CN106842496A - Method for automatically adjusting focus based on frequency domain comparison method - Google Patents
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Abstract
The invention relates to a method for automatically adjusting focus based on a frequency domain comparison method, which comprises the following steps: the computer controls the mechanical telescopic lens cone to adjust the position of the digital camera, or the imaging aspheric lens or the microscope objective, the telescopic lens cone moves telescopically while the camera shoots to obtain K pieces of image data I0,…,Ii,…,Ij,…,Ik‑1And when the telescopic position of the mechanical lens cone to be focused is obtained through a series of calculations, the mechanical lens cone is controlled to be telescopic to the position, and then automatic focusing can be realized. By comparing and analyzing frequency domain components, the adverse effect of image noise on the automatic focus adjustment precision can be avoided, the method is more suitable for automatic focus adjustment of transmission imaging, and particularly for capillaries in human skin under the condition of no wound, the shape and distribution of the capillaries have periodicity, and the characteristics in the frequency domain are more obvious, so that the automatic focus adjustment of the method is more accurate. In the aspect of device operation, the method is simpler and more convenient and is easy to use practically.
Description
Technical Field
The invention relates to the field of image processing, in particular to a method for automatically adjusting focus based on a frequency domain comparison method.
Background
The micro-lens system, the camera and the like can realize the acquisition of images, but the acquired images are blurred due to the problem of inaccurate focus in the acquisition process of the images, and the significance of image acquisition cannot be achieved. Therefore, the adjustment of the focus is crucial.
In order to facilitate the user to adjust the focus and improve the use efficiency of the device, a mechanical braking telescopic lens barrel is added in the device, and the user controls the telescopic of the mechanical lens barrel by adjusting a rocker to adjust the distance between the microscope lens system and a camera or adjust the distance between the microscope objective and an aspheric lens for imaging. Although the mechanically actuated telescopic lens barrel can reduce the focusing complexity of the device to a certain extent, a user without microscopic observation experience still has difficulty in confirming the focus, and in such a case, an automatic focus adjustment algorithm is required to calculate the optimal focus position, so that the mechanically actuated telescopic lens barrel is automatically controlled to complete the automatic focusing process of the device.
Currently, the methods for automatically adjusting the focus are mainly divided into two main categories, namely active and passive automatic adjustment. The active focus adjustment method needs to install an infrared or ultrasonic transceiver on the equipment, actively measure the distance of an observed object by using an infrared or ultrasonic ranging method, and automatically adjust the positions of all lenses in a lens system through a mechanical device according to the calculated distance so as to realize the automatic focus adjustment of the equipment. Although the active focus adjustment method has high focusing accuracy, a hardware device only used for focusing needs to be added in the equipment, which is not beneficial to the low cost and miniaturization of equipment products.
The passive focus adjusting method directly utilizes the image received by the equipment, utilizes the image processing technology to automatically analyze the definition of an observed object in the image, automatically calculates the position of a lens with the clearest image and realizes the automatic focus adjustment of the equipment. The methods for analyzing sharpness by image processing are most commonly used as a phase method and a contrast method. The phase method is realized by utilizing an automatic focusing sensor, the automatic focusing and imaging optical system is separately arranged in equipment, the automatic focusing sensor detects the phase offset of an observed object image, and the position of an imaging lens with the minimum offset is a focusing position. The auto-focus sensor has a small volume, which facilitates miniaturization of the device, but increases the cost of the device, which is not favorable for cost reduction of the device. The contrast rule is that the automatic focusing is realized by continuously shooting images from focusing to defocusing and then from defocusing, and automatically detecting the definition of the outline edge of an observed object in imaging by utilizing image processing. The contrast method does not need to additionally increase a special device for automatic focusing, and is beneficial to the low cost and miniaturization of equipment. In the conventional image contrast automatic focus adjustment method, automatic detection of sharpness is mostly realized by using an image edge detection algorithm such as a laplacian image filter, and these image processing algorithms are very sensitive to noise existing in an image while calculating the edge sharpness, so that accurate automatic focus adjustment cannot be realized under the condition of large image noise such as imaging the inside of an object through a surface.
Disclosure of Invention
According to the defects, the invention provides a method for automatically adjusting the focus based on a frequency domain comparison method.
In order to achieve the purpose, the technical scheme of the invention is as follows: the method for automatically adjusting the focus based on the frequency domain comparison method is characterized in that: the method comprises the following steps:
the computer controls the mechanical telescopic lens cone to adjust the position of the digital camera, or the imaging aspheric lens or the microscope objective, the telescopic lens cone moves telescopically while the camera shoots to obtain K pieces of image data I0,…,Ii,…,Ij,…，IK-1Performing discrete Fourier transform on horizontal or vertical component of each image, and performing Fourier transform on the dataRepresented by formula 1:
wherein,a horizontal or vertical component representing the y-th row/column of the i-th photographic image, with i in units of frames; n is the number of pixels in the horizontal/longitudinal direction of the image, the unit is pixel, x is the horizontal/longitudinal coordinate of the image, y is the vertical/horizontal coordinate of the image, the unit is pixel, u is the frequency domain coordinate of the image data, and the unit is Hz;
converting the above transformation result into energy spectrum with dB as numerical unit, and averaging the energy spectrum to obtain the horizontal/vertical energy spectrum of the image, i.e. the frequency domain component of the image, in Gi(u)[dB]Expressed, the calculation formula is as follows:
distance d (G) between horizontal/vertical frequency domain components of ith image and jth image in imagei，Gj) Using the square root of the second-order average, as shown in equation 4:
wherein d represents the frequency domain distance between two images, with the unit of [ dB ]]，Gi，GjAre the frequency domain components of the two images, respectively, and have the unit of [ dB]；
Theoretically, when the blurring degrees of the two images are the same, that is, the distance between the defocus positions is the same, the frequency domain distance is 0, and in this case, equation 5 can be obtained according to equation 4:
due to [ G ]i(u)-Gj(u)]20 or more, therefore:
(the subscript j represents the position of the image in terms of frame; in the formula, j represents a complex number symbol)
When shooting images, the digital camera or the imaging aspheric lens or the microscope objective lens is moved to shoot images, so that the position of each image in the image group corresponds to the position of the digital camera or the imaging aspheric lens or the microscope objective lens, that is, the telescopic position of the mechanical telescopic lens barrel, and according to equation 6, the relationship between the two non-focused images at the same focusing position and the telescopic position of the mechanical telescopic lens barrel satisfies the functional relationship shown in equation 7:
|fu(i)|＝|fu(i) equation 7
According to the geometrical optics principle, the image blur generated by non-focusing can be regarded as low-pass gaussian filtering processing on the focused image, so that the function in equation 7 can be approximately regarded as a gaussian function, as shown in equation 8:
here, i denotes the telescopic position of the mechanically telescopic lens barrel, and is in units of um, icExpressing the central position of the Gaussian function, namely the focusing position, and the unit is um, K expresses the number of shot images and the unit is amplitude, and sigma expresses a constant parameter related to the lens;
absolute value and natural logarithm value calculation are performed on the formula 8, and a formula 9 can be obtained:
by replacing the computational basis of equation 9 with 10, equation 9 can be changed to the form of equation 10:
from equation 10, a function for i in equation 7 is obtained, as shown in equation 11:
|fu(i)|＝log|Fu(i)|＝ai2+ bi + c formula 11
Bringing formula 11 into formula 7 gives:
therefore, by comparing the image frequency domain, the following two minima can be obtained:
according to formula 13, the two minimum values respectively represent two straight lines on the plane coordinate, the intersection point coordinate of the two straight lines is the only minimum value, and the only minimum value is the telescopic position of the mechanical lens cone during focusing to be obtained;
parameters of two straight lines in the formula 13 can be obtained by calculating the line-by-line minimum value of the two-dimensional data, and then intersection point solving is carried out on the two straight lines, and the coordinate value of the horizontal axis/the vertical axis is the telescopic position of the mechanical lens cone during focusing;
the mechanical lens barrel is controlled to stretch to the position, and then automatic focusing can be achieved.
The invention also provides a method for carrying out automatic focusing by using the calculation method, which comprises the following steps:
(1) a user controls the mechanical lens barrel to stretch and retract by using the rocker, and the positions of the imaging module, the imaging unit or the lens unit are adjusted to finish coarse focusing adjustment;
(2) the user sends an automatic focusing instruction to the computer through the rocker and other instruction devices;
(3) the computer receives the instruction and starts automatic focusing;
(4) the computer controls the mechanical lens cone to move up/down according to a preset range by taking the current position as a center, and stops moving after reaching the preset position; the camera starts recording an image while the mechanical barrel starts moving down/up with the stopped position as a starting point; the mechanical lens barrel moves, the camera records images at the same time, and the recorded images are stored in a memory or a memory of a hard disk;
(5) after shooting is finished, reading the image information by the computer, and processing the image information by using a frequency domain comparison method to obtain an RMS distribution map;
(6) the computer uses an algorithm to calculate minimum value points line by line in the RMS distribution diagram and connects the points to obtain two crossed straight lines;
(7) the computer calculates and obtains the coordinates of the intersection point of two crossed straight lines by using an algorithm, and calculates the telescopic position of the mechanical lens cone during focusing by using the transformation from the RMS distribution map coordinates to the action coordinates of the mechanical lens cone;
(8) and the computer controls the telescopic lens cone to move to the position obtained by the calculation, and automatic focusing is completed.
The invention has the beneficial effects that: the method of using passive automatic focus adjustment does not require a special focus adjustment device, and is easy to reduce the cost and size of the apparatus. Compared with a contrast method using image edge detection in the same category, the method can avoid the adverse effect of image noise on the automatic focus adjustment precision through comparison and analysis of frequency domain components, is more suitable for automatic focus adjustment of transmission imaging, and particularly has periodicity in shape and distribution and more obvious characteristics in frequency domain for micro blood vessels in human skin under the condition of no wound, so that the automatic focus adjustment of the method is more accurate. In the aspect of device operation, the method is simpler and more convenient and is easy to use practically.
Drawings
Fig. 1 is a frequency domain distance (spectrum RMS) value distribution diagram.
Detailed Description
The present invention is further illustrated by the following specific examples.
The method for automatically adjusting the focus based on the frequency domain comparison method is characterized in that: the method comprises the following steps:
the computer controls the mechanical telescopic lens cone to adjust the position of the digital camera, or the imaging aspheric lens or the microscope objective, the telescopic lens cone moves telescopically while the camera shoots to obtain K pieces of image data I0,…，Ii，…，Ij，…，IK-1Performing discrete Fourier transform on horizontal or vertical component of each image, and performing Fourier transform on the dataRepresented by formula 1:
wherein,y-th image representing the i-th photographed imageHorizontal or vertical component of row/column, unit of i is web; n is the number of pixels in the horizontal/longitudinal direction of the image, the unit is pixel, x is the horizontal/longitudinal coordinate of the image, y is the vertical/horizontal coordinate of the image, the unit is pixel, u is the frequency domain coordinate of the image data, and the unit is Hz;
converting the above transformation result into energy spectrum with dB as numerical unit, and averaging the energy spectrum to obtain the horizontal/vertical energy spectrum of the image, i.e. the frequency domain component of the image, in Gi(u)[dB]Expressed, the calculation formula is as follows:
distance d (G) between horizontal/vertical frequency domain components of ith image and jth image in imagei,Gj) Using the square root of the second-order average, as shown in equation 4:
wherein d represents the frequency domain distance between two images, with the unit of [ dB ]]，Gi,GjAre the frequency domain components of the two images, respectively, and have the unit of [ dB]；
Theoretically, when the blurring degrees of the two images are the same, that is, the distance between the defocus positions is the same, the frequency domain distance is 0, and in this case, equation 5 can be obtained according to equation 4:
due to [ G ]i(u)-Gj(u)]20 or more, therefore:
(subscript j represents the position of the image in frames; j in the formula represents a complex number symbol);
when shooting images, the digital camera or the imaging aspheric lens or the microscope objective lens is moved to shoot images, so that the position of each image in the image group corresponds to the position of the digital camera or the imaging aspheric lens or the microscope objective lens, that is, the telescopic position of the mechanical telescopic lens barrel, and according to equation 6, the relationship between the two non-focused images at the same focusing position and the telescopic position of the mechanical telescopic lens barrel satisfies the functional relationship shown in equation 7:
|fu(i)|＝|fu(j) equation 7
According to the geometrical optics principle, the image blur generated by non-focusing can be regarded as low-pass gaussian filtering processing on the focused image, so that the function in equation 7 can be approximately regarded as a gaussian function, as shown in equation 8:
here, i denotes the telescopic position of the mechanically telescopic lens barrel, and is in units of um, icExpressing the central position of the Gaussian function, namely the focusing position, and the unit is um, K expresses the number of shot images and the unit is amplitude, and sigma expresses a parameter related to a lens constant;
absolute value and natural logarithm value calculation are performed on the formula 8, and a formula 9 can be obtained:
by replacing the computational basis of equation 9 with 10, equation 9 can be changed to the form of equation 10:
from equation 10, a function for i in equation 7 is obtained, as shown in equation 11:
|fu(i)|＝log|Fu(i)|＝ai2+ bi + c formula 11
Bringing formula 11 into formula 7 gives:
therefore, by comparing the image frequency domain, the following two minima can be obtained:
according to formula 13, the two minimum values respectively represent two straight lines on the plane coordinate, the intersection point coordinate of the two straight lines is the only minimum value, and the only minimum value is the telescopic position of the mechanical lens cone during focusing to be obtained;
parameters of two straight lines in the formula 13 can be obtained by calculating the line-by-line minimum value of the two-dimensional data, and then intersection point solving is carried out on the two straight lines, and the coordinate value of the horizontal axis/the vertical axis is the telescopic position of the mechanical lens cone during focusing;
the mechanical lens barrel is controlled to stretch to the position, and then automatic focusing can be achieved.
The method for carrying out automatic focusing by the calculation method comprises the following steps:
(1) a user controls the mechanical lens barrel to stretch and retract by using the rocker, and the positions of the imaging module, the imaging unit or the lens unit are adjusted to finish coarse focusing adjustment;
(2) the user sends an automatic focusing instruction to the computer through the rocker and other instruction devices;
(3) the computer receives the instruction and starts automatic focusing;
(4) the computer controls the mechanical lens cone to move up/down according to a preset range by taking the current position as a center, and stops moving after reaching the preset position; the camera starts recording an image while the mechanical barrel starts moving down/up with the stopped position as a starting point; the mechanical lens barrel moves, the camera records images at the same time, and the recorded images are stored in a memory or a memory of a hard disk;
(5) after shooting is finished, reading the image information by the computer, and processing the image information by using a frequency domain comparison method to obtain an RMS distribution map;
(6) the computer uses an algorithm to calculate minimum value points line by line in the RMS distribution diagram and connects the points to obtain two crossed straight lines;
(7) the computer calculates and obtains the coordinates of the intersection point of two crossed straight lines by using an algorithm, and calculates the telescopic position of the mechanical lens cone during focusing by using the transformation from the RMS distribution map coordinates to the action coordinates of the mechanical lens cone;
(8) and the computer controls the telescopic lens cone to move to the position obtained by the calculation, and automatic focusing is completed.
In fig. 1, the horizontal axis represents an image i to be frequency-domain compared in units of frames, and the vertical axis represents a sample image j to be frequency-domain compared in units of frames. The luminance in the figure represents the frequency domain distance, i.e. the magnitude of the RMS value of the spectrum, the brighter value, i.e. the white part of the image, the larger frequency domain distance, which indicates that the similarity between the image i and the image j is lower. The darker value, i.e. the black part of the image, has a smaller frequency domain distance, indicating that the similarity between image i and image j is higher. Since the image at the focal position is sharp and has the lowest similarity to the image at another position, the intersection of two black regions having a higher similarity in the oblique direction is the desired focal position according to equation 13.
Claims (2)
1. The method for automatically adjusting the focus based on the frequency domain comparison method is characterized in that: the method comprises the following steps:
the computer controls the mechanical telescopic lens cone to adjust the position of the digital camera or the imaging aspheric lens or the microscope objective, the telescopic lens cone moves telescopically while the camera shoots to obtain K pieces of image data I0，…,i,…,j,…,K-1Performing discrete Fourier transform on horizontal or vertical component of each image, and performing Fourier transform on the dataRepresented by formula 1:
wherein,a horizontal or vertical component representing the y-th row/column of the i-th photographic image, with i in units of frames; n is the number of pixels in the horizontal/longitudinal direction of the image, the unit is pixel, x is the horizontal/longitudinal coordinate of the image, y is the vertical/horizontal coordinate of the image, the unit is pixel, u is the frequency domain coordinate of the image data, and the unit is Hz;
converting the above transformation result into energy spectrum with dB as numerical unit, and averaging the energy spectrum to obtain the horizontal/vertical energy spectrum of the image, i.e. the frequency domain component of the image, in Gi(u)[dB]Expressed, the calculation formula is as follows:
distance d (G) between horizontal/vertical frequency domain components of ith image and jth image in imagei,Cj) Using the square root of the second-order average, as shown in equation 4:
wherein d represents the frequency domain distance between two images, with the unit of [ dB ]]，Gi,CjAre the frequency domain components of the two images, respectively, and have the unit of [ dB]；
Theoretically, when the blurring degrees of the two images are the same, that is, the distance between the defocus positions is the same, the frequency domain distance is 0, and in this case, equation 5 can be obtained according to equation 4:
due to [ G ]i(u)-Gj(u)]20 or more, therefore:
wherein, the subscript j represents the position of the image in units of frames; j in the formula represents a complex symbol;
when shooting images, the digital camera or the imaging aspheric lens or the microscope objective lens is moved to shoot images, so that the position of each image in the image group corresponds to the position of the digital camera, the imaging aspheric lens or the microscope objective lens, that is, the telescopic position of the mechanical telescopic lens barrel, and according to equation 6, the relationship between the two non-focused images at the same focusing position and the telescopic position of the mechanical telescopic lens barrel satisfies the functional relationship shown in equation 7:
|fu(i)|＝|fu(j) equation 7
According to the geometrical optics principle, the image blur generated by non-focusing can be regarded as low-pass gaussian filtering processing on the focused image, so that the function in equation 7 can be approximately regarded as a gaussian function, as shown in equation 8:
here, i denotes the telescopic position of the mechanically telescopic lens barrel, and is in units of um, icExpressing the central position of the Gaussian function, namely the focusing position, and the unit is um, K expresses the number of shot images and the unit is amplitude, and sigma expresses a constant parameter related to the lens;
absolute value and natural logarithm value calculation are performed on the formula 8, and a formula 9 can be obtained:
by replacing the computational basis of equation 9 with 10, equation 9 can be changed to the form of equation 10:
from equation 10, a function for i in equation 7 is obtained, as shown in equation 11:
|fu(i)|＝log|Fu(i)|＝ai2+ bi + c formula 11
Bringing formula 11 into formula 7 gives:
therefore, by comparing the image frequency domain, the following two minima can be obtained:
according to formula 13, the two minimum values respectively represent two straight lines on the plane coordinate, the intersection point coordinate of the two straight lines is the only minimum value, and the only minimum value is the telescopic position of the mechanical lens cone during focusing to be obtained;
parameters of two straight lines in the formula 13 can be obtained by calculating the line-by-line minimum value of the two-dimensional data, and then intersection point solving is carried out on the two straight lines, and the coordinate value of the horizontal axis/the vertical axis is the telescopic position of the mechanical lens cone during focusing;
the mechanical lens barrel is controlled to stretch to the position, and then automatic focusing can be achieved.
2. A method for performing auto-focus using the computing method of claim 1, wherein: the method comprises the following steps:
(1) a user controls the mechanical lens barrel to stretch and retract by using the rocker, and the positions of the imaging unit, the imaging unit or the lens unit are adjusted to finish coarse focusing adjustment;
(2) a user sends an automatic focusing instruction to a computer through a rocker and other instruction devices, and the computer receives the instruction and starts automatic focusing;
(3) the computer controls the mechanical lens cone to move up/down according to a preset range by taking the current position as a center, and stops moving after reaching the preset position; the camera starts recording an image while the mechanical barrel starts moving down/up with the stopped position as a starting point; the mechanical lens barrel moves, the camera records images at the same time, and the recorded images are stored in a memory or a memory of a hard disk;
(4) after shooting is finished, reading the image information by the computer, and processing the image information by using a frequency domain comparison method to obtain an RMS distribution map;
(5) the computer uses an algorithm to calculate minimum value points line by line in the RMS distribution diagram and connects the points to obtain two crossed straight lines;
(6) the computer calculates and obtains the coordinates of the intersection point of two crossed straight lines by using an algorithm, and calculates the telescopic position of the mechanical lens cone during focusing by using the transformation from the RMS distribution map coordinates to the action coordinates of the mechanical lens cone;
(7) and the computer controls the telescopic lens cone to move to the position obtained by the calculation, and automatic focusing is completed.
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| CN108376394B (en) * | 2018-01-12 | 2021-08-10 | 上海大学 | Camera automatic focusing method and system based on frequency domain histogram analysis |
| WO2021082688A1 (en) * | 2019-10-31 | 2021-05-06 | 广州市浩洋电子股份有限公司 | Machine-vision-based intelligent focusing method for moving head computer lamp |
| US12031704B2 (en) | 2019-10-31 | 2024-07-09 | Guangzhou Haoyang Electronic Co., Ltd. | Intelligent focusing method of intelligent moving head light based on machine vision |
| CN113852761A (en) * | 2021-09-27 | 2021-12-28 | 宁波华思图科技有限公司 | Automatic focusing method of intelligent digital microscope |
| CN113852761B (en) * | 2021-09-27 | 2023-07-04 | 宁波华思图科技有限公司 | Automatic focusing method for intelligent digital microscope |
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