Hardware system architecture
Color monitors
Accurate, highly stable, calibrated color monitors: A true virtual proofing system requires both a very high base level of display design and performance and a calibration method for the RGB display system to ensure consistency between any two displays over the lifetime of the devices. (The word consistency means a nearly identical side-by-side or day-to-day match for any RGB image displayed.) Display to display consistency applies equally whether the displays are immediately adjacent or geographically remote from each other. In theory, this technology could be built into a display system. Acceptable display-to-display consistency has been demonstrated and is the subject of Dr. Christopher Edge’s U.S. Patent 6,775,633 entitled “Calibration Techniques for Imaging Devices”.
The suitable display must have a high degree of color uniformity across the entire screen. A desired goal is that any measured area exhibits <1 delta E from the center in a*b* for white and any value of gray, and <2 delta E from center L*.
An accurately calibrated system requires an accurate measurement device. Note that most low cost emissive colorimeters and spectrophotometers commonly vary from each other by +/-2 delta E or more. For some of these instruments, the device-to-device variability is much higher than the random noise of each device. Colorimeters or spectrophotometers for use in a virtual proof calibration system must be calibrated to a defined, highly reproducible standard such that the accuracy of the local measurement device relative to the standard is comparable to the error due to noise and random measurement error of the instrument . Calibrating the device to a reference standard is achieved and periodically re-confirmed through highly structured manufacturing procedures and references.
Viewer for hard copy
Since virtual proofing can periodically entail a side-by-side visual confirmation of colors displayed on the screen with those of CMYK hard copy proofs, an accurate illuminated viewing environment is necessary. Note that light booths historically have exhibited considerable variability with regard to color temperature, even among those indicating a standard viewing environment such as D50. However, since light booths are almost never placed side by side, this variability has not been a major issue. To a large degree, human visual perception adapts to the slight differences in color temperature and defines the current viewing condition as “white” when viewing images of snow, white lace, or other “white” back grounds.
A challenge for successful virtual proofing is a display device which simulates both the effects of the illuminant as well as the paper and color properties of the hard copy proof. When a display is adjacent to a small light box viewer, slight differences between the systems are readily apparent. This problem is handled by selecting viewing equipment which:
- Has uniform intensity, particularly from top to bottom,
- Contains light bulbs or fluorescent tubes that are consistent over time and from lot to lot.
Commercially available D50 (i.e., 5000K color temperature) fluorescent tubes from different manufacturers (or even different lots from the same supplier) can vary significantly from the design center point of 5000K. KPG evaluation has shown that narrow tolerances on these bulbs are necessary to insure color consistency among virtual proofing locations. Additionally, the illumination level in the hard copy viewing equipment must be reduced somewhat to yield an overall perceived brightness comparable to the display device. Note that color temperature or spectral characteristics cannot vary as illumination level is adjusted.
To ensure color, appearance and contrast of CRT display images are not compromised by stray light, a viewing environment which eliminates or nearly eliminates ambient light (e.g., a kiosk enclosed by an opaque curtain or walls) is necessary. Matchprint Virtual Proof - LCD systems (by virtue of their significantly higher brightness) enable viewing under less restrictive ambient light conditions, though the user’s perception of color can be impacted by ambient or incident light.
Standard PC, server, and network connectivity
At a minimum, the virtual proof system requires a local PC (note that “PC” can mean either Macintosh or Windows based computers) to process the image data file and render the images on the display system. The levels of connectivity can be as follows:
- Stand alone workstation - requires a suitable viewing environment (e.g., kiosk or subdued light area), calibrated display system, optional illuminated viewer for hard copy, and (if required) colorimetric measurement device for the viewer and PC system. All software and color control resides on the PC.
- Interconnected workstations – essentially this includes two or more of the previously mentioned stand-alone workstations supported by some form of network infrastructure for ease of file transfer.
- Server-based solution - multiple display systems interconnected with a server acting as the central, authoritative intermediary. In this scenario, the software components for processing files and converting color data can be distributed. The generalized conversions, including RIP’ing and conversion to standard RGB space can be best performed at the server. The non-general conversions can be performed at each local system using software in the form of a browser plug-in or native viewing client software and other local applications.
This third example would address issues such as image enhancement or CPU-intensive image processing for a particular display device and conversion from standard RGB to local RGB for that display device.
Software system architecture
The server-based solution, which is the most complex and powerful system for remote virtual proofing, is described here. The simpler forms of this system mentioned previously would be similar in many respects, except that the core processing software would reside locally on one or more systems rather than separating software and image functions among server and clients.
The following subsection, “Soft Proofing - Database for storing page image files and metadata,” describes the basic infrastructure of a system that implements soft proofing. This technology exists today in KPG’s line of RealTimeProof products.
The next subsection, “Specific requirements for virtual remote proofing,” describes the enhancement, additions and modifications required to expand a soft proofing environment to achieve virtual proofing.
Soft Proofing - Database for storing page image files and metadata
Most image databases provide standard features such as the ability to search by various fields such as job name, image name, customer name, date, etc., as well as the ability to view thumbnail versions of each. By design, image databases generally permit metadata (e.g., annotations) or data associated with each page to be archived and retrieved easily. Both KPG’s RealTimeProof and Matchprint Virtual Proofing System products provide a professional infrastructure for the image database and access via Web browsers.
Upload/download: For a distributed system, a simple means of transferring high-resolution image files to and from the server is required. Most databases permit metadata to be associated and transferred simultaneously with the image file itself at the time of upload. In the case of KPG’s RealTimeProof and Matchprint Virtual Proofing System, metadata are associated with the destination “folders” that can be designated as workspaces, projects, files, versions, etc. In general, the central requirement is that metadata be easily associated and retrieved.
Page file interpreter: The local or remote system must have the means to interpret and process the necessary image file formats. Most commonly, some form of raster image processor (RIP) is incorporated into the system. Well-known examples of this are the CPSI PostScript RIP software module by Adobe Systems and the Harlequin RIP from Global Graphics. CMYK image data and vector commands are converted to a CMYK bit map (“rasterized” file). Alternatively, the image data and vector commands can be converted to CT/LW format if systems at the remote end are capable of properly interpreting and displaying files in this format.
Processing parameters: Each file or job processed by this system must have an associated set of parameters. Some soft proofing systems allow the user to set resolution as a parameter while others allow color CMYK simulation to be set. Note that most high-end CMYK proofing systems allow both.
Examples for setting processing parameters in soft proofing systems as well as in high-end digital proofing systems are:
- Provide a menu of predetermined sets of parameters such as resolution, color target, etc. (e.g., output CMYK controllers for Kodak Approval and KPG Matchprint Digital Halftone) in the print window.
- Provide a setup window for resolution for each hot folder into which files or jobs are loaded or uploaded (e.g., KPG RealTimeProof or Matchprint Virtual Proof).
Once processing parameters are set, a means of verifying the parameters after the fact must be provided. In the case of hard copy output, it is useful to automatically provide the setup conditions on the border of the proof (often alongside a color bar or other quality checking element). In the case of files in a database or server used for soft proofing, metadata must be associated with each file to provide information on the parameters used for processing and displaying the file.
The resulting data are compressed and transmitted from server to the remote soft proofing location. Of course, truly loss-less compression is highly desirable, but often difficult to achieve.
Viewing of images from multiple nodes on the network: If the mechanism used to perform virtual proofing is a server-based model, then a means of viewing the image must be provided. Since general-purpose Internet browsers are not optimized to view high-resolution images, the solution provided by KPG RealTimeProof and Matchprint Virtual Proofing System is convenient and effective. This approach utilizes data compression technology to send only the necessary image data at screen resolution, which can be supplemented (essentially transparently to the viewer) from the server as users zoom in and out of a high-resolution image file.
The value of the server-based approach is that multiple users can simultaneously access the same or different files at the same time from the same server database, even if they are geographically separated. The central server is not used to display imagery – rather all images are displayed remotely via browser and plug-in modules or native client viewing software.
Password protected annotations: Both the KPG RealTimeProof and Adobe Acrobat systems offer the feature of password protection combined with annotations. Each participant making annotations has both a unique password and an on-screen color for the annotations performed. Each color-coded annotation has a label with the annotator’s name to identify the source of comments or image mark-ups.
Color management: Both the KPG RealTimeProof and Adobe Acrobat systems offer a degree of color management support. The Acrobat system allows each PDF file to be tagged with a CMYK ICC profile. The Acrobat system renders each CMYK pixel to the RGB display by communicating with the color management system in the operating system (e.g., ColorSync, ICM v2.0, etc.) to determine the default local RGB display profile and to convert the CMYK data to RGB pixels by means of CMYK and local RGB profile.
Specific requirements for remote virtual proofing
The previous subsection, “Database for storing page image files and metadata,” describes the state of soft proofing technologies today. In many cases, soft proofing is adequate in itself without need for the additional critical color performance enabled by virtual proofing. Of course, virtual proofing encompasses all the requirements and capabilities of soft proofing, with the following additional requirements:
Color Administrator: Each virtual proofing site should have a designated color administrator and a system administrator (who can be the same person). The color administrator must have the flexibility to adjust the unique process parameters described in the following sections (in particular, color simulation targets). Participants without administrator privileges are not able to view or change the options available to the administrator. This ensures all participants will see an image or page in the same way with the same degree of color precision.
Selection of color target simulation: Each page, project, or group of jobs needs a designated CMYK proof simulation. The administrator for the project must choose which ICC profile is associated with the each viewed image or page.
For remote virtual proofing using the server-based approach, KPG’s Matchprint Virtual Proofing System has added CMYK simulation (i.e., the CMYK ICC profile) as a process parameter for the job, selected by the administrator using password access. Non-administrators can view and confirm which color simulation was chosen for the job, but are not allowed to modify the choice. This arrangement ensures flexibility and the assurance of consistent color viewing among sites which can be geographically separated.
Optimized color management for print CMYK to display RGB: The profile and CMYK to local display RGB conversion must include corrections to XYZ and is the subject of KPG’s published U.S. patent application entitled “Correction Techniques for Soft Proofing”. This patent application describes correction of XYZ data measured on displays to correlate with XYZ data measured on CMYK hard copy.
When accurate CMYK profiling is combined with the RGB transformations described in these invention records and with accurate display calibration, critical color viewing (i.e., virtual proofing) is achieved.
Image enhancement for screen viewing: In order to provide visual contrast and detail, RGB image data rendered on the display device should be dynamically enhanced. Because display screen resolution is usually limited to approximately 100 dpi (for both CRT and LCD monitors), image enhancement is needed to achieve appearance comparable to the corresponding hard copy proof. As a higher zoom ratio is used, less enhancement (or none) is required since the rendering of the image detail is now visually equivalent between hard copy and virtual proof.
Control of time between calibrations: The display system (display hardware plus calibration software/hardware) must employ accurate calibration technology as described in KPG’s U.S. Patent 6,775,633 “Calibration Techniques for Imaging Devices”.
One of the difficulties in remote hard copy proofing is the challenge of getting remote proofing devices to match each other or a common standard. There is a further challenge to enforce periodic calibration to ensure a continuing satisfactory match between multiple proofing systems.
Matchprint Virtual Proof enforces a 24-hour calibration interval to ensure trustable, consistent color proof to proof, day to day and location to location.
Critical color mode indicator: Systems implementing virtual proofing can also be used in an adjunct, non-color critical mode.
An example could be an advertising agency professional who wishes to view images for content from a laptop computer while traveling. In this case, the capabilities of soft proofing (but not virtual proofing) are available from the system, though critical color viewing is missing. The browser plug-in or viewing application can recognize that critical color viewing capabilities are not currently enabled. Although images can be accessed, displayed and annotated, critical color judgments are not possible. Optimally, an indicator to advise the user that the local viewing system is not currently in critical color mode would be present. An example might be a banner or mark on or adjacent to the image being viewed. In a similar fashion, all annotations performed while the system is in non-critical color mode should indicate (for all viewers) they were made in non-color mode.
Indicator of measurement CIELAB values: In order to relate color of virtual proof images to quality assurance and process control systems already in place, an “eye-dropper” tool that supports CIELab values is highly useful.
The virtual proofing system described in this document would best provide an “eye-dropper” tool with the characteristics indicated below. KPG’s Matchprint Virtual Proof provides an enhanced eye-dropper tool to meet these requirements for virtual proofing:
- Displays the original CMYK values when the tool is pointing toward a particular pixel location in the displayed image.
- Displays an accurate prediction of the measured value of L*a*b* along with the value of CMYK. This value should be consistent with a particular color measurement device such as a Gretag SPM50 at D50 illumination, 2-degree observer and should be calculated and displayed to an accuracy of one decimal place (e.g., it should read L*a*b* = (97.5, 0.3, -0.2) rather than L*a*b* = (98, 0, 0)).
The reason for displaying the predicted color measurement value is to permit a press operator or other user to measure a specific color to confirm whether it matches the L*a*b* color value intended in the original image file.
Requirement for system warm-up: A final quality assurance mechanism required in a virtual proofing system is adequate warm-up time. Laboratory studies have shown that both CRT and LCD displays often emit more light and different chromatic characteristics when first switched on, but equilibrate and remain very stable for long periods of time once temperature stability is achieved.
Consider a display set to a particular target white point value (for example, an illumination white point of D50). If the display is switched off, allowed to cool and then switched back on, brightness can typically be initially higher by 5 or more delta units in L*.
To address this issue, the virtual proofing system must include a local startup application that detects whether the system has been shut down and rebooted. Off time can be used to determine the needed warm-up time.
There are several ways of preventing premature use of the system using this startup application:
- The startup application could create a time stamp file that is read by the browser or image viewer application. The viewer application would then preclude image viewing in “color critical” mode until the appropriate warm-up time has transpired.
- The calibration application could inhibit calibration until warm-up has occurred.
- The startup application could launch the calibration application, which can in turn alert the user that warm-up is required. The user can leave the area and return after the remaining warm-up time indicated on-screen has transpired. On returning, the user would find an on-screen prompt to follow the calibration procedure for each attached color-critical display. The system is then “good to go”. This approach is implemented in Matchprint Virtual Proof.
One possible approach for connecting the display to the computer is to disable the on/off switch on the display itself so that is it always in the “on” state. Monitor and PC are then connected to a single power source (with the monitor connected to an auxiliary outlet on the PC).
As an extra precaution, the startup application can run continuously in the background and periodically communicate with the display via protocols such as VESA specified DDC/CI or USB to ensure the display is in fact on and functioning.
