|Publication number||USRE42089 E1|
|Application number||US 11/347,787|
|Publication date||Feb 1, 2011|
|Filing date||Feb 3, 2006|
|Priority date||Mar 1, 2000|
|Also published as||US6686953|
|Publication number||11347787, 347787, US RE42089 E1, US RE42089E1, US-E1-RE42089, USRE42089 E1, USRE42089E1|
|Original Assignee||Joseph Holmes|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Non-Patent Citations (2), Referenced by (13), Classifications (12), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims the benefit of U.S. Provisional Application No. 60/185,969, filed Mar. 1, 2000.
This application includes a one-disk CD-R Appendix, the full contents of which are incorporated by reference herein. The disk contains the following files, listed by file name, creation date and file size in bytes: “ColorBlind Video Startup” created Jun. 22, 1999, 100K; “ColorBlind_Video_Startup.exe” created Sep. 16, 1999, 160K; “Complete_Text.txt” created Feb. 22, 2001, 40K; “Contents_of_CD.txt” created Feb. 22, 2001, 4K; “Lab Color Space Profile” created Aug. 4, 1997, 7K; “Prove it!” created Aug. 27, 1999, 4,116K; “Prove it! Installer” created Jul. 20, 1999, 2,251K; “Prove it_Beta.exe” created Jul. 16, 1999, 11,555K; and “Prove_it_Setup.exe” created Nov. 18, 1999, 7,432K.
This invention relates to the calibration of computer monitor displays, more particularly to a method of monitor calibration using target set screen displays and no supplemental light-measuring instrument.
The invention makes possible a new and superior method for calibrating visual display devices. When using computer systems to view, control and/or print graphics or photographs, it is often critically important that the computer displays be calibrated to a chosen standard condition.
Once a display (also called a “monitor,” particularly when referring to computer displays) is calibrated to a known state, techniques can then be employed to cause the display to accurately simulate the appearance of an image or graphic as it appears on, or when printed from, or as seen by another digital imaging device. Absent correct calibration, such simulations become inaccurate in like degree. The capability for accurate simulation is of tremendous importance to digital imaging in general. Therefore, display calibration is an important issue for many people.
The current invention can also be applied to the calibration of television set displays and all other types of analog or digital displays having many levels of intensity in each color channel—typically three channels, one each for red, green and blue.
The current invention is embodied in commercially available software for display calibration called ColorBlind Prove it! from ITEC Color Solutions, San Diego, Calif. The software exists in both Macintosh and Windows versions.
The prior art makes visual (instrumentless) calibration possible, but in no case does it teach a complete method for obtaining high-quality calibration. The prior art furthermore fails to address a variety of significant problems inherent in display calibration.
The Knoll Gamma version 2.0 application (see FIG. 2), published by Adobe Systems Incorporated of San Jose, Calif., provides an incomplete system, which is capable of visual calibration of low to moderate quality.
U.S. Pat. No. 5,298,993 by Albert Edgar and James Kasson teaches certain useful principles of visual calibration, including the use of targets.
U.S. Pat. No. 5,638,117 by Peter Engledrum and William Hilliard uses areas of parallel lines in a visual characterization process.
The Default Calibrator application from Apple Computer, Inc. of Cupertino, Calif, (see FIG. 3), which is a part of the program entitled ColorSync 2.5 and later, teaches a crude method for display calibration. This is prior art for only that part of the present invention called Gray Balance Method One (see
The image file “Gamma_Estimation” (see FIG. 4), from Candela, Ltd. (now Pictographics International Corporation) of Burnsville, Minn., teaches a method of identifying the current overall gamma from a broad range of possible overall gammas with the use of a gradient.
The image file “Current Gamma” (see FIG. 5), published by Adobe Systems Incorporated of San Jose, Calif., in the program entitled PageMaker 6.5, teaches another method to identify the current overall gamma from a broad range of possible overall gammas.
The image file “Gamma 1.8.tif” (see FIG. 6), also included with Adobe PageMaker version 6.5 software, from Adobe Systems Incorporated of San Jose, Calif., together with Knoll Gamma 2.0.1 and an explanatory text file entitled “Gamma Read Me,” teaches an improvement in the accuracy of the verification of a fixed gamma. However, “Gamma 1.8.tif” has significant limitations with regard to its ability to reliably reveal the correct gamma within each of several distinct subsegments of the tone scale because of the way in which tones were chosen to construct the target and the limited number of sub-targets used. “Gamma 1.8.tif” looks at six regions of the tone scale which all overlap a great deal and therefore can obscure the true nature of any observed departure from the gamma curve being sought, thus hindering efficient adjustment to achieve ideal gamma 1.8 tonality. Like all visual gamma adjustment targets, “Gamma 1.8.tif” also cannot be used to verify conformity of any display to other gamma curves, such as 2.2. The target of
The standard condition to which a display is calibrated is defined partially by the inherent colors, or chromaticities, of the display's purest red, green and blue colors. In the case of a common CRT (cathode ray tube) type display, these colors of red, green and blue are determined primarily by the colors, or chromaticities, of the phosphors used in the tube. In the case of a flat panel display (FPD), these colors are determined by the mechanism of the display which created the primary colors, which is always different from that of CRTs.
The three other principal aspects of display calibration are not fixed by the nature of the hardware itself (that is, the display or monitor). These three other aspects of display calibration are:
1) Calibrating the white point, i.e. the color of the display's white (independent of its brightness), also known as its Hue and Chroma, also known as its x,y coordinates from the X+Y+Z=1 plane of the CIE XYZ color space;
2) Calibrating its gray balance so that each gray that it displays—from black (the darkest color the display can display in a given state of calibration) all the way to white (the brightest color the display can display in a given state of calibration)—has the same color as the white (also known as the same Hue and Chroma, also known as the same x,y coordinates); and
3) Calibrating the “gamma” or tone curve of the display so that the way it displays the full range of input values from black to white follows the desired progression of luminous intensities. Typically, the full range of digital input values sent from the computer's video circuitry to the display is the range from RGB (0, 0, 0) to RGB (255, 255, 255) where each color channel has a range of 256 (two to the eighth power) values. Gamma curves are a subset of all possible tone curves and have certain mathematical properties. Displays generally need to be calibrated to a gamma curve to function properly as a calibrated display in a color managed system of imaging devices.
Two other aspects of display calibration are:
1) Calibration of absolute white intensity; and
2) Calibration of absolute black intensity.
In addition to the aspects of display calibration mentioned above, there are particular adjustments of so-called hardware controls, such as the Brightness, Contrast, Color, Bias, and Gain controls on CRT displays, which adjustments affect and/or are part of the processes of calibration mentioned above. The affected processes include calibration of the white point, the gray balance, the gamma, the absolute while intensity, and the absolute black intensity.
Flat panel displays (FPDs) exhibit different natural tone curves and white points from CRTs, and typically have different kinds of hardware controls, which affect the appearance of data displayed by these displays. FPDs also require calibration, for essentially the same reasons that CRT displays do.
Display calibration techniques in the prior art can be broken down into two main categories of calibration.
One such category is instrumented calibration, where many steps in the whole process of calibrating a display are carried out by attaching a photometer, colorimeter or spectrophotometer to the surface of the display. The attendant display calibration software causes a variety of colors to be displayed by the display. Color measurement readings are taken by the colorimeter (or other instrument), and the software utilizes these readings to facilitate most or all aspects of the calibration process.
The other main category of display calibration is visual calibration, which is accomplished without a photometer, colorimeter, spectrophotometer, or spectroradiometer for the measurement of the display and instead relies on a variety of techniques, methods and processes to accomplish essentially the same things as the instrumented calibration processes. Central to the visual calibration processes are visual targets which provide visual feedback regarding the state of the display, which then allow the user to make informed adjustments to the display using both the built-in display controls and visual calibration software. The visual calibration software modifies the video card LookUp Tables (LUTs) in response to user adjustments of sliders and like on-screen software controls. The software may also perform a variety of related functions such as automated sequential presentation of the visual calibration targets and their attendant tool interfaces, presentation of user instructions, and user education in relevant matters.
Each type of calibration, instrumented and visual, can be implemented with widely varying degrees of success, and the visual methods are potentially and generally more economical due to the lack of the need for a light measuring instrument. The present application concerns itself primarily with visual calibration and not with instrumented calibration.
Both types of calibration can be accomplished with a myriad of variations in the exact details of implementation and methods, but the prior art in visual calibration is inadequate in several key respects, which frequently makes it inadequate for high-quality work. This is due to limitations on its ability to detect and to overcome inherent problems with the nature of the display hardware's behavior and also due to limitations in the ability of the user to have confidence in the accuracy of a calibrated state achieved by prior art visual calibration methods. These problems are solved by the present invention. To a significant degree, the same problem of user uncertainty about the accuracy achieved by instrumented calibration methods also exists and is solved by the present invention.
The present invention relates to a novel process of visual calibration of computer displays. The current invention addresses all of the significant shortcomings of the prior art and, for the first time, provides a method for complete, high quality calibration of displays, particularly all computer displays. It successfully addresses each of the five specific problems in the prior art.
First, the current invention provides an objective visual method for determining the precisely optimal brightness setting for CRT displays, which method is also applicable to the setting of the “Black Level” control on some FPDs. Accurate brightness or black level setting or its equivalent tone control in the video card LUTs is a requirement for achieving any of the necessary standard tone curves. Prior art solutions to setting the brightness gave vague and subjective assessments of the shadow tonalities, which derive from the range of possible brightness settings. By finding a certain tonal relationship between two adjacent or slightly overlapping tone regions close to black, the desired curve shape can be found, by user adjustment of the brightness control while looking at the target, which is appropriate to the level of flare in their system. The system includes at least the hardware, the video LUTs and the viewing environment.
Second, the current invention provides a precise method to visually determine conformity of a display's tonality to a given standard tone curve, for example a gamma 1.8 curve, which is embodied in one of the preferred calibration target sets of the invention. The current invention clearly reveals conformity with the standard tone curve for each relevant sub-region of the tone scale, thus assuring a visual tone match between the displayed image and the image data when simulations are performed (see FIGS. 13-17). The prior art methods provide only limited ability to verify the actual tonality of a display and its conformity to a standard tone curve, because the methods of the prior art do not reveal the tonality of each relevant subsection of the tone curve. Also, some prior art methods rely on a gridded or halftone pattern of mixed dark and light tones instead of a pattern of alternating horizontal lines, rendering them essentially useless for reliable tone calibration of CRT displays because of limitations of the electronics of CRT displays. The present invention relies primarily on patterns of mixed light and dark tones, which consist of horizontal lines containing only one value, for gamma or tonality assessment. The invention relies primarily on such lined patterns, which complement the nature of currently ubiquitous display hardware used for imaging. The invention also makes it feasible to implement a solution which, in the visual calibration targets, includes the mixing of pixels of different values in individual, horizontal rows of pixels, especially in the Gray Balance Method Two procedure and target. Flat panel displays, which are not in widespread use for imaging, are likely to be much better suited to use with such mixed pixel values in individual horizontal rows of pixels in the visual calibration targets than are CRT displays of the present day. The invention also makes it possible to combine the gamma and gray balance adjustment functions into a single target set which relies on a combination of patterns such as described below in the Gray Balance Method Two targets (see FIGS. 9 and 23).
Third, the current invention is unique in meeting the need to sense and therefore be able to control and to verify the correct gray balance of the entire tone scale of the display. A process of visual comparison of the color (Chroma and Hue, or x,y coordinates) of each major region of the tone scale to that of the white is provided. The prior art at best only facilitated visual assessment of the gray balance of a portion of the tone scale. Side-by-side comparison of most of the colors in question is used, with each seen at the same lightness, instead of highly disparate lightnesses. Most importantly, a pattern of alternating tones arranged in lines is used which extends the ability to view adjacent, same-lightness gray regions not only to the midtones or upper midtones, but to the three-quartertone and quartertone regions of the tone scale as well. A second preferred embodiment extends this reach to a darker value still, essentially covering the entire tone scale. The three-quartertone, midtone and quartertone regions are not merely the regions of 25%, 50% and 75% luminance, rather, they are the more widely spaced regions of roughly 25%, 50% and 75% of the zero to 255 RGB input values scale of a display calibrated to a common gamma standard, such as 1.8 or 2.2 (see
The second preferred embodiment of the gray balance capability of the invention (see
The same extension of the user's ability to see into the three-quartertones and quartertones provided by the current invention as seen in
Fourth, to solve the problem of verifying the similarity of the tone curve in the display profile and that of the actual calibration, a new method is presented in the current invention. By converting a preferred RGB gamma target, such as that discussed above and shown in
Fifth, the above features of the current invention make the use of a visual calibration method based on most or all of the above features a practical, realistic, economical, and highly effective solution to high-quality display calibration. Even the sum of all prior art does not teach a complete solution to high-quality visual display calibration, and so visual calibration has always been relegated to a strictly second-class role. Now visual calibration can be of such high quality as to be the best method to verify instrumented calibration success or failure under most circumstances.
To complete the process of making a display ready for imaging work, a profile must be made or obtained which complements the calibration. When instrumented display calibration is performed, it is typical of software used for this purpose to also make a profile from information about the display obtained by measurement. For the visual approach to work well, the remaining information which describes the correct absolute color of the display white and the display's pure red, green and blue must be obtained. Fortunately, all four of these numerical values can readily be obtained with sufficient precision to complete the process of readying a display for performing high-quality simulations and color matching, without the necessity of user measurement of the display.
Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawing, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for illustration and description only and is not intended as a definition of the limits of the invention. The various features of novelty that characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified.
There thus has been broadly outlined the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form additional subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based readily may be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the invention of this application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.
Certain terminology and derivations thereof may be used in the following description for convenience in reference only, and will not be limiting. For example, words such as “upward,” “downward,” “left,” and “right” would refer to directions in the drawings to which reference is made unless otherwise stated. Similarly, words such as “inward” and “outward” would refer to directions toward and away from, respectively, the geometric center of a device or area and designated parts thereof. References in the singular tense include the plural, and vice versa, unless otherwise noted.
BRIEF DESCRIPTION OF THE DRAWINGS
Gray scales are old and their use well known. One version among many, this one drawn and used by the inventor of the present invention, is shown in FIG. 1. In the context of this invention, it is true that a simple gray scale can provide crude visual feedback for gray balance and tone adjustment. However, the following novel and improved targets and methods comprise a unitary procedure for full calibration of a monitor and like visual displays.
Referring to the Detailed Flow Chart of
One or more means of obtaining precise display gamma from amongst existing means must be utilized as a standard, from which the calibration thus derived can be “cloned” by making visual calibration targets which appear correctly when viewed on a display with the correct calibration state.
A group of as many as six, and a minimum of three, different kinds of visual calibration targets preferably are used, unless a target is drawn which combines the gray balance and gamma functions, in which case as few as two uncombined targets could be used. The latter possibility is within the scope of the invention, but may not yield the most precise results.
The target designs are digital image data with fixed RGB or grayscale values. When viewed on a perfectly calibrated display, they will exhibit particular tonal values, and in most cases are appropriate for only one desired gamma, such as gamma 1.8 or gamma 2.2. In addition to the targets, visual calibration software offers an appropriate set of tools and a user interface for the adjustment of the display. To calibrate a display, the user makes each of the two to six targets appear correctly when displayed on the display which is being calibrated, by using the targets and the tools provided by the software.
All of the calibration targets should be presented with appropriate surrounding colors on the screen so as to maximize their accuracy and so as to optimize the calibration for those circumstances which best typify the nature of the screen while critical work is being performed. For example, it is usually best to use grays of slightly to moderately sub-dued lightness around the targets when designing a software interface for use with CRT displays.
The Brightness target (
Referring also to the tone curve 2280 of
Note that the four dark gray values used in the brightness target or sub-target are not of sequentially increasing RGB value in the sample given. In
Three separate preferred embodiments of the brightness target portion of the current invention are shown in
Each of the three Brightness targets presented in
The following are two examples of values used in preferred embodiments of the Brightness target of the invention. The RGB values in the gamma 1.8 target for ambient lighting of about 200 lux,
The Contrast target is not mandatory for CRTs but is recommended. It is mandatory for liquid crystal flat panel displays (LCDs). Such a target is shown in the middle section of
Two sources of tonal separation failure in each of CRTs and LCDs are known which this target can reveal. The first is setting the contrast control too high. Most CRT displays do not fail to separate the highlight tones well when set to maximum contrast, but some do. Many if not most LCDs fail to separate the highlight tones correctly when wet to maximum contrast. The second is using 16-bit (or fewer) color display, instead of 24-bit or 32-bit color display, so that only 32 (or fewer) shades of gray can be displayed. The typical number of grays which can be displayed is 256, as when using 24-bit or 32-bit color with a CRT, and this number of grays facilitates precision results with all of the targets of the present invention, which results are otherwise impossible, except by dithering the lesser number of colors to simulate the larger number.
The Gamma target, which is mandatory, works only for the chosen gamma, for example gamma 1.8 or 2.2, but not both. Any preferred embodiment of the gamma target of the present invention must represent a full range of different tonal sub-sections of the tone scale of the display. Such a preferred embodiment could include a gray balance target designed to provide adequate control over both gray balance and gamma by virtue of having enough sub-targets and having those sub-targets reveal color and tonality at input values as needed to accomplish good calibration.
When the display is correctly calibrated to the target tone curve, the solid field region of the sub-target thus designed must appear to blend by having the same lightness as the adjacent lined region. The hue and chroma of the solid region will appear to be a mismatch in this target if the gray balance is incorrect. Refer to the input values shown in
Compare the numbered sub-targets in
Inspection of the curve segments thus identified will reveal the way in which this preferred embodiment of the gamma target of the invention separately characterizes each of the many most significant regions of the tone curve of the display. Note that the shadow region of the tone scale of a CRT display is much more likely to exhibit significant departures from the optimal curve shape than other regions, and that the target therefore emphasizes inspection of this region. Unlike the prior art target “Gamma 1.8.tif” (see FIG. 6), many of the sub-targets of this design are unaffected by such incorrect tonality in the deep shadows and thus, taken together, the eight sub targets of this target reveal the overall tonality clearly to facilitate efficient adjustment for correction. When the eight sub-targets of this preferred embodiment of the gamma target of the invention all appear to blend perfectly, as after successful calibration, the display can be known to have essentially perfect tonality.
The software which gives the user control over the display's tonality (as distinct from the on-screen user controls built into the display itself), does so by giving the user control over the LookUp Tables in the video card of the computer. These simple, two-dimensional tables, one for each of red, green and blue, have one output value for each input value. As the user makes gamma adjustments, the tables are altered according to mathematical functions, such as gamma functions or bezier curve functions, which cause the end points of curves representing the tables to remain unchanged, but various sections of the curves to be moved up or down. These adjustments are not gray balance adjustments. Thus, they have the same effect on all three color channels (red, green and blue) and maintain constant gray balance as adjustments are being made. The output values in the LUTs corresponding to inputs of 255 or zero are always unchanged (being 255 or zero, respectively).
A preferred tool and interface design, namely, adjustment tool 2212 having software sliders of commonly available form, namely, overall slider 2213, darks slider 2214, and lights slider 2215 is shown at the bottom of
Preferred embodiments of the Gray Balance Method Two target 2000 and Gray Balance Method One target 2100 are shown in FIGS. 9 and
The Gray Balance Method Two target 2000, shown in
First, the target 2000 contains a means to match the color of the display's white to the midtone color. This is made by creating in midtone sub-target 2024 (
Second, the target 2000 contains a means to match the color of the three-quartertone region (or similar dark region) to that of the midtone. In this preferred embodiment, this is made by creating in three-quartertone sub-target 2004 a blended region 2005 of two rows of black pixels (for a total height of preferably two black pixels) to one row of midtone color (for a preferred height of one midtone pixel) (FIG. 9A). In this example, the midtone color rows 2001 that are mixed with the double rows of black pixels 2002 have the RGB value (118, 118, 118) and the gray solid or field 2003, used as the three-quartertone gray solid, is RGB (63, 63, 63).
Third, the target 2000 contains a means to match the color of the quartertone region (or similar light region) to the color of the display's white by creating in quartertone sub target 2045 a blended region 2046 comprising one row of one-pixel-high white, RGB (255, 255, 255), one row of a very bright one-pixel-high gray color such as RGB (225, 225, 225), and one row of one-pixel-high black (0, 0, 0), in an alternating repeating pattern. The very bright gray color cannot be used in this manner with LCDs because of the typically strong gray balance shift that occurs naturally in the bright end of the tone scale of such displays, requiring a different blending of tones for such a light region sub-target. Referring to
The basis of the method of operation of the Gray Balance Method Two target 2000 of
The blend of midtone value and black (found in the three-quartertone sub target 2004, see
The blend of white, near white (bright gray), and black (found in the quartertone sub-target 2045, see
When all three sub targets look correct, A=B=C=D and the entire tone scale is very closely gray balanced and/or gamma adjusted, with the remaining possible exception of the black itself and colors very near black, in which colors errors are most difficult to see anyway. Accordingly, gray balance calibration and/or gamma adjustment is accomplished using the Gray Balance Method Two target 2000 when, after manipulation of the adjustment tool 2010 and/or a gamma adjustment tool such as the sliders 2213, 2214 and 2215, the solids 2003, 2023, and 2044 appear to the user to blend in with (disappearing as separate entities) the surrounding blended regions 2005, 2025 and 2046, respectively. When such blending occurs and calibration is achieved, a blended-region-and-solid target is said to appear fully blended visually.
By using an alternating pattern of three rows per cycle for the blended regions of the Method Two Gray Balance target, sufficient smoothness is maintained so as to allow the user to squint and/or move back from the display in order to smoothly perceive the hue and chroma of the blended region. This facilitates comparison with the hue and chroma of the adjacent solid region for careful adjustment of the gray balance and/or gamma of each, major region of the tone scale with the tool interface provided. This avoids the need to use alternating pixel values within a single row of pixels, while maintaining adequate smoothness to support all displays typical of this era, including predominantly CRT monitors. As displays increase in resolution, the tolerance for coarse patterns of alternating lines increases: Also, as flat panel displays begin to be used for imaging work, the limitations imposed by display technology on the preferred target designs of the invention change. It is highly probable that such changes will include a diminution of problems associated with using alternating pixel values in a given horizontal row. Thus other preferred embodiments of the invention may include targets similar to the one shown in
The Gray Balance Method Two target includes (see the top of
With the inclusion of the gray scale 2006 as described to the Gray Balance Method Two target 2000, a complete solution is provided for attaining perceptually excellent gray balance for the entire tone scale, which is one of the key goals of display calibration.
A second preferred embodiment of the gray balance features of the present invention is referred to as the Gray Balance Method Two, Increased Precision target 2300 (see
The light or quartertone sub-target 2304 comprises a light or quartertone solid 2307 adjacent to a blended region 2306 having a mixture of midtone pixels and light (preferably white) pixels. Preferably, the two preferred tones of pixels are formed into alternating horizontal lines, as in similar light sub-targets.
The midtone sub-target 2303 comprises a midtone solid 2309 adjacent to a blended region 2308 having a mixture of substantially black pixels and light (preferably white) pixels. Preferably the two tones are formed into alternating horizontal lines, as before.
The dark or three-quartertone sub-target 2302 comprises a dark or three-quartertone solid 2311 adjacent to a blended region 2310 having a mixture of substantially black and white pixels. Preferably the white pixels are grouped into dot-like patterns surrounded by a background of the substantially black pixels.
The very dark or five-sixths-tone sub-target 2301 comprises a very dark or five-sixths-tone solid 2313 adjacent to a blended region 2312 having a mixture of substantially black and midtone pixels. Preferably the midtone pixels are grouped into dot-like patterns surrounded by a background of the substantially black pixels.
By using a pattern of between one and three white pixels per contiguous group of about twelve total pixels, with the rest being black, a uniform pattern is made which blends with sufficient visual smoothness on higher-resolution displays (see the dotted blended region 2310 of the three-quartertone sub-target 2302 of FIG. 25 and FIG. 23). This same pattern of pixels, which may be said to comprise a uniform pattern of small fields of a light tone surrounded by a larger background of a dark tone, yields an apparent brightness at a gamma 2.2 calibration state, which approximately matches that of the three-quartertone value of (65, 65, 65). Because the color of this blend is coming approximately 92% from the few white pixels and approximately 8% from the black pixels, the blended region 2310, when compared with a dark or three-quartertone solid 2311 of a gray solid color of (65, 65, 65) enables direct matching from the white to the three-quartertone, thus doubling the matching precision. A=D directly, instead of A=C=D.
By further modifying the last uniform repeating pattern so as to blend between one and three pixels of the value from the midtone sub-target's gray solid instead of white, which in this case is (155, 155, 155), per contiguous group of about twelve total pixels, with the remainder of the group being black pixels, a fourth sub-target is created which allows indirect gray balance matching of a tone which is considerably darker than the three-quartertone, for example, about five-sixths-tone. In
By taking the tendency of flat panel displays to have strong, light tone color crossovers into account while making the quartertone sub-target of a Gray Balance Method Two target of the invention, a different result is obtained, which is shown as the quartertone or light tone sub-target 2304 in FIG. 23. This sub-target is made with a repeating pattern of one row of white pixels plus two rows of midtone pixels (each midtone row being the same height as one white row) forming the blended region 2306 and with a light solid 2307. In other variations of this target, there could be one row of black, one of midtone and one of white, as in FIG. 9.
Once the midtone sub-target 2303 has been used to match the color of its gray midtone solid 2309 to the hue and chroma of the white, the combination of white and midtone colors used in sub-target 2304 then makes it possible to achieve enhanced accuracy for the light quartertone solid 2307 gray balancing. The values chosen for the quartertone sub target 2045 of the first embodiment of the Gray Balance Method Two (
Note that the Gray Balance Method Two and Gray Balance Method One targets of the present invention can also be used as gamma adjustment targets if the tools provided in the software interface allow it, but they are not as well-suited for this purpose as the preferred Gamma targets of the invention unless the number of sub-targets in them is increased to between five and eight.
A preferred software control or interface tool 2010 is shown at the bottom of FIG. 9. The overall button 2012 of this tool allows the user to make a full-scale gray balance adjustment to cause the center sub-target (the midtone sub-target 2024) to blend in hue and chroma. This adjustment affects the entire tone scale, with the greatest effect in approximately the midtone (127, 127, 127) and zero effect at white and at black. This effect is very similar to the effect of a typical prior art color balance tool, such as that in Adobe Photo-shop, including the Constant Lightness option of that tool, with the tool used in full-scale mode.
The present tool 2010 also allows the user to then optimize the appearance of each of the other two sub-targets by making gray balance adjustments which primarily affect each of the bottom and top halves of the tone scale, using the darks button 2013 and the lights button 2014, respectively. The object is to make all three sub-targets blend perfectly in hue and chroma. In the case of the tool interface shown, the user merely clicks the button (2012, 2013 or 2014) corresponding to the region of the tone scale he or she wishes to adjust and then moves (by dragging with a computer mouse) the white cross 2015 in the color balance tool 2011 toward the color needed, as far as needed. If the three sub-targets 2004, 2024 and 2045 blend perfectly in lightness as well as hue and chroma, it indicates the absence of a certain degree of incorrect gamma calibration, as well as indicating correct gray balance calibration, though a target with more patches would be required for correct gray balance calibration with a typical LCD flat panel display. The preferred gamma target of the invention, above, is better optimized for verification and setting of the gamma of a display.
The user controls are the same as for the first embodiment, but the instructions must refer to four sub-targets instead of three.
A preferred embodiment of the Gray Balance Method One target 2100 of the present invention for use at gamma 1.8 is shown in FIG. 11. Preferred embodiments of this target for use at gamma 2.2, for example, differ only by virtue of differing choices of RGB values for the colors of the target, as is the case with all of the prior targets, except for the Contrast target. The Gray Balance Method One target 2100 uses identical structures and tone values to the Gray Balance Method Two target 2000 above—however, the Gray Balance Method One target has split the red, green and blue channels apart and represents each channel as one third of the target: bright red sub targets row 2101, bright green sub-targets row 2102, and bright blue sub-targets row 2103. Each third of the target thus split has only values of zero for the other two channels. For example, the red third of the target, 2101 bright red sub-targets row, has only numerical values of zero for green and blue, and so on. Also, the Gray Balance Method One target need not include a gray scale when the calibration software implementing the invention includes the Gray Balance Method Two target 2000, with gray scale 2006.
The user will simply use a nine-way adjustment tool (itself a split version of the preferred tool for the Gray Balance Method Two target) to adjust the tone curve of each channel. The object is to make the lightness relationships in each channel be the same as in. the other two, and to make the gamma of each channel correct, as indicated by optimal blending of each of the nine sub-targets. This assures correct gray balance, even without the ability of the user to actually see color, since the tool is relying on perception of lightness only.
The target 2100 depicted in
Thus, the target depicted in
Each colored sub-target of Gray Balance Method One target 2100 comprises a central solid of the sub-target's primary color surrounded by a blended region of alternating lines of that primary color and lines of black. For example, the three-quartertone red sub-target 2097 has a red solid 2116 surrounded by alternating red lines 2117 and black lines 2118.
Accordingly, the 12 sub-targets of
The target depicted in
This target (not illustrated) is optional for the calibration process, but improves the reliability of the display system by readily verifying whether the tonality of the display profile matches that of the calibration. To create the target, one need only convert the appropriate Gamma target's RGB data file into CIE Lab data using an accurate conversion through a display profile of the correct tonality. If the gamma being calibrated to is gamma 1.8, then a gamma 1.8 target must be converted into Lab through a gamma 1.8 display profile. No tool is required for use of this target, except that the user should be informed as to the identity of the profile being used for the simulation of the Lab image data in RGB space on the display. There is no separate figure for the Lab Gamma target because its appearance is precisely identical to that of the Gamma target, such as shown in FIG. 13. This is not to imply that its appearance when used for calibration would be precisely identical.
A preferred embodiment of the present invention necessarily includes software for calibration of the display, which preferably has several useful features. These include:
Automatic management of the calibration targets which shows the targets together with an appropriate tool interface as the user clicks through a sequence of steps.
Tools to facilitate adjustments of the video card LUTs.
Viewing aids such as lots of neutral-colored surrounding colors to prevent unwanted colors from interfering with visual judgements.
Ways to save and manage user adjustments as discreet settings.
The option to create display profiles by specification, rather than by measurement, so that the software will make appropriate display profiles based on user answers to questions and on stored values for red, green and blue chromaticity.
Options to save and manage the profiles generated by the program and other display profiles on the user's system.
Options to allow the user to calibrate to the most important tone curve standards, such as gamma 1.8 and gamma 2.2
Options to allow the user to calibrate to a wide range of white points if practical.
To use a first preferred embodiment of the invention, one completes the following steps, having reference to
20) Boot the computer and turn on the display.
40) Disable any conflicting software, which may affect the video LUT or communicate directly with the display's hardware settings.
60) Let the display warm up to stabilize its color output.
80) Launch the visual calibration software.
100) Adjust ambient illumination to subdued levels, with most light behind the plane of the display's front surface, if possible. Allow only ambient light of high color quality and consistency if possible.
120) Begin calibration.
140) Adjust the display's “Color” (white point) setting to best value using factory Gain settings or user settings. This process may involve matching the color of the ambient viewing light and may be done with a colorimeter for verification of the x,y, XYZ, or Kelvin value, or by setting to a factory-saved standard value. This requires the user to adjust the display's built-in controls in most cases.
160) Adjust the display's Bias controls, if any, in concert with Gain settings, if any, to obtain the best gray linearity of the display using these hardware adjustments alone. This process may be done with a calorimeter at this point in the process, or later, with the visual gray balance adjustment steps below, or both.
180) Choose the preferred gamma for calibration from among the gamma values for which calibration targets have been made and are available in the software for use (the Brightness, Gamma, Gray Balance Method One, Gray Balance Method Two, and Lab Gamma targets are gamma-specific). The usual target gammas for display calibration are 1.8 and 2.2. This may involve clicking a button for the desired gamma so that the software knows which target set to display later.
200) Choose a target white point for calibration in the software (typically either 5000K, 6500K or the same color as the preferred color critical viewing light, simply the color of the ambient light, or the native white point of the display, as appropriate). This step is used to determine the correct white point for the display profile that will be built by the software, if any.
220) If not already done in step 140 above, set the hardware white point (“Color”) setting in the display's built-in controls, if any, and then indicate the choice made to the software so that the final white point can be automatically adjusted by a predetermined amount in the video LUTs if necessary.
240) Optionally adjust the display's white point by using software adjustments, which have a linear affect on the entire tone scale of each color channel (red, green and blue), with maximum effect at the white end and zero effect at the black end. This kind of adjustment can substitute for user-adjustable Gain controls when setting the final white point to a value different from those available as factory-saved settings in the display itself or those obtainable in steps 200 and 220 above. The adjustment is carried out by moving two of three sliders for two of blue, green or red as needed, while looking at a white area on the screen. The adjustment can also be made by simple numerical entry into a box, by clicking arrows, etc.
260) Adjust the display's Contrast control to maximum or just below maximum for a CRT, depending on user preference and on visual feedback from the Contrast target (see contrast sub-target 2262 of the Gamma 1.8 Brightness and Contrast target 2260, FIG. 18). The contrast level which is appropriate is normally the maximum setting for a CRT. However, sometimes the display appears unnecessarily or uncomfortably bright at the maximum contrast setting, in which case the user may prefer a lower setting, such as 90 out of 100. When calibrated, the contrast sub-target 2262 should appear to contain four distinctly different tones of white and near white values, one in each of tone solids 2269, 2270, 2271 and 2272, with similar amounts of difference between the first and second, the second and third, and the third and fourth solids. Some displays loose hilight tonal separation, usually causing the two brightest steps of the Contrast target to blend and both appear white, when the Contrast control is set at maximum. This may be alleviated by lowering the Contrast setting. Also, a lack of difference between the two lightest steps of the Contrast target can indicate that the video card is set to 16-bit color (thousands of colors) instead of 24-bit or 32-bit color, causing the display to be capable of displaying only 32 levels of gray. This is not enough for proper visual calibration. With flat panel displays, it is often the case that the Contrast control must be set far below maximum to achieve the same desired tonal separation between all four steps of the Contrast sub-target 2262.
280) Adjust the display's Brightness to obtain the correct target appearance. See
Note that the correct setting of the Brightness must be done iteratively with the gamma adjustment. Each adjustment affects the other a lot. Therefore, depending on the amount of correction necessary to obtain correct calibration, one may have to use each target and its respective controls) a few times, switching from adjusting brightness to adjusting gamma and back, before the correct result can be obtained with both simultaneously.
300) Adjust the display's overall Gamma (see
320) Re-adjust display's Brightness in light of any Gamma adjustment effects on Brightness (make iterative adjustments of Brightness and overall Gamma until both are correct).
340) Re-adjust display's overall Gamma, in light of Brightness adjustment effects on Gamma (returning to FIGS. 13 and 14). Once overall Gamma and Brightness are both optimized, proceed to step 360.
360) Adjust the display's three-quartertone gamma if needed, using the darks slider 2214 in the software adjustment tool 2212, which slider primarily affects the dark tones between black and the midtone. This Darks slider 2214 causes the greatest movement in the three-quartertone range, and no movement at either black or at the midtone, very little movement between the midtone and white, and none at white. This adjustment offers the option to improve the blending of the second and third darkest sub-targets of the central six sub targets of the Gamma target 2200, namely, sub-target 2203 and sub-target 2204, respectively, with little effect on the rest of the target. Adjust until these sub-targets blend optimally.
380) Adjust the display's quartertone gamma if needed, using the lights slider 2215 in the software adjustment tool 2212, which slider primarily affects the light tones between white and the midtone, with the greatest movement occurring in the quartertone. This adjustment offers the option to improve the blending of the lightest and second lightest of the central six sub-targets of the Gamma target, namely, sub-target 2206 and sub target 2207, respectively, with little effect on the rest of the central six patches. This step may be reversed in sequence with step 360.
400) Adjust the display's overall gray balance by adjusting the gamma of each channel independently (red, green and blue) using the Gray Balance Method One target 2100, with greatest adjustment in the midtone (value 128 in each channel) and no adjustment at maximum or at minimum, i.e. white or black (see FIG. 11). The object of this adjustment is to cause the best overall blending of the central region of each of the nine sub-targets of the target, especially the three midtone sub-targets in column 2112. The lightness of the lined region is to be matched to the lightness of the tone solid within or adjacent to it. First, attempt to make the overall result correct by selecting the Overall button in the software (not shown) and then by moving a slider for each of red, green and blue (not illustrated). Once the blending results in the column of midtone sub-targets 2112 appearing to be as good as this tool will allow (FIG. 12), proceed to step 420.
420) Adjust the three-quartertone (see
440) Adjust the quartertone (value 192) for each color channel (red, green and blue), as needed (see
Either perform steps 400, 420 and 440, or steps 460, 480 and 500, or both sets of three steps (most likely steps 460, 480 and 500 if only performing one set of three steps, depending on the user's visual preference). Alternatively, as appropriate to the hardware, as when using a high-resolution flat panel display, perform steps 461, 481, and 501 only, or perform these three steps in conjunction with steps 400, 420 and 440.
460) Adjust the display's overall gray balance using the Gray Balance Method Two target 2000, until the central sub-target 2024 blends optimally with respect to hue and chroma. See
This target 2000 reveals gamma errors as well as gray balance errors, but it is optimized for gray balance error detection and adjustment, just as the Gamma target 2200 is optimized for gamma error detection and adjustment, although the Gamma target also reveals gray balance errors. Note that
461) Alternate step to step 460 for use with Gray Balance Method Two, Increased Precision target 2300 (see FIG. 23). First adjust the midtone and three-quartertone gray balance while referring to midtone sub-target 2303 and three-quartertone sub-target 2302 respectively. Accomplish this by means of making adjustments with the tool of the visual calibration software user interface to the “overall” and “darks” portions of the tone scale, as labeled in the sample interface adjustment tool 2010 shown at the bottom of FIG. 9. Proceed to step 481.
480) Proceed to adjust the gray balance of the dark half of the tone scale, making the three-quartertone sub-target 2004 of the Gray Balance Method Two target 2000 blend optimally with respect to hue and chroma, as needed (see FIG. 9). Select the Darks button 2013 to cause the tool 2010 to access the darker half of the tone scale before beginning. Proceed to step 500.
481) Alternate step to step 480 for use with Gray Balance Method Two, Increased Precision target 2300 (see FIG. 23). Adjust the 5/6th tone gray balance while referring to the five-sixths-tone sub-target 2301. Make this adjustment with the “Darks” adjustment button shown in the sample interface and with the Gray Balance curves 2286 of the Curves tool 2285 (
500) Proceed to adjust the gray balance of the light half of the tone scale, making the right quartertone sub-target 2045 blend optimally with respect to hue and chroma, as needed (see the Gray Balance Method Two target 2000 of FIG. 9). Select the Lights button 2014 to cause the tool 2010 to access the lighter half of the tone scale before beginning adjustment. When all three sub-targets 2004, 2024 and 2045 blend optimally, examine the hue and chroma of the darkest gray patch or solid 2007 of the gray scale 2006 across the top of the target (the step beside the black patch). If any noticeable color cast remains there, despite the three sub-targets and the rest of the gray scale appearing to be neutral, then further adjustment of the deep shadow color should be performed using the Gray Balance curves 2286 of the Curves Tool 2285 (see gray balance button 2284 of FIG. 22). Inspection of the Luminosity curve 2287 in the Curves Tool 2285 (see luminosity button 2288 of
501) Alternate step to step 500 for use with Gray Balance Method Two, Increased Precision target 2300 (see FIG. 23). Adjust the quartertone tone gray balance while referring to light tone (roughly quartertone) sub-target 2304. Proceed to step 520.
520) Verify the match between the overall gamma of the calibration with that of the currently active display profile using the Lab Gamma target (not illustrated because it looks identical to the target shown in FIG. 13). The appearance of this target will ideally match that of the earlier Gamma target precisely, indicating a perfect agreement between the tonality described by the display's profile and that of the actual calibration. If there is a substantial degree of mis-match, select a more appropriate display profile for use with the calibration just achieved.
530) Step back through all of the targets to make sure they all look right, and proceed to the next step if they do. If they do not, continue to make-adjustments to any of the above controls until they do.
540) Save calibration adjustments and “lock” the resulting video LUTs into the computer's video card, so that the LUTs will not revert to the LUTs that were present before calibration began. The LUTs will revert to the LUTs that were present before calibration if the calibration is not both completed and accepted by the user.
560) Inspect curves which show the effects of the above user adjustments to the video LUTs (see FIGS. 21 and 22). The hardware adjustments to Contrast, Brightness, “Color” (white point), and Gain or Bias do not affect the video LUTs, though they do affect the display's appearance and thus work together with the adjustments to the LUTs to result in correct target appearance and display calibration. Make any adjustments to individual control points on the curves as needed to overcome any perceived imperfection in the calibration not previously dealt with
580) If desired, double check the appearance of all targets now or at any later time to verify correct calibration.
600) Periodically view all targets to verify continued calibration state.
To use a second preferred embodiment of the invention, one completes the following steps, having reference to
1020) Boot the computer and turn on the display.
1040) Disable any conflicting software, which may affect the video LUT or communicate directly with the display's hardware settings.
1060) Let the display warm up to stabilize its color output.
1080) Launch the visual calibration software.
1120) Begin calibration.
1140) Adjust the display's “Color” (white point) setting to a best value using factory Gain settings or user settings. This process may involve matching the color of the ambient viewing light and may be done with a colorimeter for verification of the x,y, XYZ, or Kelvin value, or by setting to a factory saved standard value. This requires the user to adjust the display's built-in controls in most cases.
1180) Choose the preferred gamma for calibration from among the gamma values for which calibration targets have been made and are available in the software for use (the Brightness, Gamma, Gray Balance Method One, Gray Balance Method Two, and Lab Gamma targets are gamma-specific). The usual target gammas for display calibration are 1.8 and 2.2. This may involve clicking a button for the desired gamma so that the software knows which target set to display later.
1260) Adjust the display's Contrast control to maximum or just below maximum for a CRT, depending on user preference and on visual feedback from the Contrast target (see
1280) Adjust the display's Brightness to obtain the correct target appearance. See
Note that the correct setting of the Brightness must be done iteratively with the gamma adjustment. Each adjustment affects the other a lot. Therefore, depending on the amount of correction necessary to obtain correct calibration, one may have to use each target and its respective control(s) a few times, switching from adjusting brightness to adjusting gamma and back, before the correct result can be obtained with both simultaneously.
1300) Adjust the display's overall Gamma to obtain correct target appearance by using the slider in the software, such as the Overall slider 2213 pictured (see FIGS. 13 and 14).
1320) Re-adjust display's Brightness in light of any Gamma adjustment effects on Brightness (make iterative adjustments of Brightness and overall Gamma until both are correct).
1340) Re-adjust display's overall Gamma, in light of Brightness adjustment effects on Gamma (see FIGS. 13 and 14). Once overall Gamma and Brightness are both optimized, proceed to step 1460.
1460) Adjust the display's overall gray balance using the Gray Balance Method Two target 2000, until the central sub target 2024 blends optimally with respect to hue and chroma. Refer to
This target reveals gamma errors as well as gray balance errors, but it is optimized for gray balance error detection and adjustment, just as the Gamma target is optimized for gamma error detection and adjustment, although the Gamma target also reveals gray balance errors. Note that
1520) Verify the match between the overall gamma of the calibration with that of the currently active display profile using the Lab Gamma target (not shown because it looks identical to the target shown in FIG. 13). The appearance of this target will ideally match that of the earlier Gamma target precisely, indicating a perfect agreement between the tonality described by the display's profile and that of the actual calibration. If there is a substantial degree of mis-match, select a more appropriate display profile for use with the calibration just achieved.
1540) Save calibration adjustments and “lock” the resulting video LUTs into the computer's video card, so that the LUTs will not revert to the LUTs that were present before calibration began. The LUTs will revert to the LUTs that were present before calibration if the calibration is not both completed and accepted by the user.
1600) Periodically view all targets to verify continued calibration state.
One preferred embodiment of the present invention can be found in the interface of the above-referenced software application, ColorBlind Prove it!. The interface specifies two series of steps, very similar to those above, that are to be followed in a particular sequence, and preferred methods for carrying out each step. Some steps require the use of an adjustment mechanism and a visual target which instructs the user as to the adjustment required. In some cases an adjustment mechanism changes the display's built-in hardware settings and in some cases it changes the video LUTs of the computer. The preferred implementation of the entire visual calibration process necessarily includes optimizing the hardware settings, prior to implementing the software-only portion of the calibration, which makes adjustments to the video card LUTs.
The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of this invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like.
For example, the exact RGB values chosen to make the targets may vary somewhat while obtaining the same effects. Also, the number of sub-targets in the several visual calibration targets may vary. Also, the kinds of patterns employed to make the various sub-targets may vary to optimize the targets for new kinds of computer displays, such as flat panel displays, including the ability to determine gray balance more accurately and further into the shadows or highlights. Also, the design of the tool interface in the software which allows the user access to the video LUTs can vary considerably and would usually employ such typical interface features as sliders, clickable arrows, numerical entry boxes, movable “handles” of one sort or another, and the like. Displays of improved resolution are in development and will support target designs with different optimizations of the detailed structure of the targets, such as the feasible number of lines in a repeating pattern, which is currently limited to a maximum of three, and Flat Panel Displays may become ubiquitous and negate the necessity for certain design compromises necessitated by the nature of CRT displays. This will likely include the necessity for long, horizontally oriented groups of pixels of identical color value in the targets.
Furthermore, the invention can be practiced with many of the sub-targets discussed herein being presented as a series of individual sub-targets, rather than as a single, combined target with multiple sub-targets. Also, it is possible to use a single sub-target for adjusting both gamma and gray balance, for the sake of simplicity.
Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defamed by the appended claims.
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|U.S. Classification||348/179, 348/181|
|International Classification||H04N17/00, H04N17/02, H04N17/04, H04N1/60|
|Cooperative Classification||H04N1/6033, H04N17/04, H04N17/02|
|European Classification||H04N17/04, H04N1/60F2, H04N17/02|
|Sep 12, 2011||REMI||Maintenance fee reminder mailed|
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|Sep 11, 2015||REMI||Maintenance fee reminder mailed|
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