CN116152864A - Image compensation circuit and method - Google Patents

Abstract

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

Description

Image compensation circuit and method

Technical Field

The present invention relates to an image compensation circuit and method, and more particularly, to an image compensation circuit and method for an optical image sensor.

Background

In recent years, optical fingerprint recognition has become one of the most popular fingerprint recognition schemes. In the optical fingerprint sensor, the difference of relative illuminance (Relative Illumination, RI) is generated on the output sensing signal due to the characteristics of the sensing component, and the difference of relative illuminance causes the back-end signal to be easily saturated. Referring to fig. 1, fig. 1 is a schematic diagram of a typical 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 to form a modular structure. The optical fingerprint sensor module 10 operates on the principle that the light source of the panel 102 emits light to the finger pressing position, when the light irradiates the finger to reflect, the light passes through the panel 102 structure and reaches the lens 104, the light is collected by the lens 104 and then reaches the pixels on the sensor 106, the pixels can convert the sensed light intensity into voltage signals, and the voltage signals are processed by the circuit at the rear end, so that a complete fingerprint image is output.

As shown in fig. 1, in the configuration of the optical fingerprint sensor module 10, light rays reflected from the finger on their way to the sensor 106 will pass through the structure of the panel 102 and the lens 104. The differences in the characteristics of the components themselves and assembly tolerances, which may cause the relative illuminance at different locations of the sensor 106 to vary widely, include variations in the distance between the individual module spaces, i.e., variations in the thickness of the material itself and variations in the stacking of the module spaces (e.g., errors may occur in the heights z1, z2, z3 of the various portions of the module in fig. 1); the absorption rate of the individual modules to the light is different; the display spot uniformity and illuminance are different, and the light passing through the panel 102 is susceptible to the structure thereof; the curvature process variation of the lens 104 itself; assembly tolerances may occur more or less during assembly of the module, possibly causing the lens 104 to tilt resulting in an increase in brightness contrast. All the above factors cause variations in the light condensing behavior of the optical fingerprint sensor module 10, resulting in errors in the output image signal.

Referring to fig. 2, fig. 2 is a schematic diagram of the relative illuminance of the lens 104. As shown in fig. 2, the central region (the region closer to the center) has a higher relative illuminance, and the peripheral region (the region farther from the center) has a lower relative illuminance, depending on the optical characteristics of the lens 104. Thus, there is a fixed offset on the lens where the central area generally has a higher brightness and the peripheral area generally has a lower brightness, assuming an equal bright circular shape as shown in fig. 2.

Referring to fig. 3, fig. 3 illustrates fingerprint imaging of different fingerprint sensor modules. Wherein, the image (a) is a normal fingerprint image, which can clearly show a circular fingerprint, and the center brightness is brighter and the surrounding brightness is darker. The effect of slight tilting of the lens is shown in fig. (b), and it can be seen that a black patch is present on the right side of the fingerprint image, which obscures the imaging position of the finger. The image (c) is a picture generated under the serious inclination of the lens, wherein the upper left of the image is obviously blackened, so that the photosensitive area of the upper left detectable fingerprint is greatly reduced.

In view of the above, it is necessary to provide an image compensation circuit and method for the optical fingerprint sensor module 10, which is used to compensate errors and offsets caused by factors such as lens characteristics, component variations, assembly tolerances, and lens tilt on an image signal.

Disclosure of Invention

Therefore, a primary objective of the present invention is to provide an image compensation circuit and method for an image sensor, which can compensate for the relative illuminance error generated by the variation between the light emission and the imaging of the sensor.

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

Another embodiment of the present invention discloses an image compensation method for an image compensation circuit. The image compensation method comprises 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 storing the plurality of compensation values in a storage unit; and converting the plurality of compensation values into a plurality of compensation voltages, respectively, to compensate the plurality of image signals using the plurality of compensation voltages.

Drawings

Fig. 1 is a schematic diagram of a typical optical fingerprint sensor module.

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

Fig. 3 shows fingerprint imaging of different fingerprint sensor modules.

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

Fig. 5 is a flowchart of an image processing procedure according to an embodiment of the invention.

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

FIG. 7 is a schematic view of a sensing region exhibiting an elliptical shape due to lens tilt.

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 an embodiment of the invention for calculating a compensation value according to the adjustment axis of the lens tilt direction.

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

Wherein reference numerals are as follows:

10. optical fingerprint sensor module

102. Panel board

104. Lens

106. Sensor device

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. Memory cell

440. Time sequence controller

IMG image signal

CMP offset value

50. 80 image processing flow

radius r_min, r_max

r_data distance data

R1, R2, R3 region

Wgt_0, wgt_1, wgt_2, wgt_3 parameters

Detailed Description

Referring to fig. 4, fig. 4 is a schematic diagram of an image compensation circuit 40 according to an embodiment of the invention. The image compensation circuit 40 includes an image sensor 400, an Analog Front End (AFE) circuit 410, an Analog-to-digital converter (ADC) 420, a compensation control circuit 430, a memory 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 (e.g., a photodiode) for detecting light and converting the intensity of the light into an image signal IMG in the form of a voltage or a current. In one embodiment, the image sensor 400 may be an optical fingerprint sensor for detecting light reflected by a finger for fingerprint sensing. The analog front-end circuit 410 is coupled to the image sensor 400, and can receive the image signal IMG from the image sensor 400 and process the image signal IMG. In detail, the Analog front-end circuit 410 includes a programmable gain amplifier (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 the gain thereof can be adjusted according to the system requirements. The digital-to-analog converter 414 may convert the compensation value CMP for compensating the image signal IMG into a corresponding compensation voltage to compensate the image signal IMG with the compensation voltage. After the image signal IMG is amplified and compensated in the analog front-end circuit 410, the image signal IMG in voltage form may be converted into a digital code by the analog-to-digital converter 420, and output to the back-end processor for fingerprint interpretation in the form of a digital code through an image output interface, which may be, for example, a serial peripheral interface (Serial Peripheral Interface, SPI), but is 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 described 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 for image sensing and outputting the image signal IMG. In detail, the image sensor 400 may be configured with a column decoder 402 and a row decoder 404, wherein the column decoder 402 may be used to drive the pixel operation row by row, and the row decoder 404 may be used to drive the pixel operation row by row. The timing controller 440 can control the sensing pixels on the image sensor 400 by controlling the column decoder 402 and the row decoder 404 to interoperate. Generally, the optical fingerprint sensing includes reset, exposure, sampling, etc., and the timing controller 440 can control each sensing pixel to perform the sensing operation according to a predetermined timing and output the image signal IMG.

The compensation control circuit 430 may be configured to generate a compensation value CMP for the image signal IMG, which may be configured to compensate for relative illuminance (Relative Illumination, RI) errors in the image signal IMG. As can be seen from fig. 2, the characteristics of the lens cause different relative illuminances between different sensing pixels, and therefore, each pixel has its corresponding compensation value CMP for compensating the difference of the relative illuminances. In this case, the timing controller 440 may be coupled to the compensation control circuit 430. The timing controller 440 may provide coordinate information of the sensing pixels to the compensation control circuit 430 in addition to controlling the timing of the sensing pixel outputting the image signal IMG, so that the compensation control circuit 430 may generate compensation values CMP corresponding to the sensing pixels, respectively, according to the coordinate information and store the compensation values CMP to the storage unit 432. The memory unit 432 may be a register implemented by, for example, a D flip-flop (D flip-flop). While the sensing pixel outputs the image signal IMG to the analog front-end circuit 410, the digital-to-analog converter 414 may take out the corresponding compensation value CMP of the sensing pixel from the storage unit 432, convert the compensation value CMP into the corresponding compensation voltage, and then add the received image signal IMG for compensation.

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

In one embodiment, the compensation voltage may be a subtraction applied to the image signal IMG, i.e., the voltage value of the image signal IMG is subtracted from the compensation voltage to eliminate the influence of the relative illumination. In this case, for the pixel with larger relative illumination, the compensation control circuit 430 can generate a larger compensation value CMP to subtract a larger magnitude voltage on the image signal IMG; for pixels with smaller relative illumination, the compensation control circuit 430 may generate a smaller compensation value CMP to subtract a smaller magnitude voltage on the image signal IMG. In this way, the influence caused by the difference of the relative illumination between different pixels can be eliminated, so that the consistency of the relative illumination is improved, and the whole image signal IMG is on a similar level.

Therefore, the image signal IMG after compensation can eliminate offset caused by lens differences and/or assembly tolerances, so that the image signal IMG output by each pixel has a level close to each other, and can be easily identified by the subsequent processing circuit after being converted into digital data by the analog-to-digital converter 420, and the difference between the peaks and the valleys can be more effectively amplified and interpreted for the fingerprint image with poor resolution. It should be noted that the present invention compensates before converting the image signal IMG into digital data, so as to avoid eliminating fingerprint information during the conversion of the analog-to-digital converter 420 caused by signal saturation.

Referring to fig. 5, fig. 5 is a flowchart of an image processing process 50 according to an embodiment of the invention. The image processing flow 50 may be used in an image compensation circuit (such as the image compensation circuit 40 shown in fig. 4) for receiving a fingerprint image signal from an image sensor 400 (hereinafter, a fingerprint sensor is taken as an example) and scanning, compensating and recognizing the fingerprint image signal.

The image processing process 50 can be divided into two parts, namely a test process and a fingerprint identification process. Firstly, an object to be detected can be placed on a sensing area of the fingerprint sensor, and meanwhile, a panel corresponding to the fingerprint sensor displays light spots so as to project light to the object to be detected, and after the light is reflected by the object to be detected, the light reaches sensing pixels on the fingerprint sensor for imaging through focusing of a lens.

In the test process, the object to be tested may be an object with a smooth surface, such as a rubber sheet, and the fingerprint sensor scans the object to be tested to obtain the image signal IMG, and transmits the image signal IMG to the image compensation circuit 40. In the test process, the image compensation circuit 40 can determine that the compensation is not completed, and sequentially obtain the image signals IMG corresponding to the sensing pixels. At this time, since the object to be measured is a smooth-surfaced object without the difference of the fingerprint peaks and valleys, the image compensation circuit 40 expects that each sensing pixel should generate the image signal IMG having the same voltage without the relative illuminance error. That is, after the light is reflected by the smooth object, the difference of the signals detected by the sensing pixels through focusing of the lens is equivalent to the difference of the relative illuminance to be eliminated. In this case, according to the image signal IMG obtained during the scanning process, the compensation control circuit 430 may be used to calculate the corresponding compensation value CMP of each sensing pixel, and store the compensation value CMP into the memory unit 432, or update the compensation value CMP data stored in the memory unit 432.

Then, when the fingerprint identification process is performed, the object to be detected on the fingerprint sensing area is a finger, and the image compensation circuit 40 determines that the relative illuminance compensation is required, so that the fingerprint sensor scans and transmits the corresponding image signal IMG to the analog front-end circuit 410, and meanwhile, 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 the compensation value CMP into a compensation voltage, and the gain amplifier 412 amplifies the image signal IMG and adds the compensation of the compensation voltage and then transmits the image signal IMG to the analog-to-digital converter 420, and the image signal IMG is converted into a digital code by the analog-to-digital converter 420 to be output, so that the subsequent processor performs fingerprint identification and interpretation.

The compensation value CMP for compensating the image signal may 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 completely determined by the distance between the pixel (x, y) and the lens center (cnt_x, cnt_y) to compensate the relative illuminance difference as shown in fig. 2. In detail, an inner circle (with a radius r_min) and an outer circle (with a radius r_max) can be set on the fingerprint sensing area with the lens centers (cnt_x, cnt_y) as the center, wherein the radius r_max of the outer circle is larger than the radius r_min of the inner circle. According to the outer circle and the inner circle, the sensing pixel region 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, the second region R2 is located between the inner circle and the outer circle, and the third region R3 is located outside the outer circle.

Then, the compensation control circuit 430 can calculate the corresponding compensation value CMP for the sensing pixels of the different regions respectively. The sensing pixels of the first region R1 are located within the inner circle, which represents that the pixels are close to the mirror center and have a larger relative illumination, so that the compensation value CMP of the pixels can be set to a maximum value, such as 255. The sensing pixels of the third region R3 are located outside the outer circle, which means that the pixels are close to the periphery of the lens and have smaller relative illumination, so the compensation value CMP of the 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 their corresponding compensation values CMP are located between the maximum value and the minimum value (i.e., between 255 and 35), and gradually decrease as the distance between the pixels and the center of the lens increases. As shown in fig. 6A, the compensation values CMP are sequentially decreased by 200, 145, 90 as the distance between the pixel and the lens center increases. It should be noted that the values shown in fig. 6A are only one of many embodiments of the present invention, and those skilled in the art can use appropriate compensation values to convert the digital-analog converter 414 into the compensation voltage according to the number of bits.

In one embodiment, the compensation value CMP of the sensing pixel of the second region R2 between the inner circle and the outer circle can be calculated by the following formula:

wherein r_data is distance data for representing the distance between the pixel and the lens center, and DAC_max and DAC_min represent the maximum and minimum compensation values, respectively. By calculation of the above formula, it is ensured that the compensation value CMP of the sensing pixel located between the inner circle and the outer circle decreases from inside to outside in a linear manner, thereby effectively compensating for 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 examples. In practice, the image sensor may include hundreds of columns and hundreds of rows of sensing pixels, and the compensation value corresponding to each pixel may be calculated and obtained in a partitioning manner.

Fig. 6B shows another compensation method for compensating for the relative illuminance difference caused by variations such as the light condensing behavior and the assembly tolerance, in addition to the relative illuminance difference caused by the lens characteristics. As described above, the light beam is reflected by the finger and then passes through the lens to reach the sensing pixel on the image sensor, and the path of the light beam is required to pass through different materials such as the panel structure and the lens, and the influence of assembly tolerance is added, so that irregular brightness difference is generated on the sensing pixel. Therefore, in the embodiment of fig. 6B, the sensing pixel can be divided into several image grids, wherein each grid corresponds to a compensation value CMP, and the overall brightness is kept similar after compensation. In the above test procedure, the image sensor 400 can be used to sense the object with a smooth surface to generate a sensing result. The compensation control circuit 430 thus calculates the compensation value CMP size required for each grid from the sensing result. The calculated compensation value CMP approximately corresponds to such a trend according to the characteristic that the relative illuminance of the central area of the lens is high and the relative illuminance of the peripheral area is low, but there is a slight irregular difference, and the compensation value CMP distribution shown in fig. 6B, for example, can be formed.

It is noted that fig. 6B shows 5×5 grids and their corresponding compensation values CMP as examples, wherein each grid may comprise any number of sensing pixels. In addition, the pixels on the image sensor may be divided into any number of grids in a suitable manner according to the size of the sensing area and the total number of sensing pixels, but is not limited thereto.

In another embodiment, the above formula (1) for calculating the compensation value CMP can be adjusted correspondingly, taking into account the possible influence of the lens tilt on the image signal IMG. Referring back to fig. 3, as shown in fig. b and fig. c, when a lens tilt is found during a Module Test (MT), a boundary on one side is blocked and a fingerprint image cannot be displayed correctly. At this time, the fingerprint image presented by the sensing region is not a perfect circle, but is a pattern resembling an ellipse, as shown in fig. 7. Wherein the blocked side shows a sharp drop in brightness and the other side has a flatter brightness drop. In other words, although the image signal IMG of the fingerprint still approximately 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, the decreasing speeds of the relative illuminance in different directions will be different.

Generally, in the fingerprint sensing process, the image sensor scans and senses the finger covered region, and presents a circular image as shown in fig. 6A, and the region scanned by the sensing pixels and acquiring the fingerprint image can be regarded as a region of interest (Region of Interest, ROI). When the lens is tilted, a part of the scanned image is blocked and fingerprint image data cannot be obtained, so that the interested area is correspondingly adjusted to exclude the blocked area, and the image signal at the blocked position is prevented from influencing the whole fingerprint identification result. In one embodiment, the fingerprint image may be observed and the degree of lens tilt determined during the module test, thereby modifying the fingerprint sensing area (e.g., the area of interest) as described above; alternatively, the image sensor or the image compensation circuit can determine the degree of inclination of the lens (for example, whether the image is shifted or the brightness of the image is suddenly reduced is detected) according to the detected image content, so as to adjust the fingerprint sensing area according to the detection result.

When the lens tilt occurs, the above-mentioned 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 between the inner circle and the outer circle can be projected onto the x-axis and the y-axis for calculation, when the lens is tilted, the x-axis and the y-axis directions for calculating the compensation value CMP can be set according to the direction of the lens tilt, and the distance data r_data (i.e. the distance between the pixel and the lens center) can be multiplied by a parameter according to the x-axis and the y-axis directions, or the value of the distance data r_data can be directly adjusted. Taking fig. 7 as an example, the x-axis and y-axis directions may be set to be inclined by about 45 degrees, the distances of the upper right and lower left may be adjusted according to the inclination direction, and the appropriate compensation value CMP may be recalculated accordingly.

In one embodiment, compensation value adjustments corresponding to lens tilt may be made in the event of a module test failure. Referring to fig. 8, fig. 8 is a flowchart of an image processing procedure 80 for compensating lens tilt according to an embodiment of the invention. As shown in fig. 8, the system may perform a module test based on the ideal compensation value CMP without lens tilt offset. When the module test fails, it can further determine whether the lens is tilted according to the gray level change of the image (e.g. determine whether the brightness of the block is suddenly reduced). If it is determined that no lens tilt has occurred, other tests may be performed to find problems. 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 direction of the lens tilt, and adjust the range of the region of interest. 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 update and store the compensation value CMP into the memory unit 432. The image sensor 400 scans again according to the updated compensation value CMP and outputs fingerprint data to the back end for fingerprint identification, so as to determine whether the fingerprint data obtained after compensation by using the new compensation value CMP is correct. In this case, if the rescanning still has an erroneous recognition, the compensation control circuit 430 may recalculate and adjust the parameters until a correct fingerprint recognition result is obtained.

Referring to fig. 9, fig. 9 is a schematic diagram illustrating an embodiment of the invention for calculating a compensation value according to an axis adjustment of a lens tilt direction. As shown in fig. 9, the x-axis and the y-axis may be tilted by an angle of 45 degrees depending on the direction in which the lens is tilted. For example, an upper right to lower left axis may be considered the x-axis and an upper left to lower right axis may be considered the y-axis. However, the replacement of the x-axis with the y-axis does not affect the description of the present embodiment either. In addition, 4 parameters wgt_0, wgt_1, wgt_2 and wft_3 are set in the lower right, lower left, upper right and upper left directions, respectively, and these parameters can be used to adjust the compensation value of the sensing pixel in the direction corresponding to the relative illuminance distribution in the axis direction. That is, the compensation value of the sensing pixel can be adjusted by the corresponding parameters WGT_0, WGT_1, WGT_2 and/or WFT_3 according to the direction and the inclination degree of the lens and the position of the pixel.

Referring further to FIG. 10, the lens is shown with different values of parameters WGT_0, WGT_1, WGT_2 and WFT_3 generated under different tilt directions. For example, the middle graph is an ideal pattern with the lens not tilted, the brightness is the brightest at the center, and the brightness is uniformly decreased to the periphery, so the parameters wgt_0 to wgt_3 in all directions can be set to 1, which represents that the calculated compensation value CMP does not need to be adjusted. In the upper left, upper right, lower left and lower right 4 graphs, the fingerprint images show the offsets of the lenses inclined towards different directions, and the inclined directions respectively correspond to the directions of 4 axes, in the illustrations, the inclination of the lenses causes the brightness in a certain direction to be extremely slow, so that the axis parameter WGT of the direction can be set to be 32/64, and the distance data r_data needs to be multiplied by the parameter 32/64 in the calculation process of the compensation value CMP of the pixel in the direction, so that the compensation value CMP of the pixel is obtained by applying the parameter to the formula (1). The luminance decrease rate in this direction corresponding to the parameter wgt=32/64 is reduced to 1/2 of the original value. In addition, as shown in the 4 graphs of up, down, left and right, the brightness decrease speed in the up, down, left and right directions is reduced, so the parameter WGT on the corresponding axis can be set to 32/64, and the compensation value CMP of the corresponding direction pixel can be calculated according to the parameter WGT. For example, the parameters WGT_2 and WGT_3 may be set to 32/64, respectively, and the compensation value CMP of the sensing pixel located in the upper half of the sensor array is 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 of the brightness decrease in the direction caused by the lens tilt. On a fingerprint image, one or more of the parameters WGT_0-WGT_3 may be adjusted according to the lens tilt. It should be noted that the above values 32/64 are only exemplary values for compensating for the decrease in brightness, the actual values may be determined according to the degree of lens tilt, and different parameter values may be set in different directions, so as to calculate a more ideal image signal compensation value.

It is an object of the present invention to provide an image compensation circuit and an image compensation method thereof for compensating an image signal output by an image sensor, which can compensate a relative illuminance difference generated on the image signal according to factors such as lens characteristics, component variations, assembly tolerances, and/or lens tilt. Those skilled in the art will recognize modifications or variations which may be made thereto and are not limited thereto. For example, in the above embodiment, the image signal IMG is transmitted to the analog front-end circuit in the form of a voltage, and the voltage signal is converted into a digital code by the analog-to-digital converter. In yet another embodiment, the fingerprint information can be carried by the current signal or other signals for transmission according to the type of the image sensor, and the analog front-end circuit can convert the current signal into the voltage signal for compensation by the compensation voltage. In addition, the above embodiments take the fingerprint sensor and the fingerprint signal as examples to illustrate the related operations of the image sensor; however, it should be understood by those skilled in the art that the embodiments of the present invention are applicable to any optical image sensor having any purpose, and any image sensor that uses light sensing to obtain sensing information may be subject to errors in image signals due to factors such as differences in lens illumination, assembly tolerances, and/or lens tilt, and thus can be compensated by the image compensation circuit and method of the present invention.

In addition, the image compensation circuit (such as the image compensation circuit 40 shown in fig. 4) of the present invention can be implemented in a sensing integrated circuit (Integrated Circuit, IC). In addition, the image sensor including the sensing pixel array may be integrated in the same integrated circuit as other circuit components in the image compensation circuit; or integrated in a display screen and realized by a panel process, and simultaneously sensing by utilizing the light source of the panel; alternatively, the image sensor may be provided separately from the display screen and the compensation circuit. In addition, the timing controller may be integrated with other circuit components in the image compensation circuit in the same integrated circuit or provided in a separately existing integrated circuit.

It should be noted that the image compensation circuit and method of the present invention are used to compensate the difference of the relative illumination corresponding to different sensing pixels in the image signal, and the difference of the relative illumination is usually represented by the difference of brightness. Therefore, the relative illuminance difference or the luminance difference mentioned in the above embodiments are all the parts to be compensated on the image signal, and the names thereof are interchangeable in the present specification, and do not affect the description of the embodiments.

In summary, the present invention provides an image compensation circuit and method for compensating for the relative illuminance difference of different sensing pixels, which can be used in an image sensor (such as an optical fingerprint sensor). The image compensation circuit can sense in the test flow and obtain the proper 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, the digital-analog converter can be utilized to take out the compensation value from the storage unit in the fingerprint identification process, and the compensation value is converted into the compensation voltage for compensation. In one embodiment, it may be determined whether the lens is tilted in a module test, so that the compensation value is adjusted in response to the lens tilt. By the compensation, the brightness consistency of the whole fingerprint image can be improved. The compensation mode of the invention can be carried out in the front-end circuit of the analog before the analog-digital converter so as to compensate before or after the gain amplifier of the analog end amplifies the image signal, thereby avoiding the image signal from reaching saturation when entering the analog-digital converter and improving the space for amplifying the signal. In addition, the fingerprint image with poor resolution can be more effectively amplified and the difference of the wave crest and the wave trough can be interpreted.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in 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 for receiving a plurality of image signals from the image sensor and amplifying the plurality of image signals;

a compensation control circuit for generating a plurality of compensation values for the plurality of image signals;

a memory unit coupled to the compensation control circuit for storing the compensation values; and

the digital-analog converter is coupled to the memory unit and the gain amplifier and is used for respectively converting the compensation values into a plurality of compensation voltages so as to compensate the image signals by utilizing the plurality of compensation voltages.

2. The image compensation circuit of claim 1, wherein the plurality of image signals are respectively derived from a plurality of sensing pixels in the image sensor, and the plurality of compensation values respectively correspond to the plurality of sensing pixels to compensate for a corresponding one of the plurality of image signals.

3. The image compensation circuit of claim 2, further comprising:

a time sequence controller coupled to the compensation control circuit for providing coordinate information of the sensing pixels to the compensation control circuit;

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 of claim 1, wherein the plurality of image signals are compensated by the plurality of compensation voltages after being amplified by the gain amplifier.

5. The image compensation circuit of claim 1, wherein the plurality of image signals are amplified by the gain amplifier after being compensated by the plurality of compensation voltages.

6. The image compensation circuit of 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 compensation values are respectively determined according to distances between the plurality of sensing pixels and a center of the lens.

7. The image compensation circuit of claim 6, wherein the plurality of sensing pixels are separated into a first region, a second region and a third region according to a distance from the center of the lens, wherein the first region is located inside an inner circle centered on the center of the lens, the second region is located between the inner circle and an outer circle centered on the center of the lens, and the third region is located outside the outer circle.

8. The image compensation circuit of claim 7, wherein the plurality of compensation values corresponding to the plurality of sensing pixels of the first region have a maximum value, the plurality of compensation values corresponding to the plurality of sensing pixels of the third region have a minimum value, and the plurality of compensation values corresponding to the plurality of sensing pixels of the second region are located between the maximum value and the minimum value and gradually decrease with increasing distance from the center of the lens.

9. The image compensation circuit of claim 7, wherein the compensation values corresponding to the sensing pixels of the second region are projected onto an axis, and the direction of the axis is set according to the direction of the lens tilt when the lens tilt.

10. The image compensation circuit of claim 6, wherein the plurality of compensation values are adjusted according to a direction of the lens tilt and a degree of the lens tilt when the lens is tilted.

11. The image compensation circuit of claim 10, wherein when the lens is tilted, a distance data of a plurality of sensing pixels corresponding to the plurality of compensation values is multiplied by a parameter to adjust the plurality of compensation values.

12. The image compensation circuit of claim 1, wherein the magnitude of the plurality of compensation values is determined based on a sensing result of sensing a smooth surface object with the image sensor in a test procedure.

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 to compensate the plurality of image signals by using the plurality of compensation voltages.

14. The method of claim 13, wherein the plurality of image signals are respectively from a plurality of sensing pixels in the image sensor, and the plurality of compensation values respectively correspond to the plurality of sensing pixels to compensate for corresponding image signals in the plurality of image signals.

15. The image compensation method of 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 of claim 13, wherein the plurality of image signals are amplified before being compensated by the plurality of compensation voltages.

17. The image compensation method of claim 13, wherein the plurality of image signals are amplified after being compensated by the plurality of compensation voltages.

18. The image compensation method of 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 method further comprises:

the magnitude of the compensation values is determined according to the distances between the sensing pixels and the center of the lens.

19. The method of claim 18, wherein the plurality of sensing pixels are divided into a first region, a second region and a third region according to a distance from the center of the lens, wherein the first region is located inside an inner circle centered on the center of the lens, the second region is located between the inner circle and an outer circle centered on the center of the lens, and the third region is located outside the outer circle.

20. The method of claim 19, wherein the plurality of compensation values corresponding to the plurality of sensing pixels of the first region have a maximum value, the plurality of compensation values corresponding to the plurality of sensing pixels of the third region have a minimum value, and the plurality of compensation values corresponding to the plurality of sensing pixels of the second region are located between the maximum value and the minimum value and gradually decrease with increasing distance from the center of the lens.

21. The image compensation method of claim 19, wherein the plurality of compensation values corresponding to the plurality of sensing pixels of the second region are projected to an axis for calculation, and further comprising:

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 of claim 18, further comprising:

when the lens is inclined, the plurality of compensation values are adjusted according to the direction and the degree of inclination of the lens.

23. The method of claim 22, wherein the step of adjusting the plurality of compensation values according to the direction of the inclination of the lens and the degree of the inclination comprises:

and multiplying the distance data of the sensing pixels corresponding to the compensation values by a parameter respectively to adjust the compensation values.

24. The image compensation method of claim 13, further comprising:

in a test process, the image sensor is used for sensing an object with a smooth surface so as to generate a sensing result; and

determining the magnitude of the plurality of compensation values according to the sensing result.

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