The present application claims priority from Japanese application serial No. 2006-329049 filed on Dec. 6, 2006, the content of which is hereby incorporated by reference into this application.
The present invention relates to an image correction method of correcting a display luminance of a display panel and an image display device.
In a display device using a liquid crystal display panel or the like, even if an image is displayed on the whole screen at the same luminance, there appears conventionally a variation (in-plane variation) phenomenon in a luminance at each position in the screen. In order to correct this in-plane variation, there has been proposed a method by which the screen plane is divided into a plurality of areas, a luminance distribution in the areas is measured, correction values calculated from the measured luminance distribution are supplied to an image processing circuit of the display device, and when an image is displayed, a luminance distribution at respective pixels in each area is generated by an interpolation function by utilizing the correction values to maintain uniformity of luminances of the display device by using the interpolated values.
This method includes a method using analog signals as disclosed in U.S. Pat. No. 6,570,611 (JP-A-2000-284773) and a method using digital signal processing as disclosed U.S. Patent Publication No. 2005/0275640 (JP-A-2003-46809). In addition, U.S. Pat. No. 6,297,791 (JP-A-11-316577) and JP-A-2006-84729 propose a method of measuring luminances and generating correction data by measuring points on a screen with a luminance sensor.
According to the above-described techniques, a luminance at the highest gradation (tonal) level is set to the lowest luminance at the highest gradation level in a panel, because the luminance at the highest level can only be adjusted only by lowering it. Similarly, a luminance at the lowest gradation (tonal) level is set to the highest luminance at the lowest gradation level in the panel, because the luminance at the lowest level can only be adjusted only by raising it. This adjustment is, however, associated with a problem that contrast is degraded. The contrast is defined as a ratio between highest and lowest luminances at the center of a panel.
An object of the present invention is to provide an image correction method and an image display device capable of maintaining a good contrast and obtaining a smooth and high display quality without stripe noise and color unevenness by correction data, on a panel after image correction.
The present invention is characterized in that a luminance is corrected to have a curved plane taking the highest luminance at the highest gradation level at the center of a panel and lowering toward the edge of the panel, and in that a luminance is corrected to have a curved plane taking the lowest luminance at the lowest gradation level at the center of the panel and raising toward the edge of the panel.
According to the present invention, it is possible to maintain a good contrast and obtain a smooth and high display quality without stripe noise and color unevenness by correction data, on a panel after image correction.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
The liquid crystal panel unit 130 is constituted of a liquid crystal panel 140 for displaying an image and its control system. The image transfer I/F 131 is I/F for inputting an image signal from an external. The control I/F 132 is used for input/output of a control signal which controls the operation of the liquid crystal panel unit 130. The backlight unit 141 is used as a light source which emits light transmitting through the liquid crystal panel 140. The power supply circuit 134 conducts voltage conversion of a power from an external power source 120 to supply voltage to each internal constituent component.
The internal structure of the liquid crystal panel unit 130 will be described. A nonvolatile memory 133 is used for storing data to be utilized by an image processing circuit 136. The image processing circuit 136 processes an image signal input via the image transfer I/F 131, and transmits a display signal to a display unit 137. The image processing circuit 136 executes an in-plane variation correction process.
The display unit 137 is constituted of a gate driver 138, a drain driver 139 and the liquid crystal panel 140. The gate driver 138 and drain driver 139 are each made of an analog circuit such as an operational amplifier for driving the liquid crystal panel 140.
In this embodiment, the liquid crystal panel 140 uses active matrix TFT liquid. The display unit 137 is not limited only to a liquid crystal display unit, but other devices such as an organic EL device may be used. In this case, the backlight unit 141 becomes unnecessary depending upon the device used.
A power supply circuit 135 generates power for driving each circuit in the liquid crystal panel unit 130. The external power source 120 is a general external power source for supplying power to the liquid crystal display device 100. Depending upon situations, power may be supplied directly to the liquid crystal display device 100 from a general power line via a plug.
A measuring apparatus 102 is an apparatus for measuring luminances of the liquid crystal display device 100, controls to display a measurement image on the liquid crystal display device 100 and generates in-plane variation correction values from measurement results of the measurement image.
The measuring apparatus 102 is constituted of: an image sensor 101 for measuring luminances of the liquid crystal panel 140; a sensor circuit 103; a correction value generator unit 104 for generating correction values from measured luminances; a measurement image generator unit 105 for generating a measurement image to be displayed on the liquid crystal panel 140; a display unit 107 for displaying information for checking a measurement state; a recording unit 108 for recording measured data and the like; an image transfer I/F 110, a control I/F 109 and a control unit 106 for controlling these constituent components. The measurement image generator unit 105 may use an image signal generator.
In the panel inspection at 202, the measuring apparatus 102 transmits a measurement image to the liquid crystal display device 100 (Step 203), and the liquid crystal display device 100 displays the measurement image (Step 204). The displayed image is picked up with the image sensor 101 and transmitted to the measuring apparatus 102 (Step 205).
Next, luminances of the picked-up image are measured at all predetermined reference points (Step 206). In this measurement, a lattice pattern may be displayed on the liquid crystal panel 140 to facilitate judgment of the reference points. Different reference points may be used depending upon the luminances to be measured.
It is judged from the measurement results of luminances during the panel inspection at 202 whether a variation (unevenness) in luminances at respective reference points is in a rated (predetermined) range (Step 207). If a luminance variation (unevenness) is in the rated range, it is judged that the panel is a quality product, and the process is terminated (Step 208). If the luminance variation is not in the rated range, a correction process at 220 is executed.
For judgment whether the luminance variation is in the rated range, for example, as shown in the following formula (1), it is judged that the luminance variation is in the rated range, if a percent value of a luminance uniformity degree Buni(g) at a gradation level g, which percent value is the lowest luminance min(g) at the gradation level g divided by the highest luminance max(g) at the gradation level g, is not smaller than a predetermined value, e.g., not smaller than 80%.
Next, the contents of the correction process at 220 will be described. In the correction process at 220, correction values are calculated from the luminance measurement results at Step 206 (Step 209). The correction values are set to the liquid crystal display device 100 (Step 210).
Panel inspection at 211 similar to the panel inspection at 202 is executed to judge whether correction of the liquid crystal display device 100 set with the correction values functions effectively and whether the luminance variation is in the rated range (Step 222). If the luminance variation is in the rated range, the panel is judged as a quality product to terminate the process (Step 213). If the luminance variation cannot be corrected sufficiently, the panel is judged as a defective product to terminate the process (Step 214).
Si(y)=ai+bi(y−yi)+ci(y−yi)2+di(y−yi)3 (2)
The condition of smoothly coupling the interpolation curve Si+1(y) at the section of yi+1<y<yi+2 at the y-coordinate yi+1 is expressed by the following formula (3).
Si(yi)=B(g,yi)
Si(yi+1)=Si+1(yi+1)=B(g,yi+1)
S′i(yi+1)=S′i+1(yi+1)
S″i(yi+1)=S″i+1(yi+1) (3)
The boundary condition at opposite ends is set so that secondary differentiation becomes 0, because of maintaining a slope of the interpolation curve between y0 and yp when the condition of obtaining the curve is set to the following formula (4) and performing extrapolation by using this function.
S″0(y0)=S″p−1(yp)=0 (4)
The following relation (5) is established by defining as yp−yi=1 and calculating coefficients ai, bi, ci and di of the function Si(y) from the above formulae (3) and (4). By solving the formula (5), the coefficients ai, bi, ci and di of Si(y) shown in the formula (2) are determined.
Next, by using Si(y) shown in the formula (2), the luminances at the detail reference points 601, 602 and 603 shown in
In the following, the operation at Step 503 shown in
In
Representing a minimum value of the luminance shown in
Bp(gmax,x,y)=Lgmaxmin(1+Agmax×COS(πx/(2xmax))COS(πy/(2ymax))) (6)
Agmax in the formula (6) is a constant and has restrictions shown in the following formulae (7).
where Xmax and Ymax are maximum values at positions x and y, respectively.
From the conditions shown in the formulae (7), a ratio between minimum and maximum target luminances at the highest gradation level is not smaller than Buni(gmax) and not larger than 1. According to the current specification, Buni(gmax)=0.85.
A curved luminance plane obtained from the formula (6) is shown in
Next, it is assumed that luminances at the lowest gradation level gmin take values shown in
Bp(gmin,x,y)=Lgminmin(1−Agmin×COS(πx/(2xmax))COS(πy/(2ymax))) (8)
Agmin in the formula (8) is a constant and has restrictions shown in the following formulae (9).
Agmin≧0
Agmin≦1−Buni(gmin) (9)
where Xmax and Ymax are maximum values at positions x and y, respectively.
From the conditions shown in the formulae (9), a ratio between minimum and maximum target luminances at the highest gradation level is not smaller than Buni(gmin) and not larger than 1. According to the current specification, Buni(gmin)=0.6.
A curved luminance plane obtained from the formula (8) is shown in
By determining the target curved luminance planes at the highest gradation level gmax and lowest gradation level gmin in the manner described above, a contrast takes a value of Bp(gmax, 0, 0)/Bp(gmin, 0, 0) and is improved considerably as compared to Lgmaxmin/Lgminmax of planar correction. Since the luminance after correction changes smoothly, it is possible to prevent a defect such as stripes on the screen after correction.
Next, description will be made on the operation at Step 503 shown in
Generally, the gradation/luminance characteristics of a display are adjusted so as to follow a predetermined function. An adjustment method most frequently used follows generally the function of the following formula (10).
L(g)=Lgmin+(Lgmax−Lgmin)×(g/gmax)2.2 (10)
An inverse function of the formula (10) is the following formula (11). The gradation data G(g, x, y) on the XY coordinate system can be calculated by using the formula (11).
If a panel has the characteristics shown in
If the luminance at the lowest gradation level of 0 is to be raised to 0.59 cd in
Although the gradation/luminance characteristics are assumed to follow the formula (10) by way of example, the present invention is not limited thereto, but is applicable to any of gradation/luminance characteristics if an inverse function is used.
By using the gradation data G(g, x, y) calculated in the manner described above, coefficients of the Y-direction third-order interpolation curve shown in
Next, with reference to
cgYi(g,j,y)=a(g,j)+b(g,j)(y−yi)+c(g,j)(y−yi)2+d(g,j)(y−yi)3 (14)
where g represents a gradation level such as g=0, 128, . . . , 255 and j represents the number of interpolation areas in the X-direction such as j=0, 1, 2, . . . , n.
Coefficients (parameters) of this formula (14) are calculated by a Sprine function interpolation method using the formulae (2), (3), (4) and (5). This calculation is executed at Step 501 shown in
Next, description will be made on the correction processing to be executed by the liquid crystal display device 100. As the liquid crystal panel 140 is activated, the image processing circuit 136 calculates the formula (14) to generate Y-direction third-order interpolation curves 1000 which interpolate luminances of pixels existing at the borders of the interpolation areas A(i, j) in the Y-direction, as shown in
Next, while the y-coordinates are changed from y=0 to y=n, the gradation data G(g, x, y) at the border of the interpolation area A(i, j) including the y-coordinates at some timing is obtained by using the Y-direction third-order interpolation curve 1000, where x=0, ax, 2ax, . . . , n.
Next, in order to correct luminances in the X-direction, an X-direction third-order interpolation curve 1100 passing the gradation data G(g, x, y) in the Z-direction is generated. A Lagrange interpolation curve is used as the X-direction third-order interpolation curve cgXj(g, i, x). An equation of this curve is expressed by the following formula (15).
cgXj(g,i,x)=aj+bjt+cjt2+djt3 (15)
where 0≦t≦3. It is assumed that x=−ax at t=0, x =0 at t=1, x=ax at t=2, x=2ax at t=3, and that the formula (15) passes four points G(g, −ax, y), G(g, 0, y), G(g, ax, y) and G(g, 2ax, y). The coefficients aj, bj, cj and dj of this curve can be obtained from the following formulae (16).
As shown in
Next, with reference to
Description has been made on the details of the luminance variation correction process of the liquid crystal display device. As the timing when gamma correction is calculated, gamma correction may be performed each time an output gradation corresponding to each pixel is obtained from the X-direction third-order interpolation curve 1100.
A Y counter 1301 indicates a Y-coordinate under processing. Namely, it indicates which horizontal scan line is processed. Each time one line is processed, the counter is counted up, and when a count takes m, it is cleared to 0 next time.
An interpolation gradation g generator circuit 1320 is a circuit for obtaining a correction value at a gradation level g by the method described above. This circuit is provided as many as the number of gradation levels for correction. Namely, if correction is performed for white, black and intermediate luminances at three gradation levels, three circuits 1320 are used and operated in parallel. This circuit reads information from the nonvolatile memory 133 when necessary.
A width ay register 1302 stores the number of vertical pixels ay in the area A(i, j) shown in
A Y-direction interpolation area judging unit 1303 judges from the y-coordinate a corresponding interpolation area A(i, j), reads from the nonvolatile memory 133 the Y-direction third-order interpolation curve generating coefficients a(g, j), b(g, j), c(g, j) and d(g, j) of the interpolation area, and sets the coordinates to a Y-direction curve coefficient register 1304.
A Y-direction interpolation calculation unit 1306 reads the coefficients of the third-order interpolation curve from the Y-direction curve coefficient register 1304 and the present Y-coordinate from the Y counter 1301, and calculates interpolation gradation at the present Y-coordinate.
An X-direction curve coefficient calculation unit 1307 reads the values calculated by the Y-direction interpolation calculation unit 1306, calculates coefficients of the X-direction third-order interpolation curve, and sets the calculation results to an X-direction curve coefficient register 1308.
Similar to the width ay register 1302, a width ax register 1305 stores the number of horizontal pixels ax of the area A(i, j) shown in
An X-direction interpolation area judging unit 1310 judges a present interpolation area A(i, j) from the width ax register 1305 and X counter 1311, and notifies the X-direction third-order interpolation calculation unit 1309 of the coefficients to be read from an X-direction curve coefficient register 1308.
An X-direction interpolation calculation unit 1309 calculates sequentially interpolation gradation of each pixel in the X-direction (horizontal scan line direction) by using the X-direction third-order interpolation curve formula (15). The calculation results are input to the gamma correction circuit 1312.
Display image data is transferred via the image transfer I/F 131 to a data buffer 1313 and stored therein. Pixel data corresponding to a count of the X counter 1311 is read from the buffer 1313, and input to a gamma correction circuit 1312. The gamma correction circuit 1312 calculates an output gradation for the input gradation of the input pixel data, and outputs the calculation result to a correction data line buffer 1314. As pixel data of one line is accumulated in this buffer 1314, the pixel data is transmitted to the display unit 137 and displayed.
[Second Embodiment]
In the first embodiment, the measuring apparatus 102 performs a process of raising the luminance at the center of the panel higher than a periphery luminance at the highest gradation level and lowering the luminance at the center of the panel lower than the periphery luminance at the lowest gradation level, in order to improve contrast. In the second embodiment, this process of improving contrast is performed by the liquid crystal display device 100.
In the following, only different points from the first embodiment will be described.
However, in the second embodiment, as shown in
Gc(g,x,y)=Ac(g)×COS(πx/(2xmax))COS(πy/(2ymax))) (17)
where x and y represent a position on the panel having an origin (0, 0) as the center of the panel, xmax and ymax represent the maximum values of x and y, with the origin (0, 0) being used as the center of the panel. Ac(g) represents a function of a gradation level g, and takes a negative value at the lowest gradation level and a positive value at the highest gradation level.
The contrast correction values generated in the manner described above are added at the adders 1802 so that a value lower than the target luminance value can be given at the center of the panel at the lowest gradation level and a value higher than the target luminance value can be given at the highest gradation level. Also in this case, Ac(g) is set so that a ratio between the minimum luminance value Bmin(gmax) and maximum luminance value Bmax(gmax) after correction at the highest gradation level becomes not smaller than Buni(gmax), and a ratio between the minimum luminance value Bmin(gmin) and maximum luminance value Bmax(gmin) after correction at the lowest gradation level becomes not smaller than Buni(gmin). Also in this embodiment, it is possible to obtain high contrast and maintain a high image quality after correction, without displaying stripes and the like because of smooth luminance change.
In the two embodiments described above, the display unit 137 may be other display devices such as an organic EL panel. As the third-order curve for in-plane luminance variation correction, functions other than the Sprine function and Lagrange function may also be used. With this configuration, it is also possible to obtain high contrast and maintain a high image quality after correction, without displaying stripes and the like because of smooth luminance change.
The correction timing may be when the panel is shipped from a panel maker or when the panel is assembled in a housing at a display maker. The luminance unevenness of the liquid crystal display panel varies at all times because of a secular change during usage by a user, a room temperature change, a temperature change by heat of a backlight during used and the like.
In the first and second embodiments, the measuring apparatus 102 and image sensor 101 are used under various conditions. For example, when a liquid crystal display panel is shipped from a factory, a measuring apparatus 102 and image sensor 101 prepared specifically by the panel maker may be used. In the inspection before shipping and after assembly at a display maker, an inspection system of the display maker loading a portion of software of the panel maker may also be used. In the inspection during usage by a user, the measuring apparatus 102 and image sensor 101 may be a luminance meter and the like connectable to a standard input/output unit of a personal computer (PC) of a user. In this case, software loaded in CD appended to the liquid crystal display panel realizes the functions of the measuring apparatus 102 on the user PC to calculate setting values when the liquid crystal display panel is activated and to rewrite the nonvolatile memory 133.
The software on PC may automatically perform measurements and correction calculations at a constant time interval, and rewrite the nonvolatile memory 133 via the control interfaces 109 and 132. With this procedure, the present invention can deal with a change in the characteristics after sealing a panel in the housing at a display maker, a color change due to a secular change, a luminance change by a temperature and the like.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
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