CN112802128A - Method for measuring windward projection area of athlete in wind tunnel skiing test - Google Patents
Method for measuring windward projection area of athlete in wind tunnel skiing test Download PDFInfo
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- CN112802128A CN112802128A CN202110375177.XA CN202110375177A CN112802128A CN 112802128 A CN112802128 A CN 112802128A CN 202110375177 A CN202110375177 A CN 202110375177A CN 112802128 A CN112802128 A CN 112802128A
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- 238000000034 method Methods 0.000 title claims abstract description 33
- 238000012360 testing method Methods 0.000 title claims abstract description 27
- 238000012545 processing Methods 0.000 claims abstract description 11
- 238000001914 filtration Methods 0.000 claims abstract description 5
- 238000005286 illumination Methods 0.000 claims description 7
- 238000000605 extraction Methods 0.000 claims description 4
- 238000003384 imaging method Methods 0.000 claims description 2
- 230000000007 visual effect Effects 0.000 claims 1
- 238000005259 measurement Methods 0.000 abstract description 6
- 239000003550 marker Substances 0.000 abstract description 2
- 230000000694 effects Effects 0.000 description 6
- 238000004364 calculation method Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 230000009699 differential effect Effects 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
- 230000000717 retained effect Effects 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 1
- 230000000295 complement effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
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- G06T7/80—Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M9/00—Aerodynamic testing; Arrangements in or on wind tunnels
- G01M9/02—Wind tunnels
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- G06T7/10—Segmentation; Edge detection
- G06T7/194—Segmentation; Edge detection involving foreground-background segmentation
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- G06T7/00—Image analysis
- G06T7/20—Analysis of motion
- G06T7/246—Analysis of motion using feature-based methods, e.g. the tracking of corners or segments
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
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Abstract
The invention discloses a method for measuring the windward projection area of an athlete in a wind tunnel skiing test, which comprises the steps of firstly calibrating, extracting and calculating the number of pixels occupied by a calibration object through an image processing algorithm, respectively carrying out difference, noise filtering and binarization processing on a background image and a foreground image of a wind tunnel after calibration is finished, obtaining the number of pixels occupied by the athlete, and calculating the windward projection area corresponding to the athlete; the method adopts a non-contact measurement mode, does not need to install any sensor or marker on the body of the athlete, has the advantages of easy realization, accurate measurement and no influence on the athlete, and can accurately obtain the resistance coefficient.
Description
Technical Field
The invention relates to the field of wind tunnel tests, in particular to a method for measuring the windward projection area of an irregular object, especially an athlete in a wind tunnel skiing test.
Background
In the speed competition skiing project, the air resistance has great influence on the sports competition result, the sliding speed can be effectively improved by reducing the air resistance, and the sports competition result is improved. The main basis for evaluating the air resistance characteristic is a resistance coefficient Cx, wherein Cx is a dimensionless quantity, and the calculation formula is as follows:
the dynamic pressure q is manually given to a wind tunnel power system and is realized and stabilized through system closed-loop control.
However, the current technical measurement means cannot provide accurate measurement of the windward projection area s of the athlete, so that the resistance coefficient Cx cannot be calculated, and the evaluation basis of the air resistance in the skiing test is lacked.
Disclosure of Invention
The invention aims to design a set of image processing method, and realizes the area of the windward side of a measured object in a wind tunnel by combining the prior art with the special test environment of the wind tunnel.
In order to achieve the purpose, the invention adopts the following technical characteristics:
a method for measuring the windward projection area of an athlete in a wind tunnel skiing test comprises the steps of firstly calibrating, placing A4 paper as a calibration object at a position to be measured for image acquisition, and aiming at calculating the proportional relation between the area and the number of pixels under the current camera parameters (the parameters refer to the focal length, the object distance, the view field and the number of pixels in the breadth), wherein the area of the A4 paper is fixed and known, and the number of pixels occupied by the A4 paper can be extracted and calculated through an image processing algorithm.
After calibration is completed, firstly, an empty wind tunnel image is collected as a background image, then an athlete is positioned at a position to be measured to collect an image as a foreground image, the number of pixels occupied by the athlete is extracted and calculated through a background image, the foreground image and a related image processing algorithm, the projection area of the object to be measured is in a direct proportion relation with the number of the pixels occupied by the athlete because the imaging surface of a camera in the test is parallel to the cross section of the wind tunnel, the windward projection area corresponding to the athlete can be calculated according to the calibrated proportion relation, and the resistance coefficient is calculated.
In summary, due to the adoption of the technical scheme, the invention has the beneficial effects that:
1. necessary parameters are provided for calculating the resistance coefficient Cx of the wind tunnel ski test athlete, and the difficulty that the resistance coefficient Cx cannot be obtained is solved.
2. The method belongs to a non-contact measurement mode, does not need to install any sensor or marker on the body of the athlete, and has the advantages of easy realization, accurate measurement and no influence on the athlete.
3. The method has wide applicability, and can be used for wind tunnel skiing tests and wind tunnel tests of other racing sports, such as racing bicycle sports.
Drawings
The invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a calibration image of a calibration object;
FIG. 2 is the effect of taggant extraction of FIG. 1;
FIG. 3 is a background image;
FIG. 4 is a foreground image;
FIG. 5 is a graph of the differential effect of a foreground image;
FIG. 6 is a complementary effect diagram of a differential effect;
FIG. 7 is a noise filtering effect map of the supplemental effect map'
Fig. 8 is a diagram of the final extraction effect.
Detailed Description
All of the features disclosed in this specification, or all of the steps in any method or process so disclosed, may be combined in any combination, except combinations of features and/or steps that are mutually exclusive.
Any feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving equivalent or similar purposes, unless expressly stated otherwise. That is, unless expressly stated otherwise, each feature is only an example of a generic series of equivalent or similar features.
In the embodiment, black bottom plates, white bottom plates and white calibration objects are adopted to measure the windward projection area of the athlete, the method is specifically divided into three steps, the first step is calibration, and the proportional relation between the area of the calibration objects and the number of pixels is obtained. And secondly, extracting the target to be detected, extracting the athlete target in the wind tunnel environment and acquiring the corresponding pixel number. And thirdly, calculating the area of the target to be measured, and calculating the projection area according to the calibration proportion.
First, demarcate
A4 paper is selected asThe A4 paper is white and pure in color, the A4 paper is pasted on a black bottom plate to facilitate identification and extraction, the A4 paper is thin, no shadow is generated on the bottom plate after pasting, the A4 paper is easy to obtain, and the area is S1Can be accurately known.
The threshold was calculated, the calibration object was placed in the center of the wind tunnel and the image was collected as shown in figure 1. Manually selecting A4 paper containing a black bottom plate as a calibration area, traversing pixels in the area, obtaining a maximum gray value maxgary (close to 255) and a minimum gray value mingray (close to 0), and calculating a threshold value, wherein the threshold value is an average value of the sum of the maximum gray value and the minimum gray value.
And carrying out binarization operation on each pixel in the calibration area, wherein the pixel with the gray value larger than the threshold value is assigned to be 255, and the pixel with the gray value smaller than the threshold value is assigned to be 0. The pixels outside the framed calibration area are all assigned with 0, and the processing effect is shown in fig. 2.
The number of pixels with the gray value of 255 in the image, namely the number of pixels N corresponding to A4 paper is calculated1。
Secondly, extracting the target to be detected
Background and foreground images are collected, and firstly, an image is collected in an empty wind tunnel state and is used as a background and is marked as B (x, y), as shown in fig. 3. An image of the athlete is again captured as the foreground, denoted as F (x, y), as shown in fig. 4.
And (3) image difference, namely, the difference is made between corresponding pixels of the foreground image and the background image, and is marked as D (x, y) = F (x, y) -B (x, y), under the condition of different illumination, the gray values of other areas are almost unchanged except for the area with the players, and the pixel with the gray difference value larger than M1 is considered as the area with the players and is reserved, as shown in fig. 5.
In the test process, the value range of the difference value needs to be adjusted in consideration of the illumination factors under the environmental condition, the value range M1 is set to be 17-25 according to the experimental experience value, and in this embodiment, the value of M1 is 20 under the same illumination environment in the whole test process.
Local compensation, as shown in the processing effect of fig. 5, the above information of the lower leg of the athlete is completely retained after the image difference, but the information of the position of the lower leg is lost, because the background corresponding to the position above the lower leg is the center of the wind tunnel, is black, and the gray value is much smaller than the corresponding foreground of the athlete, and the background corresponding to the position below the lower leg is the floor of the wind tunnel, is white, and the gray value is higher than the corresponding foreground of the athlete. Therefore, in order to inversely differentiate the region, pixels having a differential value greater than M2 are retained, which are denoted as R (x, y) = B (x, y) -F (x, y). The treatment effect at this time can better preserve the overall appearance of the athlete, as shown in fig. 6.
In the test process, the value range of the difference value needs to be adjusted in consideration of the illumination factors under the environmental condition, the value range M2 is set to be 30-40 according to the experimental experience value, and in this embodiment, the value of M2 is 20 under the same illumination environment in the whole test process.
Noise filtering, because the shadow produced by the athlete on the ground brings error to the image difference, a small amount of noise is introduced and mainly concentrated in the middle position of two skis, as shown in fig. 7, the noise is characterized by dispersion and small area, the connected number of pixels can be set as filtering condition, and the part with the number of pixels less than 3000 in the connected area is filtered.
It should be noted that 3000 in this embodiment is an empirical value, which is related to the number of camera pixels, the focal length, and the distance between the camera and the player, and needs to be modified when the above parameters are changed.
By binarizing the image after the above processing, the characteristics of the athlete are well extracted, and the extracted image is shown in fig. 8. Calculating the number N of pixels corresponding to the athlete2。
Thirdly, calculating the area of the target to be measured
Calculating the number of pixels N of the athlete's projected area through A4 paper1Sum area S1Number of pixels N of athlete2Then the projected area of the motion person can be calculated as S2The calculation formula is as follows:
thus, the projection area S of the athlete is obtained2Provides necessary parameters for calculating the air resistance coefficient Cx.
The embodiment can be expanded, not only can be used for wind tunnel tests of skiing sports, but also can be applied to tests of all racing sports.
The invention is not limited to the foregoing embodiments. The invention extends to any novel feature or any novel combination of features disclosed in this specification and any novel method or process steps or any novel combination of features disclosed.
Claims (8)
1. A method for measuring the windward projection area of an athlete in a wind tunnel skiing test is characterized by comprising the following steps: the industrial camera is fixedly installed behind an object to be measured in a wind tunnel test section along the wind tunnel incoming flow direction, and the imaging surface of the camera is parallel to the cross section along the axis of the wind tunnel, so that the following operations are performed:
the method comprises the following steps: selecting a calibration object and calibrating the calibration object
S11: selecting an object with an accurately measurable area as a calibration object, and fixing the calibration object on the bottom plate, wherein the color of the calibration object is different from that of the bottom plate, and the two objects have poor visual identification;
s12, placing the bottom plate fixed with the calibration object in the center of the wind tunnel, carrying out image acquisition on the calibration object through an industrial camera, selecting a bottom plate area containing the calibration object as a calibration area, traversing pixels in the calibration area, acquiring a maximum gray value and a minimum gray value, and calculating a threshold value;
s13, performing binarization operation on each pixel in the calibration area, wherein the pixel with the gray value larger than the threshold value is assigned with a value of 255, the pixel with the gray value smaller than the threshold value is assigned with a value of 0, and the pixels outside the calibration area are assigned with values of 0;
s14: calculating the number of pixels with the gray value of 255 in the image, and obtaining the number of pixels N corresponding to the calibration object1；
Step two: image extraction of measured object
S21: collecting an image only containing a bottom plate as a background image in a state that the wind tunnel is vacant, placing an athlete to be tested in the center of the wind tunnel, and collecting an image containing the athlete and the bottom plate as a foreground image;
s22: performing difference processing on pixels corresponding to the foreground image and the background image, and reserving an area corresponding to the pixel with a difference value larger than M1 under the condition of different illumination, wherein the value range of M1 is any value between 17 and 25;
s23: performing local compensation on the image processed in the S22, performing difference processing on a background image and a foreground image on an image fuzzy area, reserving an area corresponding to a pixel with a difference value larger than M2, wherein the value range of M2 is any value between 17 and 25;
s24: on the basis of S23, filtering out the part with the number of pixels less than 3000 in the area communicated with the player image;
s25: binarizing the player image obtained in the step S24 to obtain the number N of pixels corresponding to the player image2；
Step three: the projected area of the athlete is calculated from the measured parameters,
wherein:
is the area of the calibration object and is,
is that
The corresponding number of pixels in the region,
is the projected area of the athlete
The number of pixels in.
2. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 1, wherein the method comprises the following steps: the calibration object is white, and the bottom plate is black.
3. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 1 or 2, wherein the method comprises the following steps: the calibration object is regular white paper, the white paper is adhered to the black bottom plate, and the white paper is free of shadow on the bottom plate.
4. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 1, wherein the method comprises the following steps: the threshold value is an average value of the sum of the maximum gray value and the minimum gray value.
5. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 2 or 4, wherein the method comprises the following steps: the maximum gray value is 255 and the minimum gray value is 0.
6. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 1, wherein the method comprises the following steps: in S23, the area under the lower leg portion of the player is subjected to the difference processing between the background image and the foreground image.
7. The method for measuring the windward projection area of the athlete in the wind tunnel ski test according to claim 1, wherein the method comprises the following steps: the communicated area is the middle position of two skis of the athlete.
8. The method for measuring the windward projection area of the athlete in the wind tunnel skiing test according to any one of claims 1, 6 and 7, wherein the method comprises the following steps: and under the same illumination environment, reserving the area corresponding to the pixels of which the difference value is greater than 20 pixels after the difference processing of the corresponding pixels of the foreground image and the background image, and reserving the area corresponding to the pixels of which the difference value is greater than 35 after the local compensation.
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