WO2001029816A1

WO2001029816A1 - Method and system for adjusting a large look up table using a small number of measurements

Info

Publication number: WO2001029816A1
Application number: PCT/US2000/028585
Authority: WO
Inventors: Harry Bowers
Original assignee: Cv Us, Inc.
Priority date: 1999-10-15
Filing date: 2000-10-16
Publication date: 2001-04-26
Also published as: AU1207001A

Abstract

A method and system for adjusting a color device (15) uses a small number of control points. The control points (Fig. 4) are reproduced by the device (15) and measured to determine an estimated color response (30). The estimated color (30) is then compared to an approximate color response (80) determined from a base standard color response predetermined from a similar color device. Differences are determined and are combined with the base color response to become an estimated color response for the device (Fig. 3). With the present invention, a large N dimensional lookup table (100) can be adjusted using only a small number of control points.

Description

METHOD AND SYSTEM FOR ADJUSTING A LARGE LOOK UP TABLE USING A SMALL NUMBER OF MEASUREMENTS

Cross-Reference To Related Applications This application claims the benefit of U.S. Provisional Application No.

60/159,749 filed October 15, 1999.

Field Of The Invention

The present invention relates to the imaging arts It finds particular application to a method and system for adjusting a large look-up table by using a small number of measurements. It will be appreciated that the present invention will find application to any imaging device such as liquid crystal displays, pnnters, monitors, scanners, digital cameras and the like. Background Of The Invention

Look-up tables are used to descnbe the color gamut's of color devices such as scanners, liquid crystal displays (LCD), pnnters, digital cameras, etc. Most three dimensional look-up tables are created by measuring, with a spectrophotometer, the output colors of a color device as a function of evenly spaced input vales.

The complete range of colors of an LCD can reproduce is controlled by Red(R), Green(G) and Blue(B) data channels that range from 0 to 255. For every RGB value that is sent to the LCD, its color (Yxy) can be measured with a spectrophotometer. A table of the measured color values is called a look-up table. The color values can be plotted in Yxy or LAB color space and the 3 dimensional plot represents all the possible colors the color device is capable of producing. It represents the color gamut of the device. Plotting the color gamut gives an indication of how many sample points must be measured to accurately represent the color gamut of the color device. The most accurate representation of the color response of the LCD would be to send every possible input color and measure the output color. This would require over 16 million measurements (255 X 255 X 255). Such a task would be very time consuming and expensive. It would be advantagous to only send a smaller number of values to the LCD, or other color device, and compute the missing values using 3 dimensional interpolation.

The present invention provides a new and unique device calibration method and system which cures the above problems and others.

Summary Of The Invention

In accordance with the present invention, a new and unique method of determining an output response for a color device is provided. An approximate output response is determined based on a small number of measurements. Using a measured base output response, an approximate response is determined from the base output response. A difference is determined therebetween and are combined with the measured base output response to become an estimated measured output response for the color device.

One advantage of the present invention is that a color device can be adjusted by making a small number of measurements.

Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

Brief Description Of The Drawings

The following is a brief description of each drawing used to describe the present invention, and thus, are being presented for illustrative purposes only and should not be limitative of the scope of the present invention, wherein:

Figure 1 is diagram of generating a standard base response look-up table from a base color device,

Figure 2 is a process diagram of generating an approximate output response for a user device in accordance with the present invention;

Figure 3 illustrates generating an estimated measured output response for the user device based on measure base response look-up table in accordance with the present invention; and

Figure 4 illustrates exemplary color output responses for a linear response, approximate user device output response, approximate base output response and the measured base output response.

Detailed Description Of The Preferred Embodiments

With reference to FIGURE 1, a process for determining the color gamut and output response of a color device is shown. A set of input values 10 are sent to a base device 15 and an output value 20 is outputted for each corresponding input value. The base device is any selected color device of a certain type, model and make that is functioning properly. It is used as the base or standard device to which other similar devices will be compared to for calibration purposes as descnbed below.

To accurately determine the output response of the base device, a large number of input values 10 are used such as hundreds or thousands or more. Of course, accuracy of the output response will increase with more input values. The output values 20 produced are then measured 25 by a color measuring device such as a spectrophotometer. The relationship between the input values 10 and the measured output values resulting from each input value is the output response 30 of the base device. This is referred to herein as the base or standard output response. An exemplary graphical representation of the measured base output response is shown in FIGURE 4. It will be appreciated that the illustrated curves are for explanatory purposes and do not reflect actual relationships between devices.

A base look-up table (LUT_B) 35 is generated for the base device 15 that represents the measure response for the device. Knowing the response for the base device 15, it can be corrected with a correction look-up table as is known in the art. Thus, when a correction lookup table is applied to the base response table 35, it adjusts the base device such that it produces output values that exactly or more accurately match their corresponding input values 10. In other words, the device produces what it is asked to produce. If the output values exactly match the input values, the resulting output response is linear as shown by curve L in FIGURE 4.

With reference to Figure 2, a process for determining the output response for a user color device 50 is shown in accordance with the present invention. The user device 50 may or may not have an associated response look-up table 55 that has been previously determined by some calibration means. In order to avoid the time and expense of recalibrating the device 50 by making thousands of measurements, only a small number of measurements are made. This is achievable by having the base response look-up table 35 from the base device 15 as a model. The pnnciple is that the shape of the color gamut (output response) does not change by much between like color devices (e.g. same model and make). In other words, similar devices should function similarly and have a similar output response.

Small changes in the color gamut will occur between similar color devices due to tolerance errors in manufacturing, dimensional and chemical changes caused by temperature and/or humidity. These small changes are, for the most part, insignificant. Therefore, it is not necessary or desirable to make a large number of measurements from color device 50 in order to adjust for small changes in its output response as compared to the base device 15 so that each device will produce similar color charactenstics. Thus, by making only a small number of measurements with device 50 and combining that information with the base response 35, a more accurate user device response for device 50 can be quickly and easily determined without making a large number of actual measurements

With further reference to Figure 2, to determine the response of a color device 50, a user selects a small number of control points from the base response lookup table 35 to be used as input values 60 To improve the results, evenly spaced control points across the color gamut should be selected. Of course, any points can be selected. For example, selecting three (3) points at 0, 128, and 255 in a device with red, blue, and green channels, would require 27 control points Of course, any number of points can be selected that is efficient for a user. These control points are inputted to the device 50 producing corresponding output values 65 which are measured at 70. The measurements indicate the actual output values reproduced corresponding their input value and form the basis for determining an approximate output response 75 for the device 50 The approximate response 75, based on three (3) control points in two dimensions, is shown in FIGURE 4 as the approximate user device response curve 75 With only three control points in two dimensions, the curve is simply two straight lines connecting the points. Because there are only a small number of actual measurements, the approximate response curve 75 is an estimate of the actual output response.

With reference to Figures 3 and 4, using the same input control points, 0, 128 and 255, an approximate base response 80 is found from corresponding output points from the base response look-up table 35. The approximate base response 80 is shown as a dashed line connecting the output points for input points 0, 128, and 255. The remaining values for the approximate user response 75 and the approximate base response 80 are determined by straight linear interpolation 85 and 90, respectively, as known in the art. For each value, a difference is determined between the two responses 75 and 80. An adjustment look-up table 95 is generated with the differences.

For example, with further reference to Figure 4, at input value 64, the corresponding output of the approximate user device response is 48. The output for the approximate base device response is 42 and the actual base device response value is 40. The difference is the difference between the approximate responses, in other words, 48 - 42 giving a difference of 6. As stated above, the difference is found for all the input values, or any desired set of values. The difference values form the adjustment look-up table 95. An estimated measured response look-up table 100 for the user device 50 is generated by combining the adjustment look-up table values with the measured base response look-up table 35. The estimated measured response look-up table 100 becomes the adjusted response look-up table 55 for the user device. Effectively, the measured base response 35 is adjusted in accordance with the small number of measurements taken from the user device. Since the assumption is that the user device 50 has a color response that is the same or very similar to the color response of the base device 15, the measure base device response 35 can be adjusted and be used as the output response for the user device 50. Once the output response is determined, a calibration or correction look-up table is for the user device 50 such that the user device will produce an output response that more closely resembles a linear response curve L. The calibration look-up table is generated as is known in the art.

The present invention reduces the time and cost necessary to adjust the output response of a color device by assuming that it should respond similar to another device of the similar make and model. The present invention is a computer implemented process such as computer program that includes instructions that perform the above- described process. The program can contain one or more base lookup tables that have been pregenerated for a number of different color devices. The base response lookup tables can also be stored externally and then retrieved by the program. The program can be distributed to users by disc, compact disc, memory or by download such that the program is maintained and executed individually by each user. The program can also be maintained and executed at a central server where users can access the server over a computer network such as the Internet and execute the calibration program.

The invention has been described with reference to the preferred embodiment Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalence thereof. Although the foregoing descnptions are based on two dimensions to simplify the explanation of the present system and method, one of ordinary skill in the art will appreciate its application to three dimensional processing and the modifications associated therewith

Claims

I claim:

1. A computer program for determining an estimated output response for a color device, the program comprising: instructions for generating an first approximate output response based on a small number of input points that produce measured output values from the color device; and instructions for generating a second approximate output response based on the small number of input points from a measured base output response determined from a base color device; instructions for determining difference values between the first and second approximate output responses; and instructions for generating an estimated output response for the color device by combining the measured base output response and the difference values.

2. A method for determining an estimated output response for a color device, the method compnsmg the steps of: generating an first approximate output response based on a small number of input points that produce measured output values from the color device; and generating a second approximate output response based on the small number of input points from a measured base output response determined from a base color device; determining difference values between the first and second approximate output responses; and generating an estimated output response for the color device by combining the measured base output response and the difference values.