Patent Publication Number: US-2023164463-A1

Title: Image compensation circuit and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image compensation circuit and method, and more particularly, to an image compensation circuit and method used for an optical image sensor. 
     2. Description of the Prior Art 
     In recent years, optical fingerprint recognition has become one of the most popular fingerprint recognition schemes. In an optical fingerprint sensor, due to the feature of the sensing element, a difference of the relative illumination (RI) may be generated in the output sensing signals. This RI difference may cause that the back-end signals are easily saturated. Please refer to  FIG.  1   , which is a schematic 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 superimposed to form the modular structure. The operational principle of the optical fingerprint sensor module  10  is that a light source of the panel  102  delivers light to the place on which a finger presses. After the light irradiates the finger and reflects by the fingerprint, the reflected light may pass through the structure of the panel  102  and then reach the lens  104 . The lens  104  collects the light so that the light can reach the pixels in the sensor  106 . The pixels may convert the sensed light intensity into voltage signals, which will be processed by the back-end circuit to output an entire fingerprint image. 
     As shown in  FIG.  1   , in the structure of the optical fingerprint sensor module  10 , the path of the light reflected from the finger to reach the sensor  106  may pass through the structures of the panel  102  and the lens  104 . Due to the differences between the devices&#39; characteristics and the assembly mismatch, the RI may appear to have significant variations on different positions of the sensor  106 . Such differences include the distance variations between respective module space such as the variations of thickness of the materials and the variations generated in staking the modules (e.g., as shown in  FIG.  1   , the height z 1 , z 2  or z 3  of each part of the module may possess an error), the difference of the light absorption rates of different modules, the difference between the uniformity and illumination of the display light spots, and the light passing through the panel  102  which is easily influenced by its structure, process variations of the curvature of the lens  104 , and the assembly mismatch that may appear in the assembly process of the modules, causing the lens  104  to tilt and increasing the contrast of brightness. All of the above factors generate variations in the light focus behavior of the optical fingerprint sensor module  10 , resulting in errors appearing in the output image signals. 
     Please refer to  FIG.  2   , which is a schematic diagram of the RI of the lens  104 . As shown in  FIG.  2   , based on the optical feature of the lens  104 , the central region (with shorter distance from the lens center) has higher RI and the peripheral region (with longer distance from the lens center) has lower RI. Therefore, there is a fixed pattern offset where the central region of the lens usually has higher brightness and the peripheral region of the lens usually has lower brightness, as the circles of equivalent brightness shown in  FIG.  2   . 
     Please refer to  FIG.  3   , which illustrates fingerprint images of different fingerprint sensor modules. Among these images, figure (a) is a normal fingerprint image, where a round fingerprint is clearly shown, and the center is brighter and the peripheral is darker. Figure (b) shows the influence caused by a slightly tilted lens, where it can be seen that the right-hand side of the fingerprint image appears a black area blocking the fingerprint image. Figure (c) is a diagram showing that the lens is tilted severely, where the upper-left corner of the image appears an obvious black image, causing that the light sensing area capable of fingerprint detection at the upper-left side shrinks significantly. 
     Thus, there is a need to provide an image compensation circuit and method used for the optical fingerprint sensor module  10 , to compensate for various factors such as the lens feature, device variations, assembly mismatches, and lens tilt that cause errors and offsets on the image signals. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide an image compensation circuit and method for an image sensor, to compensate for the errors of the relative illumination (RI) generated from various variations from light emission to imaging in the sensor. 
     An embodiment of the present invention discloses an image compensation circuit for an image sensor. The image compensation circuit comprises a gain amplifier, a compensation control circuit, a memory 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, coupled to the compensation control circuit, is used for storing the plurality of compensation values. The DAC, coupled to the memory and the gain amplifier, is used for converting the plurality of compensation values into a plurality of compensation voltages, respectively, to compensate the plurality of image signals with 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 comprises steps of: 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 memory; and converting the plurality of compensation values into a plurality of compensation voltages, respectively, to compensate the plurality of image signals with the plurality of compensation voltages. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a general optical fingerprint sensor module. 
         FIG.  2    is a schematic diagram of the RI of the lens. 
         FIG.  3    illustrates fingerprint images of 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 an embodiment of the present invention. 
         FIGS.  6 A and  6 B  are schematic diagrams of a sensing pixel array and the corresponding compensation values according to embodiments of the present invention. 
         FIG.  7    is a schematic diagram of the lens tilt causing an elliptic sensing area. 
         FIG.  8    is a flowchart of an image processing process for compensating for the lens tilt according to an embodiment of the present invention. 
         FIG.  9    is a schematic diagram of calculating the compensation values by adjusting the axes based on the tilt direction of lens according to an embodiment of the present invention. 
         FIG.  10    illustrates generating different parameter values under different tilt directions of the lens. 
     
    
    
     DETAILED DESCRIPTION 
     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 (AFE) circuit  410 , an analog-to-digital converter (ADC)  420 , a compensation control circuit  430 , a memory  432  and a timing controller  440 . The image sensor  400  includes a plurality of sensing pixels arranged as an array, where each sensing pixel includes alight sensing element (such as a photodiode) capable of sensing light and converting the light intensity into image signals IMG in voltage or current form. In an embodiment, the image sensor  400  may be an optical fingerprint sensor, for detecting light reflected from a finger to perform fingerprint sensing. The AFE circuit  410 , coupled to the image sensor  400 , may receive the image signals IMG from the image sensor  400  and process the image signals IMG. In detail, the AFE circuit  410  includes a programmable gain amplifier (PGA)  412  and a digital-to-analog converter (DAC)  414 . The PGA  412  may amplify the image signals IMG received from the image sensor  400 , where the gain magnitude may be adjusted according to system requirements. The DAC  414  may convert compensation values CMP used for the image signals IMG into corresponding compensation voltages, to use the compensation voltages to compensate the image signals IMG. After the image signals IMG are completely amplified and compensated in the AFE circuit  410 , the ADC  420  may convert the image signals IMG in voltage form into digital codes, to output the image data in the digital form through an image output interface to a back-end processor to perform fingerprint recognition. The output interface may be, for example, a serial peripheral interface (SPI), but not limited thereto. 
     The timing controller  440 , coupled to the image sensor  400 , is used for controlling sensing operations of the image sensor  400 . As mentioned above, the image sensor  400  includes an array of sensing pixels, and the timing controller  440  is used for controlling the timing of image sensing performed by the plurality of sensing pixels and outputting the image signals IMG. In detail, the image sensor  400  may be deployed with a column decoder  402  and a row decoder  404 . The column decoder  402  may drive the operations of the pixels column by column, and the row decoder  404  may drive the operations of the pixels row by row. The timing controller  440  may control the sensing pixels in the image sensor  400  through interactive controls of the column decoder  402  and the row decoder  404 . In general, the optical fingerprint sensing operations include reset, exposure and sample operations, and the timing controller  440  may control each sensing pixel to follow a predetermined timing scheme to perform the above sensing operations and output the image signals IMG. 
     The compensation control circuit  430  may generate the compensation values CMP used for the image signals IMG. The compensation values CMP may be used to compensate for the relative illumination (RI) errors of the image signals IMG. As can be seen in  FIG.  2   , the lens feature causes that different sensing pixels have different RI; hence, each pixel has its corresponding compensation value CMP, to compensate for the difference of RI. In such a situation, the timing controller  440  may be coupled to the compensation control circuit  430 . In addition to controlling the timing of the sensing pixels outputting the image signals IMG, the timing controller  440  may also provide the coordinate information of the sensing pixels for the compensation control circuit  430 , allowing the compensation control circuit  430  to generate the compensation value CMP respectively corresponding to each of the sensing pixels according to the coordinate information and store the compensation values CMP in the memory  432 . The memory  432  may be a buffer implemented with D flip-flops, for example. When the sensing pixels output the image signals IMG to the AFE circuit  410 , the DAC  414  may extract the compensation values CMP corresponding to the sensing pixels from the memory  432 , convert the compensation values CMP into the corresponding compensation voltages, and then add the compensation voltages to the received image signals IMG to perform compensation. 
     In an embodiment, the image signal IMG may first be amplified by the PGA  412 , and then compensated through the compensation voltage. Alternatively, the image signal IMG may first be compensated through the compensation voltage, and then amplified by the PGA  412 . Those skilled in the art may select an appropriate compensation method according to practical requirements, and the compensation methods should not be used to limit the scope of the present invention. 
     In an embodiment, the compensation voltage may be a deduction value to be applied to the image signal IMG; that is, the voltage value of the image signal IMG may deduct the compensation voltage to eliminate the influences of the RI. In such a situation, as for a pixel having greater RI, the compensation control circuit  430  may generate a greater compensation value CMP to deduct a larger level of voltage from the image signal IMG. As for a pixel having smaller RI, the compensation control circuit  430  may generate a smaller compensation value CMP to deduct a smaller level of voltage from the image signal IMG. As a result, the influences resulting from RI variations between different pixels may be eliminated, so as to increase the uniformity of RI, so that the overall image signals IMG may fall on similar levels. 
     Therefore, after the image signals IMG are compensated, the deviation caused by lens difference and/or assembly mismatch may be eliminated, so that the levels of the image signals IMG output by the pixels may be close to each other. These image signals IMG converted by the ADC  420  to the digital data will then be easily recognized by the subsequent processing circuit. Those fingerprint images having worse resolution may also be amplified more effectively to differentiate the difference between ridges and valleys. Note that the present invention performs compensation before the image signals IMG are converted into digital data; this approach avoids signal saturation that causes the fingerprint information to be eliminated during the conversion process of the ADC  420 . 
     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 process  50  may be used for an image compensation circuit such as the image compensation circuit  40  shown in  FIG.  4   , for receiving fingerprint image signals from the image sensor  400  (which will be considered as a fingerprint sensor as an example hereinafter) and performing scan, compensation and recognition on the fingerprint image signals. 
     The image processing process  50  may be divided into two parts: a test process and a fingerprint recognition process. First, a test sample is placed on the sensing area of the fingerprint sensor, and simultaneously the panel corresponding to the fingerprint sensor displays a light spot to forward light to the test sample. After being reflected by the test sample, the light is gathered by the lens and then reaches the sensing pixels in the fingerprint sensor to form the image. 
     During the test process, the test sample may be an object having a flat surface, such as a rubber sheet. The fingerprint sensor scans the test sample to obtain the image signals IMG, and transmits the image signals IMG to the image compensation circuit  40 . In the test process, the image compensation circuit  40  may determine that the compensation is not completed yet, and sequentially obtain the image signal IMG corresponding to each of the sensing pixels. At this time, since the test sample is a flat surface object without any ridge-to-valley difference of fingerprint, the image compensation circuit  40  may expect that each sensing pixel should generate the image signal IMG having the same voltage when there is no RI error. In other words, after the light is reflected by the flat object, the difference of signals sensed by the sensing pixels after lens focus may be equivalent to the RI difference that needs to be eliminated. In such a situation, based on the image signals IMG obtained by scanning, the compensation control circuit  430  may calculate the compensation value CMP corresponding to each of the sensing pixels, and store the compensation values CMP in the memory  432  or update the data of the compensation values CMP stored in the memory  432 . 
     Subsequently, during the fingerprint recognition process, the object put on the fingerprint sensing area is a finger. At this moment, the image compensation circuit  40  determines that there is a need to perform RI compensation. Therefore, the fingerprint sensor scans and transmits the corresponding image signals IMG to the AFE circuit  410 , and the DAC  414  in the AFE circuit  410  may take the corresponding compensation values CMP from the memory  432  and convert them into compensation voltages. The PGA  412  amplifies the image signals IMG which are compensated by the compensation voltages, and the image signals IMG are then transmitted to the ADC  420 . The ADC  420  converts the image signals IMG to digital codes to be output, in order to perform fingerprint recognition and interpretation through the back-end processor. 
     The compensation values CMP for compensating the image signals may be generated in various manners. In an embodiment, the required compensation value CMP may be determined according to the distance between the sensing pixel and the lens center. As shown in  FIG.  6 A , the compensation value CMP is determined based on the distance from the pixel (x, y) to the lens center (cnt_x, cnt_y) only, to compensate for the RI difference as shown in  FIG.  2   . In detail, in the fingerprint sensing area, an inner circle (whose radius is r min) centered on the lens center (cnt_x, cnt_y) and an outer circle (whose radius is r_max) centered on the lens center are set, wherein the radius r_max of the outer circle is greater than the radius r min of the inner circle. Based on the outer circle and the inner circle, the sensing pixels may be separated into a first area R 1 , a second area R 2  and a third area R 3 , wherein the first area R 1  is inside the inner circle, the second area R 2  is between the inner circle and the outer circle, and the third area R 3  is outside the outer circle. 
     Subsequently, the compensation control circuit  430  may calculate the corresponding compensation values CMP for the sensing pixels in different areas. In detail, the sensing pixels in the first area R 1  are located inside the inner circle, which means that these pixels are closer to the lens center and have larger RI; hence, the compensation values CMP for these pixels may be set to a maximum value such as 255. The sensing pixels in the third area R 3  are located outside the outer circle, which means that these pixels are closer to the peripheral of lens and have smaller RI; hence, the compensation values CMP for these pixels may be set to a minimum value such as 35. The sensing pixels in the second area R 2  are located between the inner circle and the outer circle, and their corresponding compensation values CMP fall between the maximum value and the minimum value (i.e., between 255 and 35), and gradually decrease with increasing distance from the lens center to the pixel. As shown in  FIG.  6 A , with the increase of distance from the lens center to the pixel, the compensation values CMP sequentially decrease from  200 ,  145  to  90 . Note that the values shown in  FIG.  6 A  are one of various implementations of the present invention, and those skilled in the art may adopt appropriate compensation values to be converted into the compensation voltages according to the bit count of the DAC  414 . 
     In an embodiment, the compensation values CMP for the sensing pixels in the second area R 2  located between the inner circle and the outer circle may be calculated through the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       C 
                       ⁢ 
                       M 
                       ⁢ 
                       P 
                     
                     = 
                     
                       
                         
                           ( 
                           
                             DAC_max 
                             - 
                             DAC_min 
                           
                           ) 
                         
                         × 
                         
                           
                             ( 
                             
                               r_max 
                               - 
                               r_data 
                             
                             ) 
                           
                           
                             ( 
                             
                               r_max 
                               - 
                               r_min 
                             
                             ) 
                           
                         
                       
                       + 
                       DAC_min 
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where r_data is a distance data representing the distance between the pixel and the lens center, and DAC max and DAC min refer to the maximum and the minimum compensation values, respectively. Through calculation of the above equation, it can be ensured that the compensation values CMP for the sensing pixels located between the inner circle and the outer circle may decrease linearly from the inside out, in order to effectively compensate for the RI variations caused by the optical feature of the lens. 
     Please note that  FIG.  6 A  only illustrates 5×5 sensing pixels and their corresponding compensation values as an example. In fact, the image sensor may include hundreds of columns and hundreds of rows of sensing pixels, where the compensation value corresponding to each pixel may be obtained by area-based calculation in the above manner. 
       FIG.  6 B  illustrates another compensation method. In addition to compensating for the RI difference resulting from the lens feature, the compensation is further used for RI difference caused by the variations of light focus behavior and assembly mismatch. As mentioned above, the light is reflected by the finger and then passes through the lens to reach the sensing pixels in the image sensor. The light path should pass through different materials of the panel structure and the lens, and the light is also influenced by the assembly mismatch; hence, irregular brightness difference may be generated in the sensing pixels. Therefore, in the embodiment of  FIG.  6 B , the sensing pixels may be divided into several blocks, and each block corresponds to one compensation value CMP, which performs compensation to make the overall brightness identical. In the above test process, the image sensor  400  may be used to perform sensing on an object having a flat surface to generate a sensing result. The compensation control circuit  430  thereby calculates the compensation value CMP required by each block according to the sensing result. Based on the feature that the center area of the lens has higher RI and the peripheral area of the lens has lower RI, the calculated compensation values CMP will roughly comply with this trend but have slightly irregular differences, such as the distribution of compensation values CMP as shown in  FIG.  6 B . 
     Note that  FIG.  6 B  illustrates 5×5 blocks and their corresponding compensation values CMP as an example, where each block may include any number of sensing pixels. In addition, according to the range of the sensing area and the total number of sensing pixels, the pixels in the image sensor may be divided into any number of blocks in an appropriate manner, which is not limited herein. 
     In another embodiment, considering that the influences on the image signals IMG that may be caused by lens tilt, the above Equation (1) used to calculate the compensation values CMP may further be adjusted correspondingly. Referring back to  FIG.  3   , as shown in figures (b) and (c), when the lens tilt is found during the module test (MT) process, the border in a certain side may be blocked and cannot show a fingerprint image accurately. At this moment, the fingerprint image in the sensing area appears to not circular but elliptic, as shown in  FIG.  7   . In the fingerprint image, the blocked part shows a sharp decrease of brightness, and the decrease of brightness on the opposite side becomes gentler. In other words, although the image signals IMG of the fingerprint still substantially comply with the distribution that the lens center has higher RI and the lens periphery has lower RI with gradual decrease from the inside out, the decreasing speed of RI in different directions will be different. 
     In general, during the fingerprint sensing process, the image sensor may scan the region covered by the finger and sense light, to generate a circular image as shown in  FIG.  6 A . The region where the sensing pixels are scanned to obtain the fingerprint image may be considered as a region of interest (ROI). When the lens tilt occurs, partial of the scanned image may be blocked and cannot be used to obtain the fingerprint image data; hence, the ROI should be adjusted correspondingly to exclude the blocked region, preventing the image signals on the blocked position from affecting the overall fingerprint recognition result. In an embodiment, the fingerprint image may be observed and the degree of lens tilt may be determined during the MT process, so as to modify the fingerprint sensing area (e.g., the abovementioned ROI) accordingly. Alternatively, the image sensor or image compensation circuit may determine the degree of lens tilt according to the detected image content (e.g., detecting whether the image has a deviation or a region where the brightness decreases sharply), to adjust the fingerprint sensing area according to the detection result. 
     When the lens tilt occurs, the above Equation (1) for calculating the compensation values CMP should also be adjusted correspondingly. In an embodiment, the compensation values CMP corresponding to the sensing pixels located between the inner circle and the outer circle may be projected to the x-axis and the y-axis to be calculated. When the lens is tilted, the directions of the x-axis and the y-axis for calculating the compensation values CMP may be set according to the tilt direction of the lens. Correspondingly, the distance data r_data (i.e., the distance between the pixel and the lens center) may be multiplied by a parameter or the value of the distance data r_data may be directly adjusted. Taking  FIG.  7    as an example, the x-axis and the y-axis may be set to be tilted by about 45 degrees, and the distances at the upper-right side and the lower-left side may be adjusted based on the tilt direction, so as to recalculate the appropriate compensation values CMP accordingly. 
     In an embodiment, the adjustment of compensation values to be adaptive to the lens tilt may be performed if the MT is failed. Please refer to  FIG.  8   , which is a flowchart of an image processing process  80  for compensating for the lens tilt according to an embodiment of the present invention. As shown in  FIG.  8   , the system may perform the MT once based on ideal compensation values CMP without deviation of lens tilt. When the MT is failed, the system may further determine whether there is a lens tilt (e.g., determining whether there is a block having brightness decreased sharply) according to the grayscale variations of the image. If it is determined that no lens tilt occurs, other tests may be performed to find out the problem. If it is determined that the lens is tilted, the compensation control circuit  430  may further calculate and set the directions of the x-axis and the y-axis according to the tilt direction of lens, and adjust the range of the ROI. Subsequently, the compensation control circuit  430  may calculate the compensation value CMP for each sensing pixel based on the new ROI, x-axis and y-axis, and then update the compensation values CMP and store the compensation values CMP in the memory  432 . The image sensor  400  thereby rescans based on the updated compensation values CMP and outputs the fingerprint data to the back end to perform fingerprint recognition, so as to determine whether the fingerprint data obtained after being compensated by the new compensation values CMP are accurate. In this embodiment, if the rescan still generates a wrong recognition result, the compensation control circuit  430  may recalculate and adjust the parameters, until the accurate fingerprint recognition result is acquired. 
     Please refer to  FIG.  9   , which is a schematic diagram of calculating the compensation values by adjusting the axes based on the tilt direction of lens according to an embodiment of the present invention. As shown in  FIG.  9   , according to the tilt direction of lens, the x-axis and the y-axis are rotated by 45 degrees. For example, the axis from upper-right to lower-left may be considered as the x-axis, and the axis from upper-left to lower-right may be considered as the y-axis. The x-axis and the y-axis may be interchanged without affecting the illustrations of the present embodiment. In addition, the  4  directions, lower-right, lower-left, upper-right and upper-left are assigned with 4 parameters, WGT_ 0 , WGT_ 1 , WGT_ 2  and WFT_ 3 , respectively. These parameters may correspond to the RI distribution of respective axis direction, and may be used for adjusting the compensation values of the sensing pixels along the direction. That is, the compensation values corresponding to the sensing pixels may be adjusted by using the corresponding parameters WGT_ 0 , WGT_ 1 , WGT_ 2  and/or WFT_ 3  according to the tilt direction and the tilt degree of the lens and also according to the position of the pixel. 
     Please further refer to  FIG.  10   , which illustrates generating different values of the parameters WGT_ 0 , WGT_ 1 , WGT_ 2  and WFT_ 3  under different tilt directions of the lens. For example, the middle figure is an ideal pattern without lens tilt, which has the maximum brightness in the center and the brightness decreases uniformly toward every direction; hence, the parameters WGT_ 0 -WGT_ 3  along all directions may be set to 1, which means that the calculated compensation values CMP need not to be further adjusted. As shown in the  4  figures at the upper-left, upper-right, lower-left and lower-right sides, the fingerprint image has a deviation resulting from different tilt directions of the lens, and the tilt directions correspond to the directions of 4 axes, respectively. In these figures, the lens tilt causes that the brightness decreases slowly along a certain direction; hence, the parameter WGT of the axis toward this direction may be set to 32/64, which means that the distance data r_data should be multiplied by the parameter 32/64 in the calculation of the compensation values CMP of the pixels in this direction, in order to apply the Equation (1) to obtain the compensation values CMP of the pixels. The parameter WGT=32/64 means that the speed of brightness decrease toward the direction is ½ of its normal speed. In addition, as shown in the  4  figures at the upper, lower, left and right sides, the speed of brightness decrease slows down toward the upper, lower, left and right directions, respectively; hence, the parameter WGT in the corresponding axis may be set to 32/64, and the compensation values CMP of the pixels in the corresponding directions may be calculated accordingly. Taking the upper figure as an example, the parameters WGT_ 2  and WGT_ 3  may be set to 32/64, and the compensation values CMP of the sensing pixels located in the upper half part of the sensor array may be calculated and adjusted accordingly. 
     In other words, the parameters WGT_ 0 , WGT_ 1 , WGT_ 2  and WGT_ 3  indifferent directions may be set according to the speed of brightness decrease toward the corresponding direction caused by the lens tilt. In a fingerprint image, one or more of the parameters WGT_ 0 -WGT_ 3  may be adjusted based on the situation of lens tilt. Note that the above value 32/64 is only an example used to illustrate the compensation for the lowered speed of brightness decrease. The practical values may be determined based on the degree of lens tilt, and different directions may have different parameter values, so as to calculate more ideal compensation values for the image signals. 
     Please note that the present invention aims at providing an image compensation circuit capable of compensating the image signals output by an image sensor and the related image compensation method, which may compensation for the RI difference generated on the image signals according to various factors such as the lens feature, device variation, assembly mismatch, and/or lens tilt. Those skilled in the art may make modifications and alterations accordingly. For example, in the above embodiments, the image signals IMG are transmitted to the AFE circuit in the voltage form, and the voltage signals are converted, by the ADC, into the digital codes. In another embodiment, current signals or other type of signals may be utilized to carry the fingerprint information to be transmitted based on the type of image sensor, and the AFE circuit may convert the current signals into voltage signals and then use the compensation voltages to perform compensation. In addition, the above embodiments illustrate the operations associated with the image sensor by taking a fingerprint sensor with fingerprint signals as an example. However, those skilled in the art should understand that the embodiments of the present invention are applicable to an optical image sensor for any purpose. As long as the image sensor obtains the sensing information by using optical sensing, it may be influenced by various factors such as the illumination difference of lens, assembly mismatch and/or lens tilt so as to generate an error in the image signals. In such a situation, the image compensation circuit and method of the present invention may be applied to perform compensation. 
     In addition, the image compensation circuit of the present invention (such as the image compensation circuit  40  shown in  FIG.  4   ) may be implemented in a sensor integrated circuit (IC). Also, the image sensor having the sensing pixel array may be integrated with other circuit elements of the image compensation circuit in the same IC, or may be integrated into a display panel to be implemented with a panel process, to perform sensing by using the light source of the panel. Alternatively, the image sensor may be disposed independent to the display panel and the compensation circuit. Further, the timing controller may also be integrated with other circuit elements of the image compensation circuit in the same IC, or disposed as a stand-alone IC. 
     Please also note that the image compensation circuit and method of the present invention are used to compensate for the RI difference corresponding to different sensing pixels in the image signals, and the RI difference usually appears as the difference of brightness. Therefore, in the above embodiments, the proposed RI difference and brightness difference are both interferences in the image signals that need to be compensated, and their names in this disclosure may be interchanged without influencing the illustrations of the embodiments. 
     To sum up, the present invention provides an image compensation circuit and method for compensating for the RI difference corresponding to different sensing pixels, to be used for an image sensor (such as an optical fingerprint sensor). The image compensation circuit may perform sensing during the test process, to obtain an appropriate compensation value for each pixel or block, or calculate a corresponding compensation value according to the position of the sensing pixel relative to the lens center. The image compensation circuit may also store the compensation values in a memory. Subsequently, during the fingerprint recognition process, the DAC is served to receive the compensation values from the memory and convert the compensation values into compensation voltages to perform compensation. In an embodiment, whether the lens is tilted may be determined in the MT, in order to adjust the compensation values to be adapted to the lens tilt. Through the above compensation, the brightness uniformity of the overall fingerprint image may be improved. The compensation method of the present invention may be performed in the AFE circuit in front of the ADC, to perform compensation before or after the gain amplifier amplifies the image signal in the analog domain. This prevents the image signal from being saturated when entering the ADC, to enhance the range of signal amplification. In addition, the fingerprint image having worse resolution may also be effectively amplified to analyze the difference between ridges and valleys. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.