Display Calibration

A portion of the disclosure of this patent application contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

This invention generally relates to improvements in the calibration of a display and more particularly to calibrating a display without the use of an auxiliary apparatus.

The invention provides an innovative approach to calibrating displays. Prior art display calibration systems either required an expensive photometer, or were very subjective. Simple and objective calibration was a desired feature but unattainable in the prior art.

Examples of prior art techniques include U.S. Pat. Nos. 4,091,411; 4,914,506 and 4,101,928 which disclose methods for adjusting saturation and color hue that are unrelated to the subject invention except in a tangential manner.

Aldus Photostyler.TM. in an example of a personal computer application running under windows that has image enhancement features. Photostyler is a professional color image processing application for use with Microsoft.RTM. Windows. It includes a rudimentary display calibration feature for initializing the display prior to enhancing the images.

U.S. Pat. No. 4,106,846 discloses a mechanical device for unrolling a test pattern over a display screen to adjust color and intensity appropriately. The test pattern is unrolled from the mechanical device on a spring loaded roller that resembles a window shade.

U.S. Pat. No. 4,760,447 discloses a method for verifying the calibration of cameras for photographing images generated from electronic data. The camera is initially calibrated using optical densitometers to bring the camera into a preselected initial calibration. After the initial calibration, a calibration pattern is displayed and photographed to produce a reference photograph. The calibration pattern includes at least two brightness and contrast calibration regions, each region having a background and a plurality of subregions. In each region, the subregions each have progressively greater contrast with the background portion such that the number of visible subregions varies with brightness and contrast settings. The number of visible subregions of each calibration region is recorded. At a subsequent time, the calibration is again verified by displaying the same calibration pattern and comparing the number of visible subregions with the recorded numbers of visible subregions.

None of the prior art references teach an objective instrumentless display calibration system or method.

Accordingly, it is a primary object of the present invention to provide an instrumentless display calibration system and method.

These and other objects of the present invention are accomplished by the operation of a process in the memory of a processor. The processor, under the control of the process, generates adjustment images with a symbol that appears light on dark when misadjusted in one direction; dark on light when misadjusted the other way; and disappears at the point when the display is adjusted correctly. The invention can be vised to calibrate brightness, color, gamma and sharpness.

The invention performs these functions by displaying two gray patterns on the monitor under test. The first gray is a solid standard shade. The second gray is formed in a half tone pattern of two shades with a large symbol overlaying the first gray. When applied to brightness adjustment, for example, the symbol appears lighter or darker than the background gray depending on the adjustment of the brightness control, and then disappears when the correct adjustment is obtained.

FIG. 1 is a block diagram of a host computer in accordance with the subject invention;

FIG. 2 illustrates a test image in accordance with the subject invention;

FIG. 3 illustrates various brightness control figures in accordance with the subject invention;

FIG. 4 illustrates a brightness curve in accordance with the subject invention;

FIG. 5 illustrates a video amplifier application of the invention in accordance with the subject invention;

FIG. 6 is a flowchart setting forth the detailed logic for display measurement in accordance with the subject invention; and

FIG. 7 is a flowchart setting forth the detailed logic for calibration in accordance with the subject invention;

FIG. 8 illustrates a phase component of the frequency response of a display application of the invention in accordance with the subject invention.

A representative hardware environment is depicted in FIG. 1, which illustrates a typical hardware configuration of a workstation in accordance with the subject invention having a central processing unit 10, such as a conventional microprocessor, and a number of other units interconnected via a system bus 12. The workstation shown in FIG. 1 includes a Random Access Memory (RAM) 14, Read Only Memory (ROM) 16, an I/O adapter 18 for connecting peripheral devices such as disk units 20 and tape drives 40 to the bus, a user interface adapter 22 for connecting a keyboard 24, a mouse 26, a speaker 28, a microphone 32, and/or other user interface devices such as a touch screen device (not shown) to the bus, a communication adapter 34 for connecting the workstation to a data processing network and a display adapter 36 for connecting the bus to a display device 38.

FIG. 2 illustrates a test image in accordance with the subject invention. The image has three tones; middle 210, light 220 and dark 230. In an image stored in a digital display adapter, these tones are contained in a plurality of pixels 240 stored in the display memory. Light 220 and dark 230 are selected to bridge a part of the gray scale to be controlled. The light and dark tones are used in a tightly repetitive pattern so when the tones are viewed from a distance, the two tones visually blend into a single tone based on principles of half-tone lithography. A third, middle 210 is selected as the background tone so that when the display is correctly adjusted, the symbol disappears when viewed from a distance.

FIG. 3 applies the invention to an adjustment of the video brightness control on a Cathode Ray Tube (CRT) display. Three cases for correct 300, slightly too high 330 and slightly too low 360 brightness are shown. For each of the three tones: light 302, 332 and 362; dark 304, 334 and 364; and middle 306, 336 and 366, the relative numeric pixel value 310 is given relative to 100% white. For example, at 300, the value for light is 50%, dark is 0% and middle is 35.6% of the value of 100% white. The lower part of the grayscale is most sensitive to change by the brightness control, so light and dark are selected to bridge the lower part of the grayscale. Middle Is selected to null the halftone symbol when brightness is correctly set. The brightness control adds a constant voltage to the video signal equivalent to adding a numeric constant to all values. In the example, brightness misadjustments add or subtract 10% from the numeric values. Standard video displays produce a lumens 320 output nearly proportional to the square of the numeric pixel value, so in this example, lumens is the square of the value.

In the area where light and dark form a tightly repetitive pattern, the eye will detect brightness corresponding to the average of lumens from the light and dark, shown in circles 370, 372 and 374. The eye will contrast the brightness with the solid middle adjacent areas 376, 378 and 380. In the case 300 where the brightness is correctly adjusted, the two regions 370 and 376 are equally bright. In the case 330 where the brightness is set 10% too high, the halftone region 372 is 12% darker than the solid middle region 378. And in the case 360 where the brightness is set 10% too low, the halftone symbol 374 emits 21% more lumens than the solid middle region 380.

In practice, the null point is so sensitive that on many displays, the symbol turns a pastel color at null, indicating that all three colors are not in balance. This condition is not a defect of the invention, because the invention exploits the human eye's tendency to average misalignments between colors. In the prior art, separately adjusting the red, green and blue brightness settings under the covers required extreme skill, or a photometer. However, the present invention allows an unskilled observer to observe color misadjustments on monitors that have been calibrated using costly prior art techniques.

The previous example applied the invention to a hardware adjustment. However, the invention is also applicable to a measurement. A plot of brightness versus numeric pixel value for a display can be used in various ways. For example, a palette lookup can precisely correct for display nonlinearities. In one scenario, a monitor is tested, and a correction palette generated for the monitor. Since customers do not have calibration equipment or the expertise to re-calibrate a display, monitor aging, repair or replacement parts could adversely affect the original calibration. Using the subject invention, the brightness curve could be regenerated quickly with the user observing and answering a computer prompt, "is the symbol lighter, darker or equal to the background?"

To span the entire brightness curve as shown in FIG. 4 at 400, assign light 410 to 100% white and dark 412 to 0% white. With monitor adjustments fixed, a user can adjust middle tone 420 until the symbol disappears. This gives the numeric pixel value that results in a lumens output 50% (middle tone 420) of the way between is white and dark. Next, the numeric value that provides 75%. lumens can be calculated in one of two ways. First, dark can be assigned to the value just found to give 50% lumens as above, and leave light assigned to 100% white. Alternatively, a halftone pattern may be used that has 75% white pixels and 25% black pixels. In either case, the null for middle occurs at 75% lumens. The carve can be further articulated employing the same technique to whatever granularity required.

Variations to the invention's application abound. A halftone pattern can be virtually any ratio of light to dark, for example 1/3. The pattern and background can be entirely in one color, like green, to measure or adjust that single color independent of the other colors. Alternatively, the edge of a symbol can be diffused to avoid visually confusing aliasing around the edges. The three tones could vary across the display, the question then being where does the null occur, not at what time or adjustment setting. The invention works with any display, including an image printer, giving a quick, accurate way to calibrate image printers or copiers without requiring expensive densitometers and operator training.

A final example, set forth in FIG. 5, uses the invention to measure frequency response of a video amplifier. In this case the dither pattern places light 510 and dark 520 tones of different widths to exercise the video amplifier of a horizontally scanned display at different frequencies. At high frequencies, light and dark merge if the amplifier response falls as in screen 532, and the halftone pattern darkens 542. If the amplifier overshoots as in screen 534, the light value becomes lighter and the dark value becomes lighter. The actual display lumens can be approximated by squaring the display voltage for each, giving a net lighter screen 544. In this may the brightness is proportional to frequency response, and the response can be measured by selecting a solid shade of gray to match, or adjusted until the halftone pattern matches a preselected solid shade of gray.

The invention only measures the electronic frequency response independent of all other optical or electron beam diffusions. This technique requires linear amplifier response. For the most accurate measurement of the brightness curve in the previous examples, the frequency effect of this example should be eliminated by selecting a halftone dither pattern with a lower frequency in the direction of the scan, such as illustrated in FIG. 5 at 530 in the low frequency halftone where the more serious overshoot is buffered with a 70% transition from black to white. On a progressive monitor every other line can alter between light and dark. However, on an interlaced display, flicker will be experienced if low frequency alteration is not employed.

FIG. 6 is a flowchart setting forth the detailed logic in accordance with the subject invention. Processing commences at function block 600 where percentages are initialized appropriately. Then, in function block 610, the grid shown in FIG. 2 is displayed and other variables are initialized. A test is, performed next at decision block 612 to determine the user's perception of the display and percentages are adjusted appropriately based on the user's input. The 75% value is an output of this initial processing.

This procedure is repeated in function is blocks 614 and a similar test is performed by the user at decision block 620. The output of this second set of processing is a value for 25%.

The procedure discussed above is repeated at function block 630 and a similar test is performed at 640 to adjust to user perception. Then, the final outputs are prepared in function block 650 for use in completing the calibration processing.

FIG. 7 is a flowchart setting forth the detailed logic for calibration in accordance with the subject invention. At function block 700, the calibration figure is presented and then at decision block 710, the user is requested to adjust to their perception. Calibration is complete at 720.

FIG. 8 illustrates a phase component of the frequency response of a display application of the invention in accordance with the subject invention. The phase component of the frequency response of a display system can also be objectively sensed using the invention. In this case, both the foreground symbol and the background consist of grids. For one, the grid is a sawtooth with a rising slope as shown in column 800. For the other, the grid is a sawtooth with a falling slope as shown in column 810. If the phase is lagging, then one will appear brighter than the other. If a leading phase, then the opposite one will appear brighter. Finally, if the phase is linear then both will appear of equal brightness. This sense can be used to align an audio circuit or to control a digital correction that predistorts the image to compensate for the display.

Pixel maps are given for the cases of rising 800 and falling 810 sawtooth patterns, and for the case of asymmetric square wave 820 for comparison. A scan line across these pixel maps translates into display control voltages such as they would appear on an oscilloscope. The effects of leading, lagging and linear phases are also shown. In the case of the sawtooth patterns, the overshoot in the direction of white makes the image lighter, while the overshoot in the direction of blackness has little effect. This phenomena is because once a light is turned off, pushing a voltage lower cannot make black any blacker. ##SPC1##

While the invention has been described in terms of a preferred embodiment in a specific system environment, those skilled in the art recognize that the invention can be practiced, with modification, in other and different hardware and software environments within the spirit and scope of the appended claims.