Display device, method of compensating for transistor aging, and method of updating parameters

Abstract

The present disclosure provides a display device, a method of compensating for transistor aging, and a method of updating parameters. Specifically, a method for compensating for transistor aging in a display device is presented. The method involves: dividing pixels into a plurality of groups including a first group, the first group comprising Z pixels, where Z > 1; sampling a pixel current of each pixel in a subset of pixels in the first group, the subset comprising M pixels, where 1 < M < Z; determining Error M using the sampled pixel currents of the M pixels and a predetermined reference current; and adjusting an input voltage V in of a transistor in more than one of the Z pixels based on Error M . Adjusting the input voltage V in may include generating a modified voltage V d , where V d = A*V in + B, and each of A and B is determined using ∑ M sign(Error m ).

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 081,700, filed September 22, 2020, the contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to an active matrix display device, and more particularly, to a display device having a thin-film switching transistor associated with each pixel. Background Technology

[0004] Currently, different types of active matrix display devices are known, including but not limited to organic light-emitting diode (OLED) display devices and liquid crystal display (LCD) devices. Figure 1 An example of a conventional active matrix addressing display device is shown, comprising an array of pixels 1, each of the pixels 1 having a display element 2. As illustrated, the display device includes a display panel having a plurality of pixels 1 arranged in rows and columns. Although for simplicity, in Figure 1 A small number of pixels 1 are shown, but the display panel includes hundreds or thousands of pixels. Pixel 1 is driven by row driver 8 and column driver 9, which receive and process data from signal controller 7 and transmit signals on scan line S and data line D.

[0005] The display panel may have a current-addressable display element 2. Various circuits exist for providing a controllable current through the display element 2, and each pixel 1 is configured to emit light based on the received current. The current received by pixel 1 is controlled by a driving transistor. To make pixel 1 output a specific color, the display device may apply a voltage to the gate of the transistor based on that specific color. A storage capacitor may be included to maintain the gate voltage after the addressing phase.

[0006] Typically, during operation, transistor characteristics (e.g., mobility and voltage threshold) can change (e.g., due to heat). Applying a high voltage above the threshold voltage causes the threshold voltage to change significantly over time. Since not all pixels 1 are used in the same way or kept on for the same amount of time, this can lead to large differences in the threshold voltage of transistors in different pixels. Therefore, a transistor might output a first current value to pixel 1 in response to a specific voltage value at one time, and a second current value to pixel 1 in response to that specific voltage value at another time. This different aging can cause problems in the display device.

[0007] Considering the large number of pixels in a display device, compensating for the variation of the transistors of each pixel individually is a daunting task. If the rate of change of temperature and ambient light is faster than the rate at which the pixels can converge, there is no convergence and no effective update. A fast and effective method for handling the different variations of the pixels in a display device is desired. SUMMARY

[0008] In one aspect, the disclosure relates to a method for compensating for transistor aging in a display device. The method involves identifying Z pixels included in a first group of a plurality of groups, where Z > 1; sampling a pixel current of each pixel in a subset of the pixels in the first group, the subset comprising M pixels, where 1 < M < Z; determining an error Error M using the sampled pixel currents of the M pixels and a predetermined reference current; and adjusting an input voltage of a transistor in more than one of the Z pixels using the error Error M .

[0009] In another aspect, the disclosure relates to a display device comprising a plurality of pixels arranged in rows and columns, each of the plurality of pixels comprising a transistor, and a sensing front-end circuit. The sensing front-end circuit is configured to sense a pixel current of a subset of M pixels of Z pixels in a group, where 1 < M < Z, and adjust an input voltage supplied to a transistor in more than one of the Z pixels using an error Error M , where Error M =∑ M sign(Error m ), and m is one of the M pixels.

[0010] In yet another aspect, the disclosure relates to a method of updating a parameter for voltage compensation in a display device. In particular, the disclosed system and method updates a parameter for voltage compensation of a first pixel in a group of Z pixels based on an error Error M determined for a subset of M pixels of the group, where 1 < M < Z, and the error Error M =∑ M sign(Error m ). BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 An example of a conventional active matrix addressed display device comprising an array of pixels is depicted.

[0012] Figure 2 An example of a conventional photodiode OLED coupled to a drive transistor M2 is depicted.

[0013] Figure 3AExamples of sensing front-end circuits incorporated into display devices are depicted.

[0014] Figure 3B Examples of sensing circuits 10 configured to compare an output current (I pixel ) to a reference current (I ref ) to generate an error (Error) are depicted.

[0015] Figure 4 Examples in which the disclosed systems and methods update parameters of a group of pixels based on errors of a subset of the group of pixels that are sensed are depicted.

[0016] Figure 5 Examples in which an entire row (or a set of Z pixels in a row, where Z > 1) is updated simultaneously are depicted.

[0017] Figure 6 Variations in pixel output as a function of the number of updates are depicted for a single pixel.

[0018] Figure 7A Variations in pixel output as a function of the number of updates are depicted for 10 pixels updated at a time.

[0019] Figure 7B Variations in pixel output as a function of the number of updates are depicted for 100 pixels updated at a time. DETAILED DESCRIPTION

[0020] Figure 2 Examples of conventional photodiode OLEDs coupled to a drive transistor M2 are depicted. The drive transistor M2 includes a first electrode connected to a first voltage ELVDD and a second electrode connected to an anode of the photodiode OLED. A cathode of the photodiode OLED is connected to a second voltage ELVSS. The drive transistor M2 is controlled by a voltage on its gate, which is stored on a capacitor C during an addressing phase. During the addressing phase, the addressing transistor Ml is turned on, allowing the desired voltage to be transferred from the data line D to the capacitor C to reach the drive transistor M2. The photodiode OLED discharges the gate voltage stored on the capacitor C. In this way, when the gate voltage on the drive transistor M2 reaches the threshold voltage, the photodiode OLED will no longer emit light, and the storage capacitor C will stop discharging.

[0021] The transistor input current (I DS ) has the following relationship with the output voltage (V GS ):

[0022] I DS = C OX *(W / L)*μ*(V GS – Vth ) 2 [Formula 1]

[0023] Among them, C OX V is a coefficient, W and L are the width and length of the transistor, respectively, μ is the mobility of the transistor, and V is a constant. GS V is the gate voltage, and V th The threshold voltage is the variable. Among these variables, mobility (μ) and threshold voltage (V) are... th This is specific to each individual transistor in the display device. Furthermore, the mobility (μ) and threshold voltage (V) of a single transistor... th For example, it can change over time and usage due to temperature variations. Therefore, the transistor characteristics between pixels can vary significantly with time and temperature. To compensate for these differences, the mobility (μ) and threshold voltage (V) between different pixels... th The algorithm monitors and tracks changes in pixels, performing calibration as needed to ensure the desired result remains. In this disclosure, the Least Mean Square (LMS) adaptive algorithm is used as an example method for calibrating pixels; however, other adaptive algorithms can be used.

[0024] Running the LMS algorithm for each individual pixel of a display device and converging each pixel to the compensation factor can be too time-consuming. For example, a pentile 120Hz QHD display has 1560×1440 pixels. If each pixel has two sub-pixels, the number of sub-pixels would equal 4,492,800. Assuming a frame time of approximately 8.33 milliseconds and assuming each pixel requires 100 senses for convergence, approximately 1% of the pixels can be sensed per frame. Convergence of all pixels would require 100×100 frames, which would take 83 seconds at 120Hz. 83 seconds is a long time for temperature and ambient light to remain constant on a display device. During this time, conditions within the display device (e.g., temperature) may change before pixel convergence, resulting in inaccurate pixel output. The system and method disclosed herein overcome this problem by determining the parameters of pixel groups rather than individual pixels. Pixels with errors in the same direction of change (+ or -) are grouped.

[0025] Figure 3A An example of a sensing front-end (SFE) circuitry (e.g., a driver and sensing IC) incorporated into a display device (e.g., incorporated into a column driver) is depicted. As shown, the sensing front-end circuitry includes sensing circuitry 10 and driving circuitry 11. Figure 3B It describes the configuration to output current (I) pixel ) and reference current (I) refA sensing circuit 10 compares the current to generate an error. The reference current is a predetermined value (e.g., 1 nA) that can be generated by the sensing front-end circuitry. Figure 3B As shown, the pixel's driving transistor M2 receives an input voltage (V) for driving the pixel. in Based on this error, the compensation unit 12 of the display device adjusts the input voltage (V) to correspond to the voltage applied to the data line D. in ), to generate the modified voltage (V) d The compensation unit 12 of the sensing circuit 10 modifies the voltage (V). d The output is fed to the gate of the driving transistor M2 to be used as V. GS .

[0026] exist Figure 3B In the example, the modified input voltage (V) to the driving transistor M2 d The following is confirmed:

[0027] V d =A*V in +B [Formula 2]

[0028] Where A is the first parameter and B is the second parameter. The compensation unit 12 can iteratively adjust the first parameter (A) and the second parameter (B) until the current (I) output to the pixel is... pixel Converging to the reference current (I) ref Until then. Therefore, the color output by each pixel can converge to the desired level. Figure 3B In the diagram, the dashed arrows above A and B indicate that the values ​​of A and B are being updated by the adaptive circuit.

[0029] The first parameter (A) and the second parameter (B) of pixel z are determined as follows:

[0030] A n+1 =A n +step*K*Error z *X n Formula 3a

[0031] B n+1 =B n +step*K*Error z Formula 3b

[0032] Where step corresponds to the step size of the Least Mean Square (LMS) algorithm, X n V corresponding to pixel z inThe associated input code word, and K corresponds to a gain factor. As used herein, the subscript "n" indicates one iteration of a pixel, and the subscript "n+1" indicates the next iteration of the same pixel. In a display device, the term Error z has a small probability of being correct. Therefore, the parameter "step" can be set small so that each measurement does not change the first parameter (A) and the second parameter (B) by a large amount. Due to the small step, several updates can be made to the first parameter (A) and the second parameter (B) so that they converge to the correct values. The small parameter step limits the tracking bandwidth of the adaptive algorithm LMS. If the pixel parameters change faster than it takes for the parameters (A) and (B) to converge, the algorithm never converges. Therefore, the speed at which convergence is reached affects how much benefit is gained from voltage adjustment.

[0033] As used herein, "Z" is the number of pixels in a group, where the same Error M value is used to update the group. Z is greater than 1, and "z" indicates a pixel of the Z pixels. "M" is a subset of Z and is a number not greater than Z, and "m" is a pixel in the subset M.

[0034] To reach convergence faster without increasing the step size, the methods and apparatuses of the present disclosure group pixels and perform a collective update rather than updating each pixel individually. Pixels can be grouped based on the probability that the pixels will experience similar environmental changes (e.g., the same temperature change) and / or the wiring of the pixels, which can make certain groupings logical / practical. In one embodiment, every "row" of pixels can be considered a group. Current is sensed for a subset of sample pixels. In one embodiment, the subset includes fewer pixels than all of the pixels in the group. One or more errors are determined based on the current of the sample pixels. Based on the one or more errors, the first parameter (A) and the second parameter (B) are updated for each pixel in the same group as the sample pixels.

[0035] For adjacent pixels, changes in threshold voltage (V th ) and mobility (μ) due to temperature and environmental conditions (e.g., amount of light) occur in the same direction (i.e., both increase or decrease). Therefore, errors from multiple pixels can be correlated and combined for a least mean square (LMS) update. The present disclosure uses this correlation to modify the LMS algorithm so that multiple pixels can be updated within one (or a small number) of frames. The present disclosure also involves using linear prediction to predict the initial value of a next pixel based on previous pixels. As will be described below, once the value of pixel 1 is determined, the value can be used to predict the initial value of the next pixel rather than using 0 or 1 as the initial value.

[0036] The systems and methods disclosed therein are based on a single sensed error. z In the example of updating a set of parameters, the disclosed system and method set the first parameter (A) and the second parameter (B) for each pixel according to the following formula:

[0037] A n+1 =A n +step*sign(Error z )*X n [Formula 4a]

[0038] B n+1 =B n +step*sign(Error z ) [Formula 4b]

[0039] Here, `step` corresponds to the step size in the Least Mean Square (LMS) algorithm, `sign` is the sign function (`sgn`), and X... n V corresponding to pixel z in Related input code words. Initial value A n and B n It is a predetermined value, and can be set to a constant or a value based on the estimated characteristics of the transistor to determine A. n+1 and B n+1 Using A n+1 and B n+1 As the first parameter (A) and the second parameter (B) in Formula 2, the modified input voltage (V) can be determined. d Because different pixels in a group may receive different input voltages (V). in Furthermore, because different pixels may have different A values. n and B n Value (A) n and B n Starting with a pixel (z), different pixels may have different modified input voltages (V). d ).

[0040] Figure 4 The system and methods disclosed therein are described using the same value, Error. M Here's an example of updating parameters for a set of pixels, where Error... M The error is based on a subset M of the sensed pixels. This is related to the error in equations 4a and 4b above. z The situation differs depending on the sampling case based on a single pixel z; formulas 5a and 5b apply to cases where errors occur. M The case is determined based on sampling of more than one pixel, where M>1 and Error M =∑M sign(Error m The disclosed system and method set the first parameter (A) and the second parameter (B) for each pixel according to the following formula:

[0041] A n+1 =A n +step*K*∑ M sign(Error m )*X n Formula 5a

[0042] B n+1 =B n +step*K*∑ M sign(Error m ) [Formula 5b]

[0043] Here, `step` corresponds to the step size of the Least Mean Square (LMS) algorithm, `sign` is the sign function, and X... n Corresponding to V in The associated input codeword. K corresponds to the gain factor. In the case of multiple pixels in the subset, Error... M It is the sum of the sign functions of the errors of the pixels in this subset. Conversely, in equations 4a and 4b, all Z pixels in the group can use the same value Error determined for the M pixels in the subset. M This updates its A and B values ​​through a "+1" iteration. However, each pixel uses its own A... n and B n Therefore, different pixels may have different A values. n+1 Value and B n+1 End of value.

[0044] refer to Figure 4 The example shown is in columns C1 to C2. Z In the diagram, the first row R1 has Z pixels. The value of a subset of pixels in the first row R1 is determined by summing their signs. Figure 4 In the code, it is called Error(R1, C1: C Z ) error (in Figure 4 In the example, M = Z). A n and B n It can be set based on the estimated characteristics, or it can be set as a constant. Using A n+1 and B n+1 As the first parameter (A) and the second parameter (B) in Formula 2, the modified input voltage (V) of pixel z can be determined. dDifferent pixels may have different A and B values. However, the same Error can be used for all pixels in the group. Z (In this specific case, Error) Z With Error M (same) to determine A n+1 and B n+1 To improve efficiency without compromising accuracy, each pixel (or at least each pixel in a subset M) is connected to a portion of the sensing front-end (SFE) circuitry in a 1:1 correspondence. Then, updates are performed on the next set. Figure 4 In the example, the next group is the second row, R2.

[0045] Figure 5 Depicts the use of the same Error Z Here's an example of updating a set of Z pixels in a row. Figure 5 It refers to Figure 4 An example, where Z = M. Reference. Figure 4 An array can be constructed using individual initial parameters A for each of the Z pixels. n B n and code X n However, the same error will be returned. Z This applies to all pixels in the first row R1. In some embodiments, an Error determined for the first group (e.g., the first row R1) can be used. Z This is used to update the A and B values ​​of the second group (e.g., the second row R2). The initial values ​​of A and B for the pixels (i.e., A1 and B1) can be set in several ways. In one example, these initial values ​​are set to hard-coded constants (e.g., A1 = 1 and B1 = 0). In another example, the initial values ​​are based on estimates of the corresponding pixel transistors (e.g., mobility, threshold voltage, etc.). K is a predetermined value, such as 1, and can be adjusted to facilitate convergence.

[0046] Figure 6 , Figure 7A and Figure 7B The changes in pixel output as a function of the number of updates are depicted. This is illustrated in the case of running the LMS algorithm once per pixel (Z=1). Figure 6 In the middle, approximately 4 × 10 5 This update is repeated until convergence is achieved. For each pixel, formulas 3a and 3b above can be used to generate... Figure 6 .

[0047] In one instance, the LMS algorithm is run on 10 pixels (Z=10). Figure 7A In the middle, approximately 4 × 10 4The next update is made to reach convergence. If only one pixel is sampled, then equations 4a and 4b can be used to generate V d for I pixel and I ref convergence. If multiple pixels are sampled, then equations 5a and 5b can be used. In a case where 100 pixels are updated at a time, 1 x 10 4 updates are needed to reach convergence. By grouping pixels for updates, a significant reduction in convergence time is achieved. By using the“collective error” of pixels that can experience changes in the same direction (e.g., adjacent / contiguous pixels), the parameters of the transistor (e.g., V th , mobility) can be effectively compensated for. Using pixel grouping, error correction can be made in near real-time. Figure 7B

[0048] Thus, the disclosed systems and methods can set parameters for pixel compensation for a pixel in a group based on one or more detected errors associated with other pixels in the group. Thus, the disclosed systems and methods can converge more quickly than systems and methods that update each pixel in a display device based on a detected error for that pixel. The concepts disclosed herein can be applied to various types of display devices, including but not limited to organic light emitting diode (OLED) display devices and liquid crystal display (LCD) devices.

[0049] Aspects of the systems and methods provided herein can be embodied in programming. Various aspects of the technology can be thought of as “products” or “articles of manufacture” typically in the form of machine (or processor) executable code and / or associated data that is carried or embodied on or in one type of machine-readable medium. Machine-executable code can be stored on an electronic storage medium, such as memory (e.g., read-only memory, random-access memory, flash memory) or a hard disk. “Storage” types media can include any or all of volatile memory, non-volatile memory, or non- transitory memory, or equivalents thereof. As used herein, unless restricted to non-transitory, tangible “storage” media, terms such as computer- or machine- “readable medium” refer to any medium that participates in providing instructions to a processor for execution.

[0050] ​The minimum mean square calculation process can be implemented using a field programmable gate array (FPGA) in a display device or driving circuit (e.g., a sense front end circuit) of a computing device. Machine-readable media, such as computer-executable code, can take many forms, including but not limited to, tangible storage media, carrier waves, or physical transmission media. Non-volatile storage media include, for example, optical or magnetic disks, such as any of the storage devices in any computer(s) illustrated in the drawings, such as those employed to implement databases, etc. Volatile storage media include dynamic memory, such as the main memory of such a computer platform. Tangible transmission media include coaxial cables; copper wire and fiber optics, including the wires that comprise bus within computer system. Carrier-wave transmission media can take the form of electric or electromagnetic signals, or acoustic or light waves such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media therefore include, for example: floppy diskettes, flexible disks, hard disks, magnetic tapes, any other magnetic medium, CD-ROMs, DVD or DVD-ROMs, any other optical medium, punch cards, paper tape, any other physical storage medium with patterns of holes, RAM, ROM, PROM, and EPROM, FLASH-EPROM, any other memory chip or cartridge, a carrier wave transporting data or instructions, cables or links transporting such a carrier wave, or any other medium from which a computer can read programming code and / or data. Many of these forms of computer readable media can be involved in carrying one or more sequences of one or more instructions to a processor for execution.

[0051] While preferred embodiments of the application have been shown and described herein, it will be apparent to those skilled in the art that many changes, modifications, and substitutions can be made thereto without departing from the application. The present application is not intended to be limited to the particular examples provided within the specification. It is now the intention of the application to cover all modifications, changes, and substitutions falling within the true spirit and scope of the present application. It is understood that all aspects of the application are not limited to the specific details, configurations, or relative proportions set forth herein, as such changes, modifications, and alternatives are apparent to one skilled in the art. It is therefore contemplated to cover any and all changes, modifications, variations, or equivalents that fall within the true spirit and scope of the present application. It is understood by those of ordinary skill in the art that additional embodiments that exercise the essential, preferred, and optional features of the application can be created, and that they are also within the scope of the application. It is therefore intended that the application not be limited to the specifically described application herein, but rather only by the following claims, which are to be construed in the broadest and most liberal way to encompass all equivalents.

Claims

1. A method for compensating for transistor aging in a display device, comprising: Identify Z pixels in the first group of multiple groups, where Z > 1; The pixel current of each pixel in a subset of pixels in the first group is sampled, the subset comprising M pixels, where 1≤M≤Z; The error is determined using the pixel currents of the sampled M pixels and a predetermined reference current. M ; as well as Using the error mentioned M Adjust the input voltage V of the transistor in more than one of the Z pixels. in , Error M =∑ M sign(Error m ), m is one of the M pixels, and Error m It is the error between the pixel current sampled based on the pixel and the reference current of the pixel.

2. The method according to claim 1, wherein, The first group is a row of pixels.

3. The method according to claim 1, further comprising: Pixels are grouped based on the direction in which pixel current changes with usage.

4. The method according to claim 1, wherein, Adjust the input voltage V in Including the use of formula V d =A*V in +B to generate the modified voltage V d The first parameter A and the second parameter B are determined as follows: A n+1 =A n +step*K*∑ M sign(Error m )*X n B n+1 =B n +step*K*∑ M sign(Error m ) Where n is the iteration of the pixels in the first group. `step` is the step size of the least mean square algorithm. K is the gain factor. X n It is an input code associated with the input voltage. sign is a sign function, and A1 and B1 are predetermined, where A1 and B1 are A n and B n The initial value.

5. The method according to claim 4, wherein, For the Z pixels in the first row R1, columns C1 to CZ, A n+1 The calculation is as follows:

6. The method according to claim 4, wherein, For the pixels in the first row R1, columns C1 to CZ, B n+1 The calculation is as follows:

7. The method of claim 1, further comprising: The parameter values ​​of the input voltage of the transistor used to adjust the first pixel are determined iteratively; as well as Linear prediction is used to predict initial parameter values ​​for iteratively determining the input voltage values ​​of the transistor for adjusting the second pixel, based on the determined parameter values ​​of the input voltage of the transistor used to adjust the first pixel.

8. The method according to claim 1, wherein, Adjust the input voltage V in This includes adjusting the input voltage V of at least one transistor in each of the Z pixels. in .

9. The method according to claim 1, wherein, Sampling the pixel current in the first group includes sampling exactly one pixel in the first group.

10. A display device, comprising: A plurality of pixels arranged in rows and columns, each of the plurality of pixels including a transistor; as well as The sensing front-end circuit is configured to sense the pixel current of a subset of M pixels out of Z pixels in a group, where 1 ≤ M ≤ Z, and uses an error. M Adjust the input voltage V supplied to the transistors of more than one of the Z pixels. in Error M =∑ M sign(Error m ), m is one of the M pixels, Error m It is the error between the sensed pixel current of the pixel and the reference current, and sign is the sign function.

11. The display device according to claim 10, wherein, The Z pixels in the group have the same orientation and vary the transistor output current as used.

12. The display device according to claim 10, wherein, The Z pixels are in a single row.

13. The display device according to claim 10, wherein, The sensing front-end circuit is configured to use formula V d =A*V in +B is used to adjust the input voltage V. in V d This is the modified voltage, where A is the first parameter and B is the second parameter, and A and B are determined as follows: A n+1 =A n +step*K*∑ M sign(Error m )*X n B n+1 =B n +step*K*∑ M sign(Error m ) Where n is the iteration of the pixels among the plurality of pixels. `step` is the step size of the least mean square algorithm. K is the gain factor. X n It is an input code associated with the input voltage. sign is a sign function, and A1 and B1 are predetermined, where A1 and B1 are A n and B n The initial value.

14. The display device according to claim 13, wherein, The sensing front-end circuit is configured to calculate A for Z pixels in columns C1 to CZ of the first row R1 as follows: n+1 :

15. The display device according to claim 13, wherein, The sensing front-end circuit is configured to calculate B for pixels in columns C1 to CZ of the first row R1 as follows: n+1 :

16. The display device according to claim 10, wherein, The sensing front-end circuit is further configured to: The parameter values ​​of the input voltage of the transistor used to adjust the first pixel are determined iteratively; as well as Linear prediction is used to predict initial parameter values ​​for iteratively determining the input voltage values ​​of the transistor for adjusting the second pixel, based on the determined parameter values ​​of the input voltage of the transistor used to adjust the first pixel.

17. The display device according to claim 10, wherein, The sensing front-end circuit is configured to adjust the input voltage V by applying a least mean square algorithm. in This is to compensate for changes in at least one of the threshold voltage and mobility of the transistor.

18. A method for updating parameters for voltage compensation in a display device, the method comprising: Error determined based on a subset of M pixels in a group of Z pixels. M To update the parameters of the first pixel in the group, where 1 ≤ M ≤ Z, and the error Error M =∑ M sign(Error m ), where m is the number of pixels in the subset of the M pixels, Error m It is the error of pixel m based on the pixel current output to pixel m and the reference current of pixel m, and sign is the sign function.

19. The method according to claim 18, wherein, The first pixel is not a pixel in the subset.

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