Camera back focus offset detection and correction method

Abstract

The invention provides a method for detecting and correcting back focus offset of a camera, which can detect and correct the back focus offset of the camera. The deviation correction process comprises the following steps that 1, under the current ZOOM motor position, a system reversely steps an FOCUS motor, and the image definition evaluation value of each stepping position is recorded step by step; 2, entering a first round of judgment, carrying out slope calculation on the image definition evaluation value, and if the maximum value of the slope is greater than or equal to a first threshold value A1, exiting the deviation correction process; 3, if the maximum value of the slope is smaller than A1, entering a second round of judgment; slope calculation is conducted again, and if the absolute value of the maximum slope value is smaller than or equal to A2, the deviation rectification process is quitted; 4, if the exit condition is not triggered in the two rounds of judgment, broadening N0 step points on the boundary of the focus following curve, and then restarting the automatic focusing module to find a new focus; and recording and calculating a difference value between the adjusted new focus position and the initial boundary position, and taking the difference value as a back focus offset calibration value.

Description

Technical Field

[0001] The present invention relates to a method for detecting and correcting the back focus offset of a camera, which is applied to an educational recording and broadcasting, video conferencing or intelligent monitoring system. Background Art

[0002] In the focusing system of a zoom camera, the back focus refers to the distance from the last optical element of the lens to the imaging sensor. Normally, the automatic focusing system of the camera adjusts the position of the lens according to this distance to ensure that the image of the target object is clearly imaged on the sensor. However, with the long-term use of the device, the back focus position of the camera may shift due to factors such as external vibration, temperature change, and device aging. In educational and conference scenarios, the camera usually needs to run for a long time and may frequently zoom or move the lens to take pictures at different angles. The complex environment in these scenarios, such as frequent device movement, repeated lens focus adjustment, and even ambient vibration, may exacerbate the occurrence of back focus offset. The back focus offset will directly affect the focusing ability of the camera. Especially at low magnifications, the imaging may become blurred, and the user cannot obtain the required clear picture. This is because once the back focus is offset, the automatic focusing system of the camera will be difficult to accurately identify the best focus, resulting in unstable imaging. In severe cases, manual recalibration may even be required, increasing the maintenance cost and usage difficulty. Therefore, how to effectively correct the back focus offset has become an important issue for improving the long-term use stability of the camera.

[0003] The existing methods mainly focus on measuring the back focus offset or alleviating the impact caused by the back focus offset through specific hardware. For example, the Chinese patent with the application number CN202122262339.7 and the name "A back focus offset measurement device" provides a back focus offset correction device. The application number CN201820101747.X and the name "A camera transparent cover and an imaging optical system having the same" provide additional hardware on the camera to limit the back focus offset amount. However, in educational and conference scenarios, users often order cameras in large quantities. The above two methods will either bring a large after-sales burden and reduce the user experience, or bring additional costs to the camera. The Chinese patent with the application number CN201610169571.7 and the name "A method and device for correcting the back focus offset of the camera module" solves the back focus offset problem from software, but this method can only take effect at a predetermined zoom magnification and does not consider the interference caused by near-object occlusion or low-texture images in the real application scenario. Therefore, it cannot be considered a practical solution. Summary of the Invention

[0004] The object of the present invention is to provide a method for detecting and correcting the back focus offset of a camera with reasonable design, which can detect and correct the back focus offset of the camera, so as to solve the problem that when the back focus of the camera is offset, some focal lengths cannot be successfully focused, affecting the normal use of the camera.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: A method for detecting and correcting the back focus offset of a camera. During the automatic focusing process of the camera, when it is found that the current focusing process enters the boundary area of the follow - focus curve, the deviation correction process is triggered. The deviation correction process is characterized by the following steps: Step 1: After the deviation correction process is started, at the current ZOOM motor position, the system steps the FOCUS motor in the reverse direction, and gradually records the image sharpness evaluation value FV at each step position. Step 2: Enter the first - round judgment of the deviation correction process. Calculate the slope of the gradually recorded image sharpness evaluation value FV, and compare the calculated slope with the first threshold A1; if the maximum value of the slope is greater than or equal to the first threshold A1, it indicates that the sharpness change in this area does not conform to the expectation, which means that the focus has not shifted too far or has reached the plateau near the focus. At this time, it is judged that there is no significant back focus offset, and the deviation correction process is exited, and the normal focusing process is restored. Step 3: If the maximum value of the slope is less than A1, enter the second - round judgment; calculate the slope of the recorded image sharpness evaluation value FV again, take its absolute value and compare it with the second threshold A2; if the absolute value of the maximum value of the slope is less than or equal to A2, exit the deviation correction process. Step 4: If the exit conditions are not triggered in both rounds of judgment, relax the boundary of the follow - focus curve by N0 step points, and then restart the automatic focusing module to find a new focus; the system records and calculates the difference between the adjusted new focus position and the initial boundary position, and uses this difference as the back focus offset calibration value for subsequent adjustment of the back focus state of the camera.

[0006] In step 1 of the present invention, the preset number of steps for the FOCUS motor to step is T.

[0007] In step 3 of the present invention, during the second - round judgment, first perform filtering processing on the recorded image sharpness evaluation value FV, and then calculate the slope of the filtered image sharpness evaluation value FV again.

[0008] In step 4 of the present invention, after obtaining the difference between the adjusted new focus position and the initial boundary position, correct both the upper - limit follow - focus curve and the lower - limit follow - focus curve in the same direction, rather than simply correcting the follow - focus curve contacted in step 1.

[0009] In the fourth step of the present invention, after obtaining the difference between the adjusted new focus position and the initial boundary position, it is necessary to relax the corresponding boundary curve limit to a certain extent to prevent the image sharpness evaluation value FV near the quasi-focus from entering the plateau period, resulting in repeated entry into the rectification process.

[0010] The image sharpness evaluation value FV described in the present invention is obtained by the image processing module of the camera through comprehensive calculation of the contrast, edge sharpness, texture complexity, and gray gradient extracted from the real-time image.

[0011] When performing filtering processing in the present invention, Gaussian filtering is used to eliminate the noise in the image signal.

[0012] Compared with the prior art, the present invention has the following advantages and effects: 1. The present invention is combined with the focusing module function of the camera, but does not impose special requirements on the function of the focusing module and does not rely on the support of a specific hardware structure. This design makes the present invention have a wider applicability and can be compatible with a variety of camera systems without complex hardware modifications.

[0013] 2. The present invention screens and sets appropriate exit thresholds through experiments to ensure that cameras with large back-focus offsets can enter the rectification process when powered on and started, without visual focusing blur.

[0014] 3. The present invention is an image-based method that detects and calculates the defocus state of the camera by obtaining the sequence of image sharpness values obtained by moving the FOCUS motor directionally at the current ZOOM motor position point by point. This method can provide real-time feedback on the focusing state and make timely adjustments to ensure the best imaging effect.

[0015] 4. The present invention has better adaptability to complex application scenarios compared with the prior art and can eliminate the influence of low-texture scenarios and near-object occlusion on back-focus rectification. 5. After two rounds of threshold judgment, the follow-focus curve limit is relaxed, and the focusing module is re-entered to calculate the estimated back-focus offset. By combining with the focusing module, the back-focus offset module will not cause obvious visual stuttering during use. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of the ZOOM-FOCUS follow-focus curve used in the camera of the embodiment of the present invention.

[0017] Figure 2 is a schematic diagram of the back-focus offset correction of the embodiment of the present invention.

[0018] Figure 3 is a flowchart of the post-embodiment of the present invention. Detailed implementation manners

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and through embodiments. The following embodiments are explanations of the present invention, and the present invention is not limited to the following embodiments.

[0021] A method for detecting and correcting the back focus offset of a camera. During the automatic focusing process of the camera, if it is found that the current focusing process enters the boundary area of the follow - focus curve, the correction process is immediately triggered. The correction process includes the following steps: Step 1: After the correction process is started, at the current ZOOM motor position, the system steps the FOCUS motor in the reverse direction with a preset number of steps T. The image sharpness evaluation value FV at each stepped position is gradually recorded, and the sharpness fluctuation in the image signal is detected in real time to determine whether there is a potential back focus offset. The image sharpness evaluation value FV is comprehensively calculated by the image processing module of the camera by extracting multiple parameters from the real - time image, including but not limited to image contrast, edge sharpness, texture complexity, and gray - level gradient, etc., and is used as a comprehensive evaluation index for the current focus sharpness of the camera.

[0022] Step 2: When the correction process enters the first - round judgment, the system calculates the slope of the gradually recorded image sharpness evaluation value FV and compares the calculated slope with the first threshold A1. If the maximum value of the calculated slope is greater than or equal to the first threshold A1, it indicates that the sharpness change in this area does not match the expectation, which means that the focus has not shifted too far or has reached the plateau near the focus. At this time, the system judges that there is no significant back focus offset, exits the correction process, and resumes the normal focusing process; Step 3: If the maximum value of the slope is less than A1, the system enters the second - round judgment. The system filters the recorded image sharpness evaluation value FV sequence to reduce the noise influence caused by texture loss or near - object interference in the image signal. After filtering, the slope of the image sharpness evaluation value FV is calculated again, and its absolute value is compared with the second threshold A2. If the absolute value of the maximum value of the slope is less than or equal to A2, it indicates that the background texture is single or there are interference factors in the image. For example, in the scene of a white - wall background or object occlusion, the system will misjudge the back focus offset. At this time, the correction process is exited; In this embodiment, during the filtering process, Gaussian filtering is used to eliminate the noise in the image signal. Especially when detecting a monotonous background (such as a white - wall or sky scene) or object occlusion (such as the camera being partially blocked by a nearby object), unnecessary correction triggers can be avoided, so as to improve the accuracy of the system in complex scenes.

[0023] Step 4: If the exit condition is not triggered in both rounds of judgment, the system automatically relaxes the boundary of the focusing curve by N0 stepping points (this preset parameter can be appropriately large), and then restarts the autofocus module to find a new focus. The system records and calculates the difference between the position of the new focus after adjustment and the initial boundary position, and uses this difference as the back focus offset calibration value for subsequent adjustment of the back focus state of the camera.

[0024] In this step, after obtaining the difference between the adjusted new focus position and the initial boundary position, both the upper limit focusing curve and the lower limit are corrected in the same direction, rather than simply correcting the focusing curve encountered in Step 1.

[0025] In this step, after obtaining the difference between the adjusted new focus position and the initial boundary position, it is necessary to relax the corresponding boundary curve limit to a certain extent to prevent the image sharpness evaluation value FV near the quasi-focus from entering the plateau period, resulting in the system repeatedly entering the rectification process.

[0026] Example 1: Back focus rectification of the camera in a dynamic education scenario.

[0027] In the multi-functional classroom or gymnasium of the school, the camera is used to record classroom interactions or student sports activities in real time. In such scenarios, the camera faces frequent scene changes and moving objects (such as the movement of students and the interaction of teachers). In addition, the camera may be installed in a corner or at a high place, where it is easily temporarily blocked (such as teaching aids, doors and windows, etc.), further increasing the difficulty of focusing. After long-term use, the camera may experience back focus offset due to vibration or equipment aging, affecting image sharpness. To ensure the stability of the recorded images, the camera enters the automatic rectification process.

[0028] During the autofocus process, when the camera detects that the focus enters the boundary area of the focusing curve, indicating that the focus offset may be outside the curve boundary, the picture quality may already be very low at this time. The system immediately triggers the rectification process to avoid more interference caused by the dynamic movement of students and complex backgrounds.

[0029] Step 1: After the rectification process is started, at the current ZOOM motor position, the system steps the FOCUS motor in the reverse direction with a preset number of steps T, and gradually records the image sharpness evaluation value FV at each stepping point. At this time, the image processing module of the camera will comprehensively calculate the current image sharpness evaluation value FV based on multiple parameters such as contrast, edge sharpness, and texture.

[0030] Step 2: Enter the first round of judgment. The system calculates the slope of the image sharpness evaluation value FV and compares it with the first threshold A1. If the maximum slope value is greater than A1, it means that the system may make a misjudgment in the dynamic background caused by the movement of students, and the actual quasi-base point has not crossed the boundary. At this time, the rectification process is exited and the normal focusing process is restored.

[0031] Step 3: If the slope is less than A1, the system enters the second round of judgment. Due to the dynamic activities of students, the interference in the image may increase. The system filters the FV sequence of the image sharpness evaluation value to reduce the random noise of the image itself and the noise caused by objects quickly passing in front of the lens or temporary occlusion (such as teaching aids, students' gestures, etc.). The maximum value of the slope of the filtered FV of the image sharpness evaluation value is compared with the second threshold A2 again. If the absolute value of the maximum slope is less than A2, it indicates that the interference is too large, and the system exits the rectification process to avoid unnecessary correction.

[0032] Step 4: If the exit condition is not triggered in both rounds of judgment, the system automatically relaxes the boundary N step points of the follow - focus curve and restarts the auto - focus module to find a new focus. At this time, the system records the difference in the adjusted boundary position and uses it as the back - focus offset calibration value to ensure that the camera continuously maintains a clear imaging effect in a complex dynamic education scenario.

[0033] Embodiment 2: Back - focus rectification of a camera in a static meeting scenario.

[0034] In a typical meeting scenario, a camera is used to capture images of participants or speakers. The meeting environment is usually relatively static, with most backgrounds being single - color backgrounds or low - texture walls (such as white walls, projection screens, etc.), and there are few dynamic moving objects interfering. However, due to the long - term operation of the device or the influence of environmental vibration, the back - focus of the camera may shift, resulting in a blurred image. To ensure that important content can be clearly presented to the participants, the camera starts the back - focus rectification process.

[0035] When the camera performs auto - focusing, it detects that the current focus enters the boundary area of the follow - focus curve. Due to the simple and less - changing background, the camera is prone to focusing errors in low - contrast areas. At this time, the system triggers the rectification process to ensure accurate focus of the image.

[0036] Step 1: After the rectification process is started, at the current ZOOM motor position, the system steps the FOCUS motor in the reverse direction with a preset number of steps T, and records the FV of the image sharpness evaluation value at each step point.

[0037] Step 2: The system enters the first round of judgment, calculates the slope of the FV of the image sharpness evaluation value and compares it with the threshold A1. If the maximum value of the slope is greater than A1, it means that the focus has not shifted or has not shifted too far, and the system exits the rectification process and resumes normal focusing.

[0038] Step 3: If the maximum slope is less than A1, the system enters the second-round judgment. Due to the random fluctuation of the image sharpness evaluation value FV caused by possible noise in the image, the system filters the FV sequence of the image sharpness evaluation value to reduce the misjudgment caused by noise. The slope of the filtered image sharpness evaluation value FV is compared with the threshold A2 again. If the absolute value of the maximum slope is less than A2, it indicates that the normal focusing in the low-texture scene is misjudged as back-focus shift, and the system exits the correction process.

[0039] Step 4: If the exit condition is not triggered in both rounds of judgment, the system relaxes the boundary N stepping points of the focus tracking curve and restarts the automatic focusing module to find a new focus. The system records the difference between the new focus and the initial boundary position and uses it as the back-focus calibration value to ensure clear and stable imaging when the camera operates for a long time in a static meeting environment.

[0040] In addition, it should be noted that for the specific embodiments described in this specification, the zero), the shape of the components), the names taken, etc. can be different. The above content described in this specification is only an example of the structure of the present invention. Any equivalent changes or simple changes made according to the structure), features, and principles described in the inventive concept of the present invention are included in the protection scope of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, as long as they do not deviate from the structure of the present invention or exceed the scope defined by this claim book, they should fall within the protection scope of the present invention.

Claims

1. A method for detecting and correcting the back focus offset of a camera. During the automatic focusing process of the camera, when it is found that the current focusing process enters the boundary area of the follow - focus curve, the deviation correction process is triggered. It is characterized in that, The rectification process includes the following steps: Step 1: After the rectification process is started, at the current ZOOM motor position, reverse step the FOCUS motor, and gradually record the image sharpness evaluation value FV at each step position. Step 2: The rectification process enters the first round of judgment. Calculate the slope of the gradually recorded image sharpness evaluation value FV, and compare the calculated slope with the first threshold A1; if the maximum slope value is greater than or equal to the first threshold A1, it indicates that the sharpness change in this area does not match the expectation, indicating that the focus has not shifted too far or has reached the plateau near the focus. At this time, it is judged that there is no significant back focus shift, and the rectification process is exited, and the normal focusing process is restored. Step 3: If the maximum slope value is less than A1, enter the second round of judgment; calculate the slope of the recorded image sharpness evaluation value FV again, take its absolute value and compare it with the second threshold A2; if the absolute value of the maximum slope value is less than or equal to A2, exit the rectification process. Step 4: If the exit conditions are not triggered in both rounds of judgment, relax the boundaries of the focus tracking curve by N0 step points, and then restart the automatic focusing module to find a new focus; the system records and calculates the difference between the adjusted new focus position and the initial boundary position, and uses this difference as the back focus offset calibration value, which is subsequently used to adjust the back focus state of the camera.

2. The method for detecting and correcting the back focal offset of a camera according to claim 1, characterized in that: In Step 1, the preset number of steps for the FOCUS motor to step is T.

3. A method for detecting and correcting the back focus offset of a camera according to claim 1, characterized in that: In Step 3, during the second round of judgment, first perform filtering processing on the recorded image sharpness evaluation value FV, and then calculate the slope of the filtered image sharpness evaluation value FV again.

4. A method for detecting and correcting the back focal shift of a camera according to claim 1, characterized in that: In Step 4, after obtaining the difference between the adjusted new focus position and the initial boundary position, correct both the upper limit focus tracking curve and the lower limit focus tracking curve in the same direction, rather than simply correcting the focus tracking curve touched in Step 1.

5. A method for detecting and correcting the back focal shift of a camera according to claim 1, characterized in that: In Step 4, after obtaining the difference between the adjusted new focus position and the initial boundary position, it is necessary to relax the corresponding boundary curve limits to a certain extent to prevent the image sharpness evaluation value FV near the quasi-focus from entering the plateau and causing repeated entry into the rectification process.

6. A method for detecting and correcting the back focus offset of a camera according to claim 1, characterized in that: The described image sharpness evaluation value FV is obtained by the image processing module of the camera through comprehensive calculation of the contrast, edge sharpness, texture complexity, and gray level gradient of the image extracted from the real-time image.

7. A method for detecting and correcting the back focus offset of a camera according to claim 3, characterized in that: During the filtering process, Gaussian filtering is used to eliminate the noise in the image signal.

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