TW201814240A - Method of length measurement for 2D photography - Google Patents

Abstract

A method of length measurement for 2d photography comprises the following steps. First, a measure object and a photographed object are respectively photographed to form a reference image and a photographed object image, wherein the measure object has a geometrical shape capable of being presented by an area calculation function saved in a computer with a predetermined size. Thereafter, a measure object image is separated from the reference image and scanned to obtain a total number of pixels included therein, so as to obtain a specific number of pixels corresponding to a unit length based on the area calculation function and the predetermined size. Therefaore, once any two points of the photographed object image are marked up on a screen of the computer, an actual length between the two points is calculated out based on a ratio of the total number of pixels to the specific number of pixels.

Description

Length measurement method for plane shooting

The present invention is a method of calculating a geometrical shape and having one of the preset dimensions as a reference photographing area.

Generally, it is impossible to measure the online size of the product on the Internet. It is difficult for the purchaser to understand the true size of the product, which may result in hesitation, purchase and sale disputes or returns. Although there are also ways to mark the size of the goods for sale, it will affect the aesthetics of the product image and the flexibility of the background. There is also a ruler placed next to the photographed object, so that the image viewer can know the size of the object to be tested by simultaneously seeing the scale of the object to be tested and the scale. However, in this case, although the image viewer can clearly see the total length and the total width of the clothes, the size of the pattern, pockets and sleeves on the clothes can only be estimated by the image viewer in proportion, and the accuracy is not high. . Moreover, when the object to be tested is thick, it is more likely to cause an error in the size estimation of the image viewer due to the difference in the shooting distance between the object to be tested and the scale relative to the lens.

In the reference object shooting stage, although it is also possible to take a ruler or measure the size of the reference object with other optical instruments, and input the reference object size as the basis for measuring the size of the object to be tested, it is necessary to manually input the reference object size, which is easy to generate. Wrong measurement results. Similarly, the dimensions not shown on the reference object can only be estimated by the image viewer in proportion, which is not only difficult to obtain the size, but also has low accuracy. In contrast, the present invention can automatically input the reference object size or the number of holes with different processing symbols to represent different reference object sizes, so as to reduce errors when shooting different sizes of the object to be tested.

One of the objects of the present invention is to provide a method for measuring a geometric shape and having one of the preset dimensions as a reference image area. Separating the reference object from a background, scanning a total number of pixels covered by the reference object on the screen, and calculating a number of pixels corresponding to the length of the reference object per unit by using the preset size and the reference object area calculation formula . The object to be tested is placed on the background, the shooting distance of the photographing device, the zooming/zooming of the lens and the shooting angle are kept fixed, and the object to be tested is taken, that is, the number of pixels can be measured on the screen. The length of any two points on the object to be tested. Aligning the reference object and the center of the object to be tested with the camera can improve the accuracy of the measurement. The preset size and area calculation formula of the reference object must be manually input first.

Generally, one of the different sizes of the object to be tested selects one of the reference objects with the same size to improve the measurement accuracy, but the reference object of different sizes and the calculation formula of the area must be manually input, which is easy to input due to the size of the reference object. Or an error in the area calculation formula causes an error in measuring the object to be tested. Therefore, another object of the present invention is to process one or more processed through-hole symbols through the reference object on the reference object, and then to correct the preset size of the reference object by processing the number of holes of the through-hole symbol. Therefore, the size and area calculation formula of the reference object pre-stored in the computer can be automatically selected through the number of holes of the processed hole-piercing symbol to avoid the influence of measurement errors caused by human input errors.

Most of the reference objects of the predetermined size are calculated by using a planar projection shape of a flat plate or a cylinder, that is, the outer edge of the reference object is projected into a planar geometric shape, and the planar geometric shape can be an area. Calculate the formula to express. Moreover, when the object to be tested is thin, a thin flat reference object is generally used, and when the object to be tested is thick, a cylindrical reference object (such as a cylinder, a triangular cylinder, etc.) similar to the thickness of the object to be tested is generally used. To improve the accuracy of the size calculation. In addition, in addition to the aforementioned flat plate or cylinder, a flat plate or a cylinder having a plurality of bumps or a block or a spherical shape on the top surface can be used as a reference object, and the geometric shape is the outer edge of the reference object. Plane projection of the camera in the shooting direction. However, in stereoscopic shooting at different angles, when the flat reference object changes the original preset size due to different viewing angles and cannot be taken as different angles, the stereoscopic object to be tested is measured at different angles. Another object of the present invention is to use a ball of a predetermined diameter as a reference object, and the projected area of the ball shot at each different angle is used as a basis for measuring the object to be tested of the shooting angle.

In order to achieve the above purpose, we can select any one or more of the geometric shapes that are easier to calculate the area, such as the square, triangle, rectangle, trapezia, lozenqe, a flat plate or a column of a circular, a star, a polygon, or a flat plate or a ball having a bump or a block on the top surface as a reference object, That is, any geometric shape corresponding to the area formula can be used as a reference object for measurement. First, the dimensions of the reference objects are preset as a, b, d and h as indicated in Fig. 2, and the square area is a × a, the rectangular area is a × b, and the area of the triangle is a × h / 2, trapezoidal The area is (a + b) × h / 2, the area of the diamond is a × h, the projected area of the cylinder is d × h, and the projected area of the circle or sphere is π × d × d / 4, where d is the diameter ( Diameter), π is the circumference ratio. On the reference object of the known size and area calculation formula, one or more processed hole marks are processed to penetrate the reference object of the selected geometry, as shown in FIG. The object is machined into a hole punching symbol of a geometric shape such as a circle, a triangle, a square, a star, etc., and the processed hole punching symbols are isolated, and then the holes are formed by the processing. The number of holes of the symbol assists the confirmation of the size of the reference object. The reference area of the fixed geometry is selected, and the area calculation formula is known, and the number of holes and the preset size of the reference object are pre-stored in the computer. The lookup table can know the size of the reference object as the basis for measuring the size of the object to be tested on the screen, and can save the trouble of manually inputting the size of the reference object of the selected geometry to the computer and avoiding the problem. Refer to the object size to enter the wrong possibility.

In order to enable the reviewing committee to have a further understanding and understanding of the features, objects and functions of the present invention, a detailed explanation will be given in conjunction with the drawings.

FIG. 1 is a flow chart showing the calculation of a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention. The geometry is a planar projection of the reference object outside of the imaging direction of the camera. Step S11 is to select a reference object that has been preset in the computer to be placed on a shooting background, and take a shooting device to obtain the reference object image and store it in the computer, and then use the computer as the reference object on the screen. The separation of the shooting background is as shown in step S12, and then the computer scans the total number of pixels of the range covered by the reference object on the screen, as in step S13, and then the predetermined size and area calculation formula in the computer is stored via the reference object. The number of pixels corresponding to the length of the reference object can be calculated. In step S14, the shooting distance of the photographing device, the enlargement/reduction of the lens, and the shooting angle are fixed. In step S15, an object to be tested is placed at the same time. Shooting on the background and taking the object to be tested by the photographing device and storing the completed object image to be tested into the computer. In step S16, the object on the object to be tested can be measured in the number of pixels on the screen. The length of two points is as shown in step S17. In the first figure, there is no processing hole-piercing symbol on the reference object. Different reference objects cannot be distinguished. When using different reference objects, the calculation formula for the size and area of the computer is reset. In the actual measurement, the object to be tested may be photographed to confirm the image range of the object to be tested and then the reference object is photographed, that is, in the first figure, S16, S15 and S11, S12, S13, S14 and S17 are executed first. FIG. 2 is a diagram showing the reference object of a flat plate, a cylinder or a sphere which may be used as a plane photographing measurement for calculating a geometric shape and having a preset size according to an embodiment of the present invention. Geometric shape style. The reference object may be any other flat shape in the form of an area formula, a bump on a flat plate, a block or a cylinder or a ball object. The sphere uses the circular projection of the shooting direction of the camera as the total number of pixels of the reference object. The circle represents a round plate or a sphere.

FIG. 4 is a diagram showing a reference object and a method for placing an object to be tested when one of the geometrical shapes and one of the preset sizes is used as a plane photographing measurement according to an embodiment of the present invention. . In Fig. 4(a), a reference object 1 is placed horizontally on a shooting background 2 and then photographed by a photographing device 3 aligned with the reference object 1. FIG. 4(b) shows that the photographing device 3 is photographed in alignment with the object to be tested 4, and then the measurement of the object to be tested is performed.

FIG. 5 is a diagram showing a reference object and a method for placing an object to be tested when calculating a geometric shape and one of the preset sizes is used as a plane shooting measurement according to an embodiment of the present invention. . Figure 5 (a) is a hanging manner of a reference object 5 attached to a strip of transparent acrylic 6 with the background of the photographing background 7, and the photographing device 3 is aligned with the reference object 5 for horizontal shooting, wherein the reference The middle height of the object 5 should be the same as the middle height of the object to be tested 9 which is to be measured, and a plurality of holes 8 are opened above the strip-shaped transparent acryl 6 as the use for adjusting the height of the reference object 5. Fig. 5(b) shows the photographing device 3 being photographed at an intermediate point of the object to be tested 9, and then measuring the object to be tested.

There are three common methods for separating the reference object and the shooting background in Fig. 1: (1) The reference object is separated from the difference in brightness of the shooting background. Common color modes (RGB mode, YIQ, HSV, YUV, YCbCr, etc.), we take the RGB color mode to illustrate, R is the abbreviation of Red; G is the abbreviation of Green; B is the abbreviation of Blue, turn it For high gray image, RGB is converted to YIQ using the following formula, Y represents luminance; I (inphase) and Q (quadrature) represent two different tones. For high-gray images, through the gray-scale distribution histogram, as shown in Fig. 12, using the drop of the peak and valley, using the brute-force method, the image is binarized, and the gray scale value is low. Turns black in the t* interval and turns the grayscale value larger than the t* interval to white. Thereby separating the shooting background and the reference object, as shown in FIG. (2) The chroma key converts RGB to YCbCr and then separates the reference object from the shooting background according to the feature value. The detailed technique of the color key can be found in the literature [1] Keith Jack, "Video Demystified", Independent Pub Group (Computer), 1996. The color keys are mainly used in the movie, film and game related industries. Mainly by the green screen and the blue screen as the background, the background color interval is adjusted, and the background color is converted into a transparent background, thereby replacing the background. (3) Separating the reference object from the background of the photograph by manual removal on the computer and the screen.

The total number of pixels covered by the reference object on the scanning screen as shown in Fig. 1 will be described below when processing one or more processed hole-punching symbols through the reference object on the reference object. In addition, the size and area calculation formula preset by the reference object can be used to calculate the number of pixels corresponding to the length of the reference object per unit. Taking the rectangle of Fig. 2 as an example, the reference object is a rectangle, and the preset side lengths are a and b, respectively, and the area is a × b. When scanning to know the total number of pixels covered by the reference object, dividing the total number of pixels by b is equal to the number of pixels corresponding to the length a, and dividing the number of corresponding pixels by a is equal to the length of the reference object per unit. The number of pixels corresponding to it. The reference object of one of the different geometries shown in Fig. 2, the labeled a, b, h, and d are preset values, and the calculation formulas of different areas and the total number of pixels covered by the reference object are all The number of pixels corresponding to the length of the reference object per unit can be calculated according to the above algorithm, and is used as a measure of the size of the object to be tested.

FIG. 6(a) and (b) are diagrams showing the processing of a hole-piercing symbol on a reference object when calculating a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention. , the vertical (hanging) of the reference object and the horizontal flat shooting mode. The objects to be tested are placed in the same manner as in Figures 4(b) and 5(b).

FIG. 7 is a flow chart showing the processing of a hole-piercing symbol on the reference object when calculating a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention. Steps S21 and S23 to S26 of Fig. 7 are not described in the steps S11 and S13 to S14 of Fig. 1 except for the steps S21 and S23 to S26 in Fig. 7 (the steps are the same). S22 and S27 to S29 are the same as steps S12 and S15 to S17, and will not be described again. Briefly, Figure 7 uses only one reference object with a machined hole symbol similar to that of Figure 1, but when there are multiple reference objects on each of the reference objects, the reference objects are The number of holes can be calculated by using different sizes, different geometric shapes and different area calculations of the reference objects. The number of holes is compared and the aforementioned data in the pre-existing computer is taken out, there is no error, and the reference object having the approximate size of the processed hole-piercing symbol can be directly placed according to the size of the object to be tested. Fig. 8 is a flow chart showing the detailed description of steps S23 and S24 of Fig. 7, and Fig. 11 is a view showing the scanning step of Fig. 8. Figure 8 firstly scans the reference object with the processed hole-piercing symbol on the shooting background from the upper left corner of the screen, from left to right, and scans the entire image from top to bottom to find the first background block. The pixel, labeled as block A, also continuously marks the background pixel adjacent to the pixel as block A until the adjacent pixel has no longer found the background, as shown in FIG. 11(a) below.

Then, continue to search for unmarked pixels (marked background will no longer be searched. If you search to the top of the first hole in the 11th figure (b), the unmarked background block is found.) .

Then, in the same manner as the search for the block A, the photographing background is marked as the block B, and the photographing background of the adjacent block B is also marked as the block B as shown in Fig. 11(c).

After that, continue to search for unmarked blocks (marked backgrounds will not be searched. If you search to the top of the second processing hole symbol in Figure 11 (d), you will find unmarked background blocks. ).

Similarly, in the same manner as the search for block A, the photographing background is marked as block C, and the photographing background of the adjacent block C is also marked as block C, as shown in Fig. 11(e).

Then, the method of searching and marking block C is repeated, and the shooting background is found and labeled as block D, as shown in Fig. 11(f).

Thereafter, blocks A, B, C, and D have been found, and the open block A is removed and three blocks B, C, and D are obtained, as shown in Fig. 11(g).

In summary, blocks B, C, and D in block A and the reference object can be simultaneously marked by scanning once, and blocks B, C, and D are counted when calculating the total number of pixels in the range covered by the reference object. In.

In step S26 of step 7, according to the number of holes obtained in step S24, the lookup table obtains the preset size and area calculation formula of the reference object to calculate the number of pixels corresponding to the length of the reference object per unit. The lengths of a, b, h, etc. corresponding to different hole numbers can be preset to any similar size according to the size of the object to be tested as follows, and then the area calculation formula of the reference object is used for calculation. Chart

In addition to comparing the dimensions of the reference object, the number of holes can also be compared with the geometry and area calculation formula. That is, when the number of different holes is different on the reference objects, the calculation formulas of the reference object sizes, the different geometric shapes, and the different areas can be compared. The user can automatically complete the number of pixels corresponding to the length of the reference object per unit by simply placing the reference objects with the processed hole-punching symbols. The machined hole sign can be a separate hole of any shape, such as a circle, a triangle, a square, a rectangle, a trapezoid, a star, or a polygon.

FIG. 9 is a schematic diagram showing hemispherical shooting in which a sphere is projected into a circle and a reference object area of one of the sphere diameters is taken as a plane photographing measurement according to an embodiment of the present invention. Figure 9 (a) places an object to be tested 10 on a turntable 11 and sets the laser pointer 14 on the hoist platform 13 against the background of the photographing background 12. The lifting platform 13 adjusts the height so that the laser marker 14 is aligned with the center point of the object 10 to be tested. One of the preset diameters d of the first figure (b) is suspended from the upper fixed block 17 by a transparent wire 16 or the ball 15 is supported by a transparent column from below, and the height of the ball 15 is adjusted to the center point thereof. Aligned with the laser light of the laser marker 14 and mounted by a plurality of cameras 19 mounted on a multi-arm, arc or circular rack. ～28 is aligned with the ball 15 and the circular projected area of the ball 15 is taken at different angles by the respective imaging devices 19-28 as the reference object area for each of the imaging devices 19-28. . Then, the object to be tested 10 is photographed by the photographing devices 19 to 28 in FIG. 9( a), and is rotated by the turntable 11 to shoot at various angles, that is, the measured object can be measured in the completed image of each shooting angle. Any two points on the object 10 length.

FIG. 10 is a schematic diagram showing the global shooting of a sphere projection or circle and presetting a reference area of the circular diameter as a plane photographing measurement according to an embodiment of the invention. The photographing method of Fig. 10(a) & (b) and Fig. 9 are placed above the turntable 30, and the object to be tested 10 is suspended in the air by the turntable 30 for global shooting and the laser marker 31 is at an intermediate height. The rest are the same.

In summary, the method for calculating the reference object area of one geometrical shape has been used as a plane shooting measurement method, and it is not necessary to manually take a scaled ruler and manually input the length thereof, which is easier to cause. When the error is measured and the size of different objects to be tested is measured, the above process must be repeated, and the present invention processes one or more processed hole-piercing symbols on a reference object, and when measuring the size of different objects to be tested, When the reference object having the different hole number processing hole punching symbols is put into the shooting, the number of pixels corresponding to the length of the reference object per unit can be automatically completed without error.

In addition, the above embodiments are exemplified by taking a reference object first and then photographing the object to be tested. However, those of ordinary skill in the art should understand that, without adjusting the shooting distance, zooming in/out of the lens, and shooting angle, first photographing the object to be tested and then photographing the reference object can not only achieve the present invention. The effect to be achieved ensures that the reference object does not exceed the shooting range of the camera when shooting. That is to say, step S16 in Fig. 1 can be reversed with steps S11 to 14, and step S28 in Fig. 7 can also be reversed with steps S21 to S26. Therefore, an example of first taking an object to be tested and then photographing the reference object should also be included in the scope of the present invention.

The length measurement method for planar shooting provided by the present invention includes, but is not limited to, the following two types: 1. placing a reference object in front of a shooting background, wherein the reference object has a geometric shape of a predetermined size, geometry The shape can be expressed by an area calculation formula, and the preset size and area calculation formula are pre-stored in one computer; the reference object and the shooting background are taken by one shooting device to form a reference image; the reference image is stored in the computer And displayed on a screen of the computer; separating the reference image into a reference object image and a shooting background image, and displaying the reference object image on the screen; scanning a range of the reference object image by the computer, and calculating the coverage range The total number of pixels, and then the computer calculates the number of pixels corresponding to one unit length of the reference object image according to the preset size and area calculation formula; placing an object to be tested before the shooting background, and taking the object to be tested by the photographing device and Shoot the background to form an image of the object to be tested, and the object to be tested The image is stored in the computer; and the image of the object to be tested is displayed on the screen and marks any two points in the image of the object to be tested, wherein a ratio of an actual length to a unit length between the two points is substantially equal to the two points A ratio of the number of pixels corresponding to the number of pixels corresponding to the unit length. 2. Place a reference object in front of a shooting background, wherein the reference object has a geometry of a preset size, the geometry can be represented by an area calculation formula, and the reference object has at least one machined hole symbol and A comparison table of the size, area calculation formula and the relative relationship between the number of holes pre-formed with the hole punching symbol is pre-stored in a computer; the reference object and the shooting background are photographed by a photographing device to form a reference image; The reference image is stored in the computer and displayed on a screen of the computer; the reference image is separated into a reference object image and a background image, and the reference object image is displayed on the screen; a cover of the reference object image is scanned by the computer Range, and calculate the total number of pixels in the coverage range, and mark the hole punching symbol on the reference object image and calculate the number of holes to know the preset size through the comparison table, and then calculate the formula according to the preset size and area calculation by the computer. The number of pixels corresponding to one unit length of the reference object image; The object is placed in front of the shooting background, and the object to be tested and the background are photographed by the camera to form an image of the object to be tested, and the image of the object to be tested is stored in the computer; and the image of the object to be tested is displayed on the screen and marked for Measuring any two points in the object image, wherein a ratio of an actual length to a unit length between the two points is substantially equal to a ratio of the number of pixels corresponding to the two points and the number of pixels corresponding to the unit length.

1, 5‧‧‧ reference objects

2, 7, 12 ‧ ‧ shooting background

3, 19 ~ 28 ‧ ‧ camera

4, 9, 10‧‧‧ Items to be tested

6‧‧‧ Strip transparent acrylic

8‧‧‧ hole

11, 30‧‧ ‧ turntable

13‧‧‧ Lifting platform

14, 31‧‧ ‧ laser marker

15‧‧‧ sphere

16‧‧‧Transparent line

17‧‧‧Fixed blocks

Blocks A, B, C, D‧‧

D‧‧‧diameter

S11～S17, S21～S29‧‧‧ steps

FIG. 1 is a flow chart showing the calculation of a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention.

FIG. 2 is a diagram showing the reference object of a flat plate, a cylinder or a sphere which may be used as a plane photographing measurement for calculating a geometric shape and having a preset size according to an embodiment of the present invention. Geometric shape style.

Figure 3 is a schematic diagram of the processing of the hole punching symbol.

FIG. 4 is a diagram showing a reference object and a method for placing an object to be tested when one of the geometrical shapes and one of the preset sizes is used as a plane photographing measurement according to an embodiment of the present invention. .

FIG. 5 is a diagram showing a reference object and a method for placing an object to be tested when calculating a geometric shape and one of the preset sizes is used as a plane shooting measurement according to an embodiment of the present invention. .

FIG. 6(a) and (b) are diagrams showing the processing of a hole-piercing symbol on a reference object when calculating a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention. , the vertical (hanging) of the reference object and the horizontal flat shooting mode.

FIG. 7 is a flow chart showing the processing of a hole-piercing symbol on the reference object when calculating a geometric shape and one of the preset sizes of the reference object area as a plane photographing measurement according to an embodiment of the present invention.

Fig. 8 is a flow chart showing the detailed description of steps S23 and S24 of Fig. 7.

FIG. 9 is a schematic diagram showing hemispherical shooting in which a sphere is projected into a circle and a reference object area of one of the sphere diameters is taken as a plane photographing measurement according to an embodiment of the present invention.

FIG. 10 is a schematic diagram showing the global shooting of a sphere projection or circle and presetting a reference area of the circular diameter as a plane photographing measurement according to an embodiment of the invention.

Figure 11 is a schematic illustration of the scanning step in Figure 8.

Figure 12 is a schematic diagram of separating a high grayscale image into a background image and a reference object image.

Claims (12)

A length measuring method for planar shooting, comprising: placing a reference object in front of a shooting background, wherein the reference object has a geometric shape of a predetermined size, the geometric shape can be expressed by an area calculation formula, and The preset size and the area calculation formula are pre-stored in a computer; the reference object and the shooting background are captured by a camera to form a reference image; the reference image is stored in the computer and displayed on the computer Separating the reference image into a reference object image and a shooting background image, and displaying the reference object image on the screen; scanning a range of the reference object image by the computer, and calculating the coverage The total number of pixels in the range, and the computer calculates the number of pixels corresponding to a unit length of the reference object image according to the preset size and the area calculation formula; placing an object to be tested before the shooting background, and The photographing device captures the object to be tested and the photographing background to form an image of the object to be tested, and deposits the image of the object to be tested In the computer; and displaying the image of the object to be tested on the screen and marking any two points in the image of the object to be tested, wherein a ratio between an actual length and the unit length between the two points is substantially equal to the two points A ratio of the number of corresponding pixels to the number of pixels corresponding to the unit length. The length measuring method for plane shooting according to claim 1, wherein the reference object is a flat surface or a flat plate or a cylinder having a bump on the top surface, and the geometric shape is One of the outer edges of the reference object is projected on a plane in a shooting direction of the photographing device, and is triangular, square, rectangular, trapezoidal, diamond, circular, star or polygonal. The method for length measurement for plane shooting according to claim 1, wherein separating the reference image into the reference object image and the shooting background image comprises using a brightness difference between the reference object image and the shooting background image. Or the characteristic value of the color key or manual back. The length measuring method for plane shooting according to claim 1, wherein the reference object is placed in a horizontally horizontal manner or a vertically suspended manner. The length measuring method for plane shooting according to claim 2, wherein the reference object is a ball, and the photographing device performs hemisphere or global shooting from a plurality of shooting angles to obtain an amount from the shooting angles. Measure the object to be tested. The length measuring method for plane shooting as described in claim 5, wherein the ball is placed by hanging or supported by a transparent column. A length measuring method for planar shooting, comprising: placing a reference object in front of a shooting background, wherein the reference object has a geometric shape of a predetermined size, and the geometric shape can be expressed by an area calculation formula, The reference object has at least one processing hole-piercing symbol, and the comparison table, the area calculation formula and a comparison table pre-set the relative relationship between the holes of the processing hole-piercing symbols are pre-stored in a computer; The photographing device captures the reference object and the photographing background to form a reference image; the reference image is stored in the computer and displayed on a screen of the computer; and the reference image is separated into a reference object image and a shooting background And displaying the reference object image on the screen; scanning a range of the reference object image by the computer, and calculating a total number of pixels in the coverage area, and marking the processing through hole on the reference object image And calculating the number of holes to learn the preset size via the comparison table, and then calculating, by the computer, the preset size and the area Calculating a number of pixels corresponding to a unit length of the reference object image; placing an object to be tested before the shooting background, and taking the object to be tested and the shooting background by the photographing device to form an image of the object to be tested And storing the image of the object to be tested into the computer; and displaying the image of the object to be tested on the screen and marking any two points in the image of the object to be tested, wherein an actual length between the two points and the unit A ratio of the length is substantially equal to a ratio of the number of pixels corresponding between the two points and the number of pixels corresponding to the unit length. The length measuring method for plane shooting according to claim 7, wherein the shape of the machined hole symbol may be a circle, a triangle, a square, a rectangle, a trapezoid, a star or a polygon. The length measuring method for plane shooting according to claim 7 , wherein separating the reference image into the reference object image and the shooting background image comprises using a brightness difference between the reference object image and the shooting background image. Or the characteristic value of the color key or manual back. The length measuring method for plane shooting according to claim 7, wherein the reference object is placed in a horizontally horizontal manner or a vertically suspended manner. The length measuring method for plane shooting according to claim 7, wherein the number of the reference objects is plural, and different reference objects have different numbers of holes to pass through the comparison table. The formula is calculated by comparing the preset size, the geometry, and the area. The length measuring method for plane shooting according to claim 7, wherein the reference object is a flat surface or a flat plate or a cylinder having a bump on the top surface, and the geometric shape is One of the outer edges of the reference object is projected on a plane in a shooting direction of the photographing device, and is triangular, square, rectangular, trapezoidal, diamond, circular, star or polygonal.