CN116152864A - Image compensation circuit and method - Google Patents

Abstract

The invention discloses an image compensation circuit and method. The image compensation circuit is used for an image sensor and includes a gain amplifier, a compensation control circuit, a storage unit and a digital-to-analog converter. The gain amplifier is used for receiving multiple image signals from the image sensor and amplifying the multiple image signals. The compensation control circuit is used to generate a plurality of compensation values for the plurality of image signals. The storage unit is coupled to the compensation control circuit for storing the plurality of compensation values. The digital-to-analog converter is coupled to the storage unit and the gain amplifier, and is used for converting the plurality of compensation values into a plurality of compensation voltages, so as to compensate the plurality of image signals by using the plurality of compensation voltages.

Description

Image compensation circuit and method

technical field

The invention relates to an image compensation circuit and method, in particular to an image compensation circuit and method applicable to an optical image sensor.

Background technique

In recent years, optical fingerprint recognition has become one of the most popular fingerprint recognition solutions. In the optical fingerprint sensor, due to the characteristics of the sensing components, there will be a difference in relative illumination (Relative Illumination, RI) in the output sensing signal, and the difference in relative illumination will cause the back-end signal to be easily saturated. Please refer to FIG. 1 , which is a schematic structural diagram of a general optical fingerprint sensor module 10 . As shown in FIG. 1 , the optical fingerprint sensor module 10 includes a panel 102 , a lens 104 and a sensor 106 , which are stacked together to form a modular structure. The operating principle of the optical fingerprint sensor module 10 is that the light source through the panel 102 emits light to the place where the finger is pressed. When the light irradiates the fingerprint of the finger and is reflected, it will pass through the structure of the panel 102 and reach the lens 104, where the light is collected by the lens 104. And then to the pixels on the sensor 106, the pixels can convert the sensed light intensity into a voltage signal, which is processed by the back-end circuit to output a complete fingerprint image.

As shown in FIG. 1 , in the structure of the optical fingerprint sensor module 10 , light reflected from the finger and reaching the sensor 106 passes through the structure of the panel 102 and the lens 104 . Due to the differences in the characteristics of the components themselves and assembly tolerances, the relative illuminance at different positions of the sensor 106 may vary greatly. Variation (as shown in Figure 1, there may be errors in the heights z1, z2, and z3 of each part of the module); individual modules have different absorption rates for light; there are differences in the uniformity and illuminance of the light spot of the display, and the light passing through the panel 102 is easily affected by its structure The influence of the curvature of the lens 104 itself; the assembly tolerance may occur more or less during the module assembly process, which may cause the lens 104 to tilt and increase the brightness contrast. The above factors all cause variations in the light-gathering behavior of the optical fingerprint sensor module 10 , resulting in errors in output image signals.

Please refer to FIG. 2 , which is a schematic diagram of the relative illuminance of the lens 104 . As shown in FIG. 2 , according to the optical characteristics of the lens 104 , the central region (the region closer to the center of the mirror) has higher relative illuminance, while the peripheral region (the region farther away from the center of the mirror) has lower relative illuminance. Therefore, there is a fixed offset on the lens where the central area usually has higher brightness and the peripheral area usually has lower brightness, presenting an isoluminous circle as shown in FIG. 2 .

Please refer to FIG. 3 , which shows fingerprint imaging of different fingerprint sensor modules. Among them, Figure (a) is a normal fingerprint image, which can clearly present a circular fingerprint, and its center brightness is brighter while the surrounding brightness is darker. Figure (b) shows the effect caused by the slight tilt of the lens. It can be seen that there is a black block covering the imaging position of the finger on the right side of the fingerprint image. Figure (c) is the screen produced under the severe tilt of the lens, in which there is an obvious black shadow in the upper left of the image, which causes the photosensitive area in the upper left to detect fingerprints to be greatly reduced.

In view of this, it is necessary to propose an image compensation circuit and method that can be used in the optical fingerprint sensor module 10 to compensate for errors caused by factors such as lens characteristics, component variations, assembly tolerances, and lens tilts on image signals. and offset.

Contents of the invention

Therefore, the main purpose of the present invention is to provide an image compensation circuit and method applicable to an image sensor, which can compensate for relative illuminance errors caused by various variations of the sensor from light emission to imaging.

An embodiment of the present invention discloses an image compensation circuit for an image sensor, the image compensation circuit includes a gain amplifier, a compensation control circuit, a storage unit and a digital-to-analog converter (Digital-to- Analog Converter, DAC). The gain amplifier is used for receiving multiple image signals from the image sensor and amplifying the multiple image signals. The compensation control circuit is used to generate a plurality of compensation values for the plurality of image signals. The storage unit is coupled to the compensation control circuit for storing the plurality of compensation values. The digital-to-analog converter is coupled to the storage unit and the gain amplifier, and is used for converting the plurality of compensation values into a plurality of compensation voltages, so as to compensate the plurality of image signals by using the plurality of compensation voltages.

Another embodiment of the present invention discloses an image compensation method for an image compensation circuit. The image compensation method includes the following steps: receiving a plurality of image signals from an image sensor, and amplifying the plurality of image signals; generating a plurality of compensation values for the plurality of image signals, and compensating the plurality of storing values in a storage unit; and converting the plurality of compensation values into a plurality of compensation voltages respectively, so as to compensate the plurality of image signals by using the plurality of compensation voltages.

Description of drawings

FIG. 1 is a schematic structural diagram of a general optical fingerprint sensor module.

Fig. 2 is a schematic diagram of the relative illuminance of the lens.

Figure 3 shows fingerprint imaging for different fingerprint sensor modules.

FIG. 4 is a schematic diagram of an image compensation circuit according to an embodiment of the present invention.

FIG. 5 is a flowchart of an image processing process according to Embodiment 1 of the present invention.

6A and 6B are schematic diagrams of a sensing pixel array and its corresponding compensation values according to an embodiment of the present invention.

FIG. 7 is a schematic diagram of an ellipse-shaped sensing area caused by tilting the lens.

FIG. 8 is a flowchart of an image processing procedure for compensating lens tilt according to an embodiment of the present invention.

FIG. 9 is a schematic diagram of adjusting the axis according to the tilt direction of the lens to calculate the compensation value according to an embodiment of the present invention.

Figure 10 shows that the lens produces different parameter values under different tilt directions.

Wherein, the reference signs are explained as follows:

10 Optical Fingerprint Sensor Module

102 panels

104 lenses

106 sensors

40 Image Compensation Circuit

400 image sensor

402 column decoder

404 line decoder

410 Analog front-end circuit

412 Programmable Gain Amplifier

414 Digital to Analog Converter

420 Analog to Digital Converter

430 compensation control circuit

432 storage units

440 Sequence Controller

IMG image signal

CMP compensation value

50, 80 image processing flow

r_min, r_max radius

r_data distance data

R1, R2, R3 regions

WGT_0, WGT_1, WGT_2, WGT_3 parameters

Detailed ways

Please refer to FIG. 4 , which is a schematic diagram of an image compensation circuit 40 according to an embodiment of the present invention. The image compensation circuit 40 includes an image sensor 400, an analog front-end (Analog Front-End, AFE) circuit 410, an analog-to-digital converter (Analog-to-Digital Converter, ADC) 420, a compensation control circuit 430, a The storage unit 432 and a timing controller 440 . The image sensor 400 includes a plurality of sensing pixels arranged in an array, wherein each sensing pixel includes a photosensitive element (such as a photodiode), which can be used to detect light and convert the intensity of the light into a voltage or an image signal IMG in the form of a current. In one embodiment, the image sensor 400 may be an optical fingerprint sensor, which is used to detect light reflected from a finger for fingerprint sensing. The analog front-end circuit 410 is coupled to the image sensor 400 and can receive an image signal IMG from the image sensor 400 and process the image signal IMG. Specifically, the analog front-end circuit 410 includes a Programmable Gain Amplifier (PGA) 412 and a Digital-to-Analog Converter (DAC) 414 . The programmable gain amplifier 412 can amplify the image signal IMG from the image sensor 400, and its gain can be adjusted according to system requirements. The digital-to-analog converter 414 can convert the compensation value CMP used to compensate the image signal IMG into a corresponding compensation voltage, so as to use the compensation voltage to compensate the image signal IMG. After the image signal IMG is amplified and compensated in the analog front-end circuit 410, the image signal IMG in voltage form can be converted into a digital code by the analog-to-digital converter 420, and output to the back-end processor in the form of digital code through the image output interface For fingerprint reading, the output interface may be, for example, a serial peripheral interface (Serial Peripheral Interface, SPI), but not limited thereto.

The timing controller 440 is coupled to the image sensor 400 and can be used to control the sensing operation of the image sensor 400 . As mentioned above, the image sensor 400 includes an array of a plurality of sensing pixels, and the timing controller 440 can be used to control the timing of the plurality of sensing pixels to perform image sensing and output the image signal IMG. In detail, the image sensor 400 can be set up with a column decoder 402 and a row decoder 404 , the column decoder 402 can be used to drive pixels to operate column by column, and the row decoder 404 can be used to drive pixels to operate row by row. The timing controller 440 can control the sensing pixels on the image sensor 400 by controlling the interworking of the column decoder 402 and the row decoder 404 . In general, optical fingerprint sensing includes operations such as reset, exposure, and sampling. The timing controller 440 can control each sensing pixel to perform the above sensing operations according to a predetermined timing and output an image signal IMG.

The compensation control circuit 430 can be used to generate a compensation value CMP for the image signal IMG, and the compensation value CMP can be used to compensate a relative illumination (Relative Illumination, RI) error in the image signal IMG. According to FIG. 2 , it can be known that the characteristics of the lens cause different relative illuminance among different sensing pixels. Therefore, each pixel has its corresponding compensation value CMP for compensating the difference in relative illuminance. In this case, the timing controller 440 can be coupled to the compensation control circuit 430 . In addition to controlling the timing of the sensing pixels outputting the image signal IMG, the timing controller 440 can also provide the coordinate information of the sensing pixels to the compensation control circuit 430, so that the compensation control circuit 430 can generate images corresponding to each sensor according to the coordinate information. The compensation value CMP of the pixel is measured, and the compensation value CMP is stored in the storage unit 432 . The storage unit 432 may be a register implemented by, for example, a D flip-flop. When the sensing pixel outputs the image signal IMG to the analog front-end circuit 410, the digital-to-analog converter 414 can fetch the corresponding compensation value CMP of the sensing pixel from the storage unit 432, and after converting the compensation value CMP into a corresponding compensation voltage, The received image signal IMG is added for compensation.

In one embodiment, the image signal IMG can be amplified by the programmable gain amplifier 412 first, and then compensated by the compensation voltage; or, the image signal IMG can be compensated by the compensation voltage first, and then compensated by the programmable gain amplifier 412 to zoom in. Those skilled in the art can select a suitable compensation method according to actual needs, which should not be used to limit the scope of the present invention.

In an embodiment, the compensation voltage may be a subtraction term applied to the image signal IMG, that is, the voltage value of the image signal IMG will be subtracted by the compensation voltage to eliminate the influence of the relative illuminance. In this case, for pixels with relatively large relative illuminance, the compensation control circuit 430 can generate a relatively large compensation value CMP to subtract a relatively large voltage from the image signal IMG; for pixels with relatively low relative illuminance, the compensation control circuit 430 can 430 can generate a smaller compensation value CMP to subtract a smaller voltage from the image signal IMG. In this way, the influence caused by the difference in relative illuminance between different pixels can be eliminated, thereby improving the consistency of the relative illuminance, so that the overall image signal IMG falls on a similar level.

Therefore, the image signal IMG after compensation can eliminate the offset caused by, for example, lens difference and/or assembly tolerance, so that the image signal IMG output by each pixel has a level close to each other, and is converted into digital data by the analog-to-digital converter 420 Afterwards, it can be easily identified by the subsequent processing circuit, and the fingerprint image with poor resolution can be more effectively enlarged and the difference between the peak and the valley can be judged. It should be noted that the present invention performs compensation before the image signal IMG is converted into digital data, so as to avoid signal saturation causing the analog-to-digital converter 420 to eliminate fingerprint information during conversion.

Please refer to FIG. 5 , which is a flowchart of an image processing process 50 according to an embodiment of the present invention. The image processing flow 50 can be used in an image compensation circuit (such as the image compensation circuit 40 shown in FIG. 4 ), which is used to receive the fingerprint image signal from the image sensor 400 (fingerprint sensor is used as an example hereinafter) and process the fingerprint image signal Scan, compensate and identify.

The image processing process 50 can be divided into two parts: a testing process and a fingerprint identification process. First, an object to be tested can be placed on the sensing area of the fingerprint sensor, and the panel corresponding to the fingerprint sensor displays light spots to project the light to the object to be tested. After being reflected by the object to be tested, it passes through the lens Focusing and reaching the sensing pixels on the fingerprint sensor for imaging.

During the test process, the object to be tested may be an object with a smooth surface, such as a rubber sheet. The fingerprint sensor scans the object to obtain an image signal IMG, and transmits the image signal IMG to the image compensation circuit 40 . During the test process, the image compensation circuit 40 may determine that the compensation has not been completed, and sequentially obtain the image signals IMG corresponding to each sensing pixel. At this time, since the object to be detected is an object with a smooth surface and there is no difference between the peaks and valleys of the fingerprint, when there is no relative illuminance error, the image compensation circuit 40 expects that each sensing pixel should generate an image signal IMG with the same voltage . That is to say, after the light is reflected by the smooth object, the signal difference detected by the sensing pixels through lens focusing is equivalent to the relative illuminance difference to be eliminated. In this case, according to the image signal IMG obtained during the scanning process, the compensation control circuit 430 can be used to calculate the compensation value CMP corresponding to each sensing pixel, and store the compensation value CMP in the storage unit 432, or update the storage unit 432 Stored compensation value CMP data.

Next, when performing the fingerprint identification process, the object to be detected on the fingerprint sensing area is a finger, and at this time the image compensation circuit 40 judges that relative illuminance compensation needs to be performed, so the fingerprint sensor scans and transmits the corresponding image signal IMG To the analog front-end circuit 410, at the same time, the digital-to-analog converter 414 in the analog front-end circuit 410 can take out the corresponding compensation value CMP from the storage unit 432 and convert it into a compensation voltage, and the gain amplifier 412 amplifies the image signal IMG and adds it to the compensation voltage after compensation The data is transmitted to the analog-to-digital converter 420, and converted into digital code by the analog-to-digital converter 420 for output, so that fingerprint identification and interpretation can be performed by a subsequent processor.

The compensation value CMP for compensating the image signal can be generated in various ways. In one embodiment, the required compensation value CMP can be determined according to the distance between the sensing pixel and the center of the lens. As shown in FIG. 6A , the compensation value CMP is entirely determined by the distance between the pixel (x, y) and the mirror center (cnt_x, cnt_y), so as to compensate for the relative illuminance difference shown in FIG. 2 . Specifically, on the fingerprint sensing area, an inner circle (with a radius of r_min) and an outer circle (with a radius of r_max) can be set with the center of the lens (cnt_x, cnt_y) as the center, wherein the radius of the outer circle r_max is greater than the radius r_min of the inner circle. According to the outer circle and the inner circle, the sensing pixels can be divided into a first region R1, a second region R2 and a third region R3, wherein the first region R1 is located inside the inner circle, and the second region R2 is located between the inner circle and the outer circle. Between the circles, the third region R3 is located outside the outer circle.

Next, the compensation control circuit 430 can calculate corresponding compensation values CMP for the sensing pixels in different regions. Wherein, the sensing pixels of the first region R1 are located within the inner circle, which means that these pixels are close to the center of the mirror and have relatively large relative illuminance. Therefore, the compensation value CMP of these pixels can be set to a maximum value, such as 255. Sensing pixels in the third region R3 are located outside the outer circle, which means that these pixels are close to the periphery of the lens and have a relatively small relative illuminance. Therefore, the compensation value CMP of these pixels can be set to a minimum value, such as 35. The sensing pixels of the second region R2 are located between the inner circle and the outer circle, and the corresponding compensation value CMP is located between the maximum value and the minimum value (that is, between 255 and 35), and with the distance between the pixel and the lens center increase and gradually decrease. As shown in FIG. 6A , as the distance between the pixel and the mirror center increases, the compensation values CMP are 200, 145, and 90, which decrease sequentially. It should be noted that the value shown in FIG. 6A is only one of many embodiments of the present invention, and those skilled in the art can use an appropriate compensation value to convert the compensation voltage according to the number of digits of the digital-to-analog converter 414 .

In an embodiment, the compensation value CMP of the sensing pixels located in the second region R2 between the inner circle and the outer circle can be calculated using the following formula:

Wherein, r_data is a distance data, which is used to indicate the distance between the pixel and the mirror center, and DAC_max and DAC_min respectively represent the maximum and minimum compensation values. Through the calculation of the above formula, it can be ensured that the compensation value CMP of the sensing pixel located between the inner circle and the outer circle decreases linearly from the inside to the outside, thereby effectively compensating the relative illuminance difference caused by the optical characteristics of the lens.

It should be noted that FIG. 6A only shows 5×5 sensing pixels and their corresponding compensation values as an example. In fact, the image sensor may include hundreds of columns and rows of sensing pixels, and the compensation value corresponding to each pixel can be calculated and obtained in the above manner.

Fig. 6B shows another compensation method, in addition to compensating for the difference in relative illuminance caused by lens characteristics, it also compensates for the difference in relative illuminance caused by variations such as light-gathering behavior and assembly tolerances. As mentioned above, the light is reflected by the finger and then passes through the lens to reach the sensing pixels on the image sensor. Produces irregular brightness differences. Therefore, in the embodiment of FIG. 6B , the sensing pixels can be divided into several image grids, wherein each grid corresponds to a compensation value CMP, and the overall brightness remains similar after compensation. In the above test process, the image sensor 400 can be used to sense an object with a smooth surface to generate a sensing result. The compensation control circuit 430 thus calculates the required compensation value CMP for each grid according to the sensing result. According to the characteristic that the relative illuminance in the central area of the lens is relatively high and the relative illuminance in the peripheral area is low, the calculated compensation value CMP roughly conforms to this trend, but there are slight irregular differences, which can form the compensation as shown in Figure 6B Value CMP distribution.

It should be noted that FIG. 6B shows 5×5 grids and their corresponding compensation values CMP as an example, wherein each grid may include any number of sensing pixels. In addition, according to the size of the sensing area and the total number of sensing pixels, the pixels on the image sensor can be divided into any number of grids in a suitable manner, without being limited thereto.

In another embodiment, considering the possible influence of the lens tilt on the image signal IMG, the above formula (1) for calculating the compensation value CMP may also be adjusted accordingly. Please refer back to Figure 3, as shown in Figure (b) and Figure (c), when the lens is found to be tilted during the Module Test (Module Test, MT), the border on one side will be blocked and cannot be displayed correctly fingerprint image. At this time, the fingerprint image presented in the sensing area is not a perfect circle, but an oval-like pattern, as shown in FIG. 7 . Among them, a sharp decrease in brightness occurs on the blocked side, while the decrease in brightness on the other side becomes more gradual. In other words, although the image signal IMG of the fingerprint still roughly conforms to the distribution that the relative illuminance near the center of the lens is larger and the relative illuminance near the periphery of the lens is smaller and gradually decreases from inside to outside, but the relative illuminance decreases in different directions. difference.

Generally speaking, during the fingerprint sensing process, the image sensor will scan and sense the area covered by the finger to present a circular image as shown in Figure 6A, and scan through the sensing pixels to obtain the area of the fingerprint image It can be regarded as a region of interest (Region of Interest, ROI). When the lens is tilted, some areas of the scanned image will be blocked and the fingerprint image data cannot be obtained. Therefore, the area of interest must be adjusted accordingly to exclude the blocked area and avoid the influence of the image signal on the blocked position. to the overall fingerprint recognition result. In one embodiment, the fingerprint image can be observed during the module test and the degree of lens inclination can be judged, thereby modifying the fingerprint sensing region (such as the above-mentioned region of interest); or, the image sensor or image compensation circuit can be based on the detection The detected image content is used to determine the degree of lens inclination (for example, to detect whether the image is shifted or if there is a block with a sharp drop in brightness), so as to adjust the fingerprint sensing area according to the detection result.

When the lens is tilted, the above calculation formula (1) about the compensation value CMP also needs to be adjusted correspondingly. In one embodiment, the compensation value CMP corresponding to the sensing pixel located between the inner circle and the outer circle can be projected onto the x-axis and y-axis for calculation. When the lens is tilted, it can be set according to the direction in which the lens tilts. Determine the x-axis and y-axis directions used to calculate the compensation value CMP, and accordingly multiply the distance data r_data (that is, the distance between the pixel and the mirror center) by a parameter, or directly adjust the value of the distance data r_data. Taking Figure 7 as an example, the x-axis and y-axis can be set to incline at about 45 degrees, and the distances between the upper right and lower left can be adjusted according to the inclination direction, and the appropriate compensation value CMP can be recalculated accordingly.

In one embodiment, the compensation value adjustment according to the tilt of the lens can be performed when the module test fails. Please refer to FIG. 8 , which is a flow chart of an image processing procedure 80 for compensating lens tilt according to an embodiment of the present invention. As shown in FIG. 8 , the system can perform a module test according to the ideal compensation value CMP without lens tilt offset. When the module test fails, it can be further judged whether there is a tilt of the lens (for example, whether there is a block with a sharp drop in brightness) according to the grayscale change of the image. If it is judged that the lens is not tilted, other tests can be performed to find the problem. If it is determined that the lens is tilted, the compensation control circuit 430 may further calculate and set the x-axis and y-axis directions according to the tilted direction of the lens, and adjust the range of the ROI. Then, the compensation control circuit 430 can calculate the compensation value CMP of each sensing pixel according to the new region of interest, the x-axis and the y-axis, and then update and store the compensation value CMP into the storage unit 432 . The image sensor 400 re-scans according to the updated compensation value CMP and outputs the fingerprint data to the backend for fingerprint recognition, so as to determine whether the fingerprint data obtained after compensation with the new compensation value CMP is correct. In this example, if the recognition error still occurs after re-scanning, the compensation control circuit 430 may recalculate and adjust parameters until a correct fingerprint recognition result is obtained.

Please refer to FIG. 9 , which is a schematic diagram of adjusting the axis according to the tilting direction of the lens to calculate the compensation value according to an embodiment of the present invention. As shown in FIG. 9 , according to the tilting direction of the lens, the x-axis and the y-axis can be tilted at an angle of 45 degrees. For example, the axis from the upper right to the lower left can be regarded as the x-axis, and the axis from the upper left to the lower right can be regarded as the y-axis. However, replacing the x-axis with the y-axis does not affect the description of this embodiment. In addition, four parameters WGT_0, WGT_1, WGT_2 and WFT_3 are respectively set for the four directions of lower right, lower left, upper right and upper left. These parameters can respectively correspond to the relative illuminance distribution in this axis direction and be used to adjust the sensing pixels in this direction compensation value. That is to say, the corresponding compensation value of the sensing pixel can be adjusted according to the direction and degree of lens inclination, and according to the position of the pixel, through corresponding parameters WGT_0, WGT_1, WGT_2 and/or WFT_3.

Please further refer to FIG. 10 , which shows that different values of parameters WGT_0 , WGT_1 , WGT_2 and WFT_3 are generated under different tilt directions of the lens. For example, the picture in the middle is an ideal pattern with the lens not tilted. Its brightness is the brightest in the center and decreases uniformly to the surroundings. Therefore, the parameters WGT_0~WGT_3 in all directions can be set to 1, representing the calculated compensation value CMP does not need to be adjusted. In the upper left, upper right, lower left, and lower right images, the fingerprint images show the offset of the lens tilted in different directions, and the tilt directions correspond to the directions of the four axes. In these illustrations, the lens tilt causes the brightness in a certain direction The decline is very slow, so the axis parameter WGT in this direction can be set to 32/64, which means that during the calculation of the compensation value CMP of the pixel in this direction, the distance data r_data needs to be multiplied by the parameter 32/64, so as to be applied to the formula (1) Obtain the compensation value CMP of the pixel. Wherein, the parameter WGT=32/64 is equivalent to reducing the speed of brightness decrease in this direction to 1/2 of the original one. In addition, as shown in the four pictures of up, down, left, and right, the speeds of brightness decline in the directions of up, down, left, and right are slowed down, so the parameter WGT on the corresponding axis can be set to 32/64, which can be Based on this, the compensation value CMP of the pixel in the corresponding direction is calculated. Taking the above figure as an example, the parameters WGT_2 and WGT_3 can be set to 32/64 respectively, and the compensation value CMP of the sensing pixels located in the upper half of the sensor array can be calculated and adjusted accordingly.

In other words, the parameters WGT_0 , WGT_1 , WGT_2 and WGT_3 can be set in different directions according to the speed at which the luminance decreases in that direction caused by the tilt of the lens. On a fingerprint image, one or more of the parameters WGT_0˜WGT_3 can be adjusted according to the tilt of the lens. It should be noted that the above value of 32/64 is only one of the sample values used to compensate for the slowing down of brightness decline. The actual value can be determined according to the degree of lens inclination, and different parameter values can be set in different directions, so that A more ideal image signal compensation value is calculated.

It should be noted that the object of the present invention is to provide an image compensation circuit and an image compensation method capable of compensating the image signal output by the image sensor, which can be based on lens characteristics, component variations, assembly tolerances, and/or Factors such as lens tilt compensate for relative illuminance differences on the image signal. Those skilled in the art may make modifications or changes accordingly, but are not limited thereto. For example, in the above embodiments, the image signal IMG is transmitted to the analog front-end circuit in the form of voltage, and then the voltage signal is converted into digital code by the analog-to-digital converter. In yet another embodiment, according to the type of the image sensor, the current signal or other signals may be used to carry fingerprint information for transmission, and the analog front-end circuit may convert the current signal into a voltage signal and compensate with a compensation voltage. In addition, the above-mentioned embodiments all take the fingerprint sensor and the fingerprint signal as an example to illustrate the related operation of the image sensor; however, those skilled in the art should understand that the embodiments of the present invention are applicable to any optical image Sensors, as long as they are image sensors that use light sensing to obtain sensing information, may be affected by factors such as lens illumination differences, assembly tolerances, and/or lens tilts to generate image signal errors, and therefore can be Compensation is performed through the image compensation circuit and method of the present invention.

In addition, the image compensation circuit of the present invention (the image compensation circuit 40 shown in FIG. 4 ) can be implemented in a sensing integrated circuit (Integrated Circuit, IC). In addition, the image sensor including the sensing pixel array can be integrated in the same integrated circuit with other circuit components in the image compensation circuit; sensing; alternatively, the image sensor can also be set independently of the display screen and compensation circuit. In addition, the timing controller can also be integrated into the same integrated circuit with other circuit components in the image compensation circuit, or be arranged in a separate integrated circuit.

It should also be noted that the image compensation circuit and method of the present invention are used to compensate the difference in relative illuminance corresponding to different sensing pixels in the image signal, and the difference in relative illuminance is usually represented by the difference in brightness. Therefore, the relative illuminance difference or luminance difference mentioned in the above-mentioned embodiments are all parts of the image signal that need to be compensated, and their names can be replaced with each other in this specification without affecting the description of the embodiments.

To sum up, the present invention proposes an image compensation circuit and method for compensating relative illuminance differences corresponding to different sensing pixels, which can be used in image sensors (such as optical fingerprint sensors). The image compensation circuit can sense during the test process and obtain the appropriate compensation value of each pixel or grid, or calculate the corresponding compensation value according to the position of the sensing pixel corresponding to the center of the lens, and store the compensation value in the storage unit. Then, a digital-to-analog converter can be used to fetch the compensation value from the storage unit in the process of fingerprint identification, and convert it into a compensation voltage for compensation. In one embodiment, it is possible to determine whether the lens is tilted during the module test, so as to adjust the compensation value according to the lens tilt. Through the above compensation, the brightness consistency of the overall fingerprint image can be improved. The compensation method of the present invention can be carried out in the analog front-end circuit before the analog-to-digital converter, so as to compensate before or after the image signal is amplified by the gain amplifier at the analog end, which can prevent the image signal from reaching saturation when it enters the analog-to-digital converter, so as to improve room for signal amplification. In addition, fingerprint images with poor resolution can be more effectively enlarged and the difference between peaks and valleys can be judged.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (24)

1. An image compensation circuit for an image sensor, the image compensation circuit comprising: a gain amplifier, used to receive a plurality of image signals from the image sensor, and amplify the plurality of image signals; a compensation control circuit for generating a plurality of compensation values for the plurality of image signals; a storage unit, coupled to the compensation control circuit, for storing the plurality of compensation values; and A digital-to-analog converter, coupled to the storage unit and the gain amplifier, is used to respectively convert the plurality of compensation values into a plurality of compensation voltages, so as to use the plurality of compensation voltages to compensate the plurality of image signals. 2. The image compensation circuit according to claim 1, wherein the plurality of image signals respectively come from a plurality of sensing pixels in the image sensor, and the plurality of compensation values respectively correspond to the plurality of Pixels are sensed to compensate corresponding image signals of the plurality of image signals. 3. The image compensation circuit according to claim 2, further comprising: a timing controller, coupled to the compensation control circuit, for providing coordinate information of the plurality of sensing pixels to the compensation control circuit; Wherein, the compensation control circuit generates the plurality of compensation values respectively corresponding to the plurality of sensing pixels according to the coordinate information. 4. The image compensation circuit according to claim 1, wherein the plurality of image signals are first amplified by the gain amplifier, and then compensated by the plurality of compensation voltages. 5. The image compensation circuit according to claim 1, wherein the plurality of image signals are compensated by the plurality of compensation voltages first, and then amplified by the gain amplifier. 6. The image compensation circuit according to claim 1, wherein the image sensor comprises a lens, the plurality of compensation values respectively correspond to a plurality of sensing pixels in the image sensor, and the plurality of The magnitudes of the compensation values are respectively determined according to the distances between the plurality of sensing pixels and the center of the lens. 7. The image compensation circuit according to claim 6, wherein the plurality of sensing pixels are divided into a first area, a second area, and a third area according to their distance from the center of the lens , wherein the first area is located within an inner circle centered on the center of the lens, the second area is located between the inner circle and an outer circle centered on the center of the lens, and the third area is located outside the outer circle. 8. The image compensation circuit according to claim 7, wherein the plurality of compensation values corresponding to the plurality of sensing pixels in the first area has a maximum value, and the plurality of sensing pixels in the third area The plurality of compensation values corresponding to the pixels have a minimum value, the plurality of compensation values corresponding to the plurality of sensing pixels in the second region are located between the maximum value and the minimum value and along with the center of the lens gradually decreases with increasing distance. 9. The image compensation circuit according to claim 7, wherein the plurality of compensation values corresponding to the plurality of sensing pixels in the second region are projected onto an axis for calculation, and when the lens is tilted, The direction of the axis is set according to the direction in which the lens is tilted. 10. The image compensation circuit as claimed in claim 6, wherein when the lens is tilted, the plurality of compensation values are adjusted according to the tilting direction and the tilting degree of the lens. 11. The image compensation circuit according to claim 10, wherein when the lens is tilted, a parameter is multiplied by a distance data of a plurality of sensing pixels corresponding to the plurality of compensation values to adjust the Multiple compensation values. 12. The image compensation circuit according to claim 1, wherein the magnitudes of the plurality of compensation values are based on a sensing result generated by using the image sensor to sense a smooth surface object in a test process results to decide. 13. An image compensation method for an image compensation circuit, the image compensation method comprising: receiving a plurality of image signals from an image sensor, and amplifying the plurality of image signals; generating a plurality of compensation values for the plurality of image signals, and storing the plurality of compensation values in a storage unit; and The plurality of compensation values are respectively converted into a plurality of compensation voltages, so as to compensate the plurality of image signals by using the plurality of compensation voltages. 14. The image compensation method according to claim 13, wherein the plurality of image signals respectively come from a plurality of sensing pixels in the image sensor, and the plurality of compensation values respectively correspond to the plurality of Pixels are sensed to compensate corresponding image signals of the plurality of image signals. 15. The image compensation method according to claim 14, further comprising: The plurality of compensation values respectively corresponding to the plurality of sensing pixels are generated according to coordinate information of the plurality of sensing pixels. 16. The image compensation method according to claim 13, wherein the plurality of image signals are first amplified, and then compensated by the plurality of compensation voltages. 17. The image compensation method according to claim 13, wherein the plurality of image signals are compensated by the plurality of compensation voltages first, and then amplified. 18. The image compensation method according to claim 13, wherein the image sensor comprises a lens, the plurality of compensation values respectively correspond to a plurality of sensing pixels in the image sensor, and the image Compensation methods also include: The magnitudes of the plurality of compensation values are determined according to the distances between the plurality of sensing pixels and the center of the lens. 19. The image compensation method according to claim 18, wherein the plurality of sensing pixels are divided into a first area, a second area, and a third area according to their distance from the center of the lens , wherein the first area is located within an inner circle centered on the center of the lens, the second area is located between the inner circle and an outer circle centered on the center of the lens, and the third area is located outside the outer circle. 20. The image compensation method according to claim 19, wherein the plurality of compensation values corresponding to the plurality of sensing pixels in the first area has a maximum value, and the plurality of sensing pixels in the third area The plurality of compensation values corresponding to the pixels have a minimum value, the plurality of compensation values corresponding to the plurality of sensing pixels in the second region are located between the maximum value and the minimum value and along with the center of the lens gradually decreases with increasing distance. 21. The image compensation method according to claim 19, wherein the plurality of compensation values corresponding to the plurality of sensing pixels in the second region are projected onto an axis for calculation, and the image compensation method further comprises : When the lens is tilted, the direction of the axis is set according to the direction in which the lens is tilted. 22. The image compensation method according to claim 18, further comprising: When the lens is tilted, the multiple compensation values are adjusted according to the tilting direction and the tilting degree of the lens. 23. The image compensation method according to claim 22, wherein the step of adjusting the plurality of compensation values according to the tilted direction and the tilted degree of the lens comprises: A parameter is respectively multiplied by a distance data of a plurality of sensing pixels corresponding to the plurality of compensation values to adjust the plurality of compensation values. 24. The image compensation method according to claim 13, further comprising: In a test process, using the image sensor to sense a smooth surface object to generate a sensing result; and According to the sensing result, the magnitudes of the plurality of compensation values are determined.

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