. Pat. No. 5,107,332, issued to Chan, and U.S. Pat. No. 5,185,673, issued to Sobol, disclose similar systems for performing closed-loop control of digital printers. Both Chan and Sobol share the following features: 1) They are oriented
toward relatively low quality, "desktop" devices, such as ink jet printers. 2) An important component in each system is a scanner, in particular, a flat-bed image digitizer. 3) The scanner and printing assembly are used as part of a closed system of
calibration. A standardized calibration form made by the printing system is scanned and "distortions" or deviations from the expected color values are used to generate correction coefficients used to improve renderings. Colorimetric calibration of the
scanner or print path to a device independent criterion in support of sharing of color data or proofing on external devices was not an objective. 4) No requirements are placed upon the spectral sensitivities of the scanner's RGB channel sensitivities.
This has ramifications for the viability of the method for sets of rendering colorants other than those used in the closed printing system, as explained below.
In Sobol, the color reproduction of the device is not modelled; rather the "distortions" are measured and used to drive compensatory changes in the actual image data, prior to rendering. In Chan, there appears to be a model of the device which
is modified by feedback to control rendering. However, colorimetric calibration for the purposes of building gamut descriptions in support of proofing relationships among devices is not disclosed.
Pertaining to item (b) of the Holub, et al. paper in J. Imaging Technology and to the foregoing patents, two articles are significant: 1) Gordon and Holub (On the use of linear transformations for scanner calibration, Color Research and
Application, 18: 218-219, 1993) and 2) Holub (Colorimetric aspects of image capture, IS&T's 48th Annual Conference Proceedings, The Society for Imaging Science and Technology, Arlington, Va.; May, 1995, pp. 449-451.) Taken together, these articles
demonstrate that, except when the spectral sensitivities of the sensor's channels are linear combinations of the spectral sensitivity functions of the three human receptors, the gamut of an artificial sensor will not be identical to that of a normal
human. In other words, the artificial sensor will be unable to distinguish colors that a human can distinguish. Another consequence is that there is generally no exact or simple mathematical transformation for mapping from sensor responses to human
responses, as there is when the linearity criterion set forth in this paragraph is satisfied by the artificial sensor.
To summarize the preceding paragraphs: The objective of measuring the colors of reproduction for the purpose of controlling them to a human perceptual criterion across a network of devices in which proofing and the negotiation of approval are
goals is best served when the image sensors are linear in the manner noted above.
Results of a colorimetric calibration of several printing presses were reported by Holub and Kearsley (Color to colorant conversions in a colorimetric separation system, SPIE vol. 1184, Neugebauer Memorial Seminar on Color Reproduction, pp.
24-35, 1989.) The purpose of the procedure was to enable workers upstream in the production process in a particular plant to be able to view images on video display devices, which images appeared substantially as they would in production, consistent with
the goals of Neugebauer in U.S. Pat. No. 2,790,844. Productivity was enhanced when design could be performed with awareness of the limitations of the production equipment. The problem was that the production equipment changed with time (even within a
production cycle) so that static calibration proved inadequate.
In U.S. Pat. No. 5,182,721, Kipphan et al. disclose a system for taking printed sheets and scanning specialized "color bars" at the margin of the sheets with a spectral colorimeter. Readings in CIELAB are compared to aim values and the color
errors so generated converted into corrections in ink density specifications. The correction signals are passed to the ink preset control panel and processed by the circuits which control the inking keys of the offset press. Operator override is
possible and is necessary when the colorimeter goes out of calibration, since it is not capable of calibration self-check. Although the unit generates data useable for statistical process control, the operator must be pro-active in sampling the press
run with sufficient regularity and awareness of printed sheet count in order to exploit the capability. The process is closed loop, but off-line and does not read image area of the printed page. Important information regarding color deviations within
the image area of the press sheet is lost by focussing on the color bars.
On page 5 of a periodical "Komori World News" are capsule descriptions of the Print Density Control System, which resembles the subject of Kipphan et al. Also described is the Print Quality Assessment System, which poises cameras over the press.
The latter is primarily oriented toward defect inspection and not toward on-line color monitoring and control.
Sodergard et al. and others (On-line control of the colour print quality guided by the digital page description, proceedings of the 22nd International Conference of Printing Research Institutes, Munich, Germany, 1993 and A system for inspecting
colour printing quality, TAGA Proceedings, 1995) describe a system for grabbing frames from the image area on a moving web for the purposes of controlling color, controlling registration and detecting defects. The application is in newspaper publishing. Stroboscopic illumination is employed to "freeze" frames of small areas of the printed page which are imaged with a CCD camera. The drawback of the Sodergard et al. system is that color control lacks the necessary precision for high quality color
reproduction.
Optical low pass filtering (descreening) technology relevant to the design of area sensors for imaging colorimetry is discussed in U.S. Pat. No. 4,987,496, issued to Greivenkamp, and Color dependent optical prefilter for the suppression of
aliasing artifacts, Applied Optics, 29: 676-684, 1990.)
Paul Shnitser (Spectrally adaptive acousto-optic tunable filter for fast imaging colorimetry, Abstract of Successful Phase I Proposal to U.S. Dept. of Commerce Small Business Innovatio | | |
