Binocular camera external parameter calibration method under large field of view based on light emitting diode target

Abstract

According to the binocular camera external parameter calibration method in the large view field based on the light emitting diode target, in order to solve the problems that in the large view field, the imaging area of the target is small, the number of effective visual features is small, and the visual features are distributed unevenly, the target is moved multiple times, and images are shot to obtain a large number of evenly-distributed visual features; secondly, aiming at the problem that the imaging quality is reduced due to environment illumination change and camera thermal noise, an image preprocessing technology is adopted to improve the signal-to-noise ratio of an image, and a light emitting diode array target is adopted to improve the saliency of an image light spot; thirdly, aiming at the problem that the light spot edge in the image is not clear, a light spot area detection technology is adopted in the proposal, a light spot area is accurately segmented from a background image, and pixel binarization is carried out; and the external parameters of the high-precision binocular camera are calculated by using the geometrical relationship between the coordinates of the light spot centroid in the image coordinate system and the coordinates of the center of the light emitting diode in the camera coordinate system.

Description

Technical Field

[0001] The present invention belongs to the technical field of computer vision, and in particular relates to a method for calibrating extrinsic parameters of a binocular camera under a large field of view based on a light emitting diode target. Background Art

[0002] In the modern industrial field, optical measurement technology has been increasingly valued due to its advantages such as high precision, low cost and non-contact. In particular, optical measurement technology based on binocular cameras is suitable for target positioning with unknown depth of field. In long-distance (such as about 30-50 meters) binocular vision measurement tasks, long-baseline (m-level) binocular cameras are used, and the relative position and posture between the two cameras need to be accurately calibrated in advance. The calibration accuracy of the external parameters of the binocular camera will directly affect whether the entire optical measurement task can be successfully completed.

[0003] Traditional binocular camera calibration methods usually use small calibration plates, which are suitable for close-up photography. However, when the calibration plate is far away from the binocular camera, the projection area of ​​the calibration plate in the camera imaging plane will be very small, and the number of effective visual features that can be extracted is small. In addition, the visual features are unevenly distributed and the extraction accuracy is low, which will lead to poor accuracy of binocular camera extrinsic calibration and cannot meet the precise measurement requirements of large objects at a distance. In addition, the calibration plates used in traditional methods are all backlight-free. Under conditions of low light intensity (such as indoors), the imaging quality is significantly reduced, the extraction accuracy of visual features is reduced, and it is difficult to ensure the calibration accuracy. Summary of the invention

[0004] In view of this, the extrinsic parameter calibration method of a binocular camera under a large field of view based on a light emitting diode target of the present invention has the technical advantages of high precision, strong robustness, low cost and easy operation.

[0005] A method for calibrating external parameters of a binocular camera under a large field of view based on a light-emitting diode target is suitable for calibrating external parameters of a binocular camera under illumination changes and large-scale scenes using a total station and a light-emitting diode array target. The method for calibrating external parameters of a binocular camera under a large field of view includes:

[0006] Step 1: Initial calibration, including,

[0007] 1.1 The binocular camera, total station and light-emitting diode array target are fixedly installed in the calibration site;

[0008] 1.2 The total station is installed in a position where it can observe the binocular camera and the light-emitting diode array target at the same time, and the total station does not block the camera from shooting the diode array target. The coordinate system of the total station is defined as the world coordinate system;

[0009] Step 2: Use the hand-eye calibration method to calibrate the internal parameters of the binocular camera respectively;

[0010] Step 3: Preset the number of times the LED target needs to move;

[0011] Step 4: After moving the target position once, keep it still;

[0012] Step 5: Use a total station to measure the position of the LED target in the world coordinate system and record it;

[0013] Step 6: The binocular camera takes pictures of the light-emitting diode targets respectively and saves the image data;

[0014] Step 7: Use image filtering method to pre-process the image data to improve the image contrast;

[0015] Step 8: Use the image threshold segmentation method to segment the light spot area of ​​the light emitting diode from the image;

[0016] Step 9: The light spot area is subjected to pixel binarization processing to obtain a processed image;

[0017] Step 10: The processed image uses a spot centroid calculation method to estimate the spot centroid coordinates and record them;

[0018] Step 11: Determine whether the preset target movement times have been reached. If the preset times have been reached, proceed to step 12; otherwise, proceed to step 4;

[0019] Step 12: By looping through steps 3 to 9, several sets of light-emitting diode target position coordinates and diode spot centroid coordinates are obtained, and the external parameters of the binocular camera are obtained using a typical binocular camera external parameter calibration method.

[0020] The technical beneficial effects of the present invention are:

[0021] In order to solve the problem of small target imaging area, few effective visual features and uneven distribution of visual features under large field of view, the target is moved multiple times and images are taken to obtain a large number of evenly distributed visual features. In order to solve the problem of image quality degradation caused by changes in ambient light and thermal noise of the camera, image preprocessing technology is used to improve the image signal-to-noise ratio, and the light-emitting diode array target is used to improve the significance of the image spot. In addition, in order to solve the problem of unclear edge of the light spot in the image, the light spot area detection technology is used to accurately segment the light spot area from the background image and perform pixel binarization, that is, the background pixel value is 0 and the light spot pixel value is 255. Next, in order to solve the problem of light spot centroid detection, this proposal adopts the re-coordinate positioning method. Finally, in order to solve the problem of external parameter calibration of the binocular camera, the light spot centroid coordinates of the target at multiple positions are detected by intermittently moving the light-emitting diode target and taking binocular images. The geometric relationship between the light spot centroid coordinates in the image coordinate system and the light-emitting diode center coordinates in the camera coordinate system is used to calculate the high-precision external parameters of the binocular camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 Extrinsic calibration framework for binocular cameras in large-scale scenes;

[0024] Figure 2 The process of extrinsic calibration of binocular cameras in large-scale scenes;

[0025] Figure 3 Example of a light emitting diode array target;

[0026] Figure 4 Example of equipment arrangement at a calibration site. DETAILED DESCRIPTION

[0027] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0028] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present disclosure.

[0029] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on the present disclosure, it should be understood by those skilled in the art that an aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects described herein can be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described herein can be used to implement this device and / or practice this method.

[0030] The method of the present invention aims at the problems of small target imaging area, few effective visual features and uneven distribution of visual features under a large field of view. This proposal moves the target multiple times and takes images to obtain a large number of evenly distributed visual features. Secondly, in order to address the problem of reduced imaging quality due to changes in ambient lighting and camera thermal noise, this proposal uses image preprocessing technology to improve the image signal-to-noise ratio, and uses light-emitting diode array targets to improve the significance of image spots. Thirdly, in order to address the problem of unclear edges of light spots in images, this proposal uses light spot area detection technology to accurately segment the light spot area from the background image and perform pixel binarization, that is, the background pixel value is 0 and the light spot pixel value is 255. Next, in order to address the problem of light spot centroid detection, this proposal uses a re-coordinate positioning method. Finally, in order to address the problem of binocular camera external parameter calibration, the light spot centroid coordinates of the target at multiple positions are detected by intermittently moving the light-emitting diode target and taking binocular images. The geometric relationship between the light spot centroid coordinates in the image coordinate system and the light-emitting diode center coordinates in the camera coordinate system is used to calculate the high-precision external parameters of the binocular camera. Figures 1 to 4The extrinsic calibration method of a binocular camera under a large field of view based on an LED target is suitable for calibrating the extrinsic parameters of a binocular camera under illumination changes and large-scale scenes using a total station and an LED array target. The extrinsic calibration method of a binocular camera under a large field of view includes:

[0031] Step 1: Initial calibration, including

[0032] 1.1 The binocular camera, total station and light emitting diode array target are fixedly installed in the calibration site respectively. The horizontal distance between the binocular camera and the light emitting diode array target is ≥30m;

[0033] 1.2 The total station is installed in a position where it can observe the binocular camera and the light-emitting diode array target at the same time, and the total station does not block the camera from shooting the diode array target. The coordinate system of the total station is defined as the world coordinate system;

[0034] Step 2: Use the hand-eye calibration method (e.g., Zhang Zhengyou's monocular camera intrinsic calibration) to calibrate the internal parameters of the binocular camera respectively;

[0035] Step 3: Preset the number of times the LED target needs to move (at least 3 times);

[0036] Step 4: After moving the target position once, keep it still, wherein the target can be moved from left to right with equal spacing, or from right to left with equal spacing;

[0037] Step 5: Use a total station to measure the position of the LED target in the world coordinate system and record it;

[0038] Step 6: The binocular camera takes pictures of the light-emitting diode targets respectively and saves the image data;

[0039] Step 7: Use image filtering methods (such as bilateral iterative filtering) to preprocess the image data to improve the image contrast;

[0040] Step 8: Use an image threshold segmentation method (such as a multi-scale threshold segmentation method) to segment the light spot area of ​​the light emitting diode from the image;

[0041] Step 9: The light spot area is subjected to pixel binarization processing to obtain a processed image;

[0042] Step 10: The processed image uses a spot centroid calculation method (such as a center of gravity positioning method) to estimate the spot centroid coordinates and record them;

[0043] Step 11: Determine whether the preset target movement times have been reached. If the preset times have been reached, proceed to step 12; otherwise, proceed to step 4;

[0044] Step 12: See Figure 3 By looping through steps 3 to 9, several groups (i.e., the pre-set number of diode target movement times) of light-emitting diode target position coordinates and diode spot centroid coordinates are obtained, and the external parameters of the binocular camera, i.e., the relative position and posture between the binocular cameras, are obtained by using a typical binocular camera extrinsic calibration method.

[0045] The method of the present invention can realize the external parameter calibration of the binocular camera by using a customized light-emitting diode target in complex lighting conditions and large-scale scenes. First, in order to address the problems of small target imaging area, few effective visual features, and uneven distribution of visual features under a large field of view, this proposal moves the target multiple times and takes images to obtain a large number of evenly distributed visual features. Secondly, in order to address the problem of image quality degradation caused by changes in ambient lighting and camera thermal noise, this proposal uses image preprocessing technology to improve the image signal-to-noise ratio, and light-emitting diode array targets to improve the significance of image spots. Thirdly, in order to address the problem of unclear edges of light spots in images, this proposal uses light spot area detection technology to accurately segment the light spot area from the background image and perform pixel binarization, that is, the background pixel value is 0 and the light spot pixel value is 255. Next, in order to address the problem of light spot centroid detection, this proposal uses a heavy coordinate positioning method. Finally, in order to calibrate the external parameters of the binocular camera, the light-emitting diode target is moved intermittently and binocular images are taken to detect the centroid coordinates of the light spot at multiple positions of the target. The geometric relationship between the centroid coordinates of the light spot in the image coordinate system and the center coordinates of the light-emitting diode in the camera coordinate system is used to calculate the high-precision external parameters of the binocular camera.

[0046] Example

[0047] Two visible light cameras Computar M2518-MPV and a custom LED array target are used. The target is a 5×3 circular backlit calibration plate. The white spots on the calibration plate are circular. The distance between the centers of two adjacent circles is 140mm. The diameter of the circle is fixed and consistent. The light emitted is white visible light. An example of a LED array target is attached. Figure 3 . The traditional binocular camera extrinsic calibration method is only applicable to the extrinsic calibration based on short-baseline (cm-level) binocular cameras, and cannot meet the requirements of long-baseline (m-level) binocular camera extrinsic parameter calibration for complex lighting, large-scale scenes, and customized targets. To this end, the method of the present invention uses a visible light camera to shoot customized light-emitting diode target images in the calibration site to test and verify the spot image area algorithm. The calibration site is a range of 30m in depth, 8m in field of view width, and 4.5m in space height. The object to be measured is within the site, and the camera is outside the site. It can image customized light-emitting diode targets in complex lighting and large-scale scenes.

[0048] The above are only specific embodiments of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present disclosure should be included in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure should be based on the protection scope of the claims.

Claims

1. A method for calibrating the external parameters of a binocular camera under a large field of view based on a light-emitting diode target, which is suitable for calibrating the external parameters of a binocular camera under illumination changes and large-scale scenes using a total station and a light-emitting diode array target, and is characterized in that: The method for calibrating the external parameters of a binocular camera under a large field of view includes: Step 1: Initial calibration, including, 1.1 The binocular camera, total station and light-emitting diode array target are fixedly installed in the calibration site; 1.2 The total station is installed in a position where it can observe the binocular camera and the light-emitting diode array target at the same time, and the total station does not block the camera from shooting the diode array target. The coordinate system of the total station is defined as the world coordinate system; Step 2: Use the hand-eye calibration method to calibrate the internal parameters of the binocular camera respectively; Step 3: Preset the number of times the LED target needs to move; Step 4: After moving the target position once, keep it still; Step 5: Use a total station to measure the position of the LED target in the world coordinate system and record it; Step 6: The binocular camera takes pictures of the light-emitting diode targets respectively and saves the image data; Step 7: Use image filtering method to pre-process the image data to improve the image contrast; Step 8: Use the image threshold segmentation method to segment the light spot area of ​​the light emitting diode from the image; Step 9: The light spot area is subjected to pixel binarization processing to obtain a processed image; Step 10: The processed image uses a spot centroid calculation method to estimate the spot centroid coordinates and record them; Step 11: Determine whether the preset target movement times have been reached. If the preset times have been reached, proceed to step 12; otherwise, proceed to step 4; Step 12: By looping through steps 3 to 9, obtain several sets of light-emitting diode target position coordinates and diode spot centroid coordinates, and use a typical binocular camera external parameter calibration method to obtain the external parameters of the binocular camera.

2. The method for calibrating external parameters of a binocular camera under a large field of view according to claim 1, characterized in that: The horizontal distance between the binocular camera and the target of the LED array in step 1.1 is ≥ 30m.

3. The method for calibrating external parameters of a binocular camera under a large field of view according to claim 1, characterized in that: The hand-eye calibration method described in step 2 is Zhang Zhengyou's monocular camera intrinsic calibration method.

4. The method for calibrating external parameters of a binocular camera under a large field of view according to claim 1, characterized in that: Step 3: The LED target needs to be moved at least 3 times.

5. The method for calibrating external parameters of a binocular camera under a large field of view according to claim 4, characterized in that: In step 4, the target is moved multiple times with equal spacing.

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