TW202109456A - Method of two-dimensional fringe-encoded pattern projection for instantaneous profile measurements - Google Patents

Abstract

A method of two-dimensional fringe-encoded pattern projection for instantaneous profile measurements is provided. The method comprises a projection step, an image capture step, a transform step, and an unwrapping step. By adopting a 2D fringe-encoded pattern with fourier transform profilometry, even if the surface color or reflectivity varies rapidly with positions, the2D fringe-encoded pattern can still distinguish the fringe order very well.

Description

Method suitable for instantaneous shape measurement of two-dimensional coded fringe projection

The present invention relates to a measurement method, in particular to a method suitable for instantaneous topography measurement of two-dimensional coded fringe projection.

At present, the topography measurement technology suitable for dynamic objects is mostly structured light projection or fringe projection technology, combined with single-shottechniques. The main measurement principle is to project a pattern with a string-like transmittance on a dynamic object to be measured, and the stripe distribution on the surface of the object to be measured is recorded by a CCD in another viewing angle. The fringe distribution captured by the CCD will be distorted with the contour of the object, so the degree of phase distortion of the fringe is related to the depth change of the object to be measured, which is called the "phase-to-depth relation" , Find this relational expression to further obtain the dynamic three-dimensional coordinates of the surface of the object to be measured. In addition, the object to be measured with a deep fault on the surface cannot be identified purely from the image, so the fringe order has been developed to identify the fringe order and achieve the purpose of phase unwrapping.

However, the fringe encoding method is susceptible to interference from the external environment. For example, poor lighting or low reflectivity leads to a low signal-to-noise ratio. Not only does it cause errors when capturing the fringe phase, it is easy to cause errors when performing fringe decoding. At this time, it will also cause misjudgment on the gray scale, and then cause the error of phase unwrapping.

Therefore, it is necessary to provide a method suitable for instantaneous topography measurement of two-dimensional coded fringe projection to solve the problems of the above-mentioned conventional technology.

The main purpose of the present invention is to provide a method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, using two-dimensional fringe coding pattern with Fourier transform, even if the surface color or reflectivity changes rapidly with position, two-dimensional fringe coding The pattern can also distinguish the order of the stripes very well.

In order to achieve the above objective, the present invention provides a method suitable for instantaneous topography measurement of two-dimensional coded fringe projection. The method includes a projection step, an imaging step, a conversion step, and an expansion step; in the projection step , Projecting a two-dimensional stripe coding pattern from a first viewing angle onto a dynamic object to be measured, so that a projection stripe is formed on a surface of the dynamic object to be measured; in the image capturing step, an image capturing device is used The projection fringe is captured from a second angle of view and captured as an image; in the conversion step, a first phase extraction is performed on the image along a horizontal direction to obtain a first phase image, and then Perform a second phase extraction on the image along the isophase line direction of the first phase image to obtain a second phase image; in the expansion step, perform quaternary decoding on the second phase image line , In order to identify the fringe sequence of the first phase image, and then achieve the purpose of unfolding the first phase image, and then obtain a contour of the dynamic object under test based on the relationship between the phase and the depth.

In an embodiment of the present invention, the image capture device is an image sensor array.

； 其中y_o 是常數值，a (x ,y )是背景強度，b (x ,y )是調製幅度，d 是該圖像擷取裝置獲得的該的周期。In an embodiment of the present invention, in the image capturing step, the gray scale obtained along an image line of the image can be expressed as:

; Where y _o is a constant value, a ( x , y ) is the background intensity, b ( x , y ) is the modulation amplitude, and d is the period obtained by the image capturing device.

In an embodiment of the present invention, the two-dimensional stripe coding pattern is configured to provide a plurality of additional information, and the additional information includes a digital stream of quaternary numbers, a stripe sequence, and a total number of stripes.

In an embodiment of the present invention, the first viewing angle and the second viewing angle form an acute angle.

In an embodiment of the present invention, in the conversion step, the first phase extraction is performed on the image along a horizontal direction using Fourier transform to obtain the first phase map of the phase distribution in the horizontal direction.

In an embodiment of the present invention, in the conversion step, Fourier transform is used to perform the second phase extraction on the image along the isophase line direction of the first phase map to obtain the direction along the isophase line This second phase map of the phase distribution.

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。In an embodiment of the present invention, in the conversion step, the following formula is used to perform the first phase extraction in the horizontal direction:

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。In an embodiment of the present invention, in the conversion step, the following formula is used to perform the second phase extraction in the isophase line direction of the first phase map:

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； 其中m 是描繪四進制數的腳註，T_o 是邊緣週期，r 是量化比率。In an embodiment of the present invention, in the projection step, the transmittance of the two-dimensional fringe code pattern can be expressed as:

; Where m is a footnote describing a quaternary number, T _o is the fringe period, and r is the quantization ratio.

In an embodiment of the present invention, the quantization ratio r is defined as: if the quaternary number m=0, then r =1.00; if the quaternary number m=1, then r =1.33; if the quaternary number m =2, then r =1.67; and if the quaternary number m=3, then r =2.00.

As mentioned above, the method of the present invention suitable for instantaneous topography measurement of two-dimensional coded fringe projection proposes the use of two-dimensional fringe coding pattern with Fourier transform, wherein the fringe pattern used for phase extraction can be directly used for unfolding without The phase unwrapping adopts additional predictions, so that the fringe order can be accurately identified to achieve the purpose of phase unwrapping. In addition, the design of the two-dimensional stripe coding pattern is simple and reliable. Even if the surface color or reflectivity changes rapidly with the position, the two-dimensional stripe coding pattern can distinguish the stripe sequence well, which is suitable for most of the Dynamic objects, and can measure multiple or complex color objects at the same time.

In order to make the above and other objectives, features, and advantages of the present invention more obvious and understandable, the preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. Furthermore, the directional terms mentioned in the present invention, such as up, down, top, bottom, front, back, left, right, inside, outside, side, surrounding, center, horizontal, horizontal, vertical, vertical, axial, The radial direction, the uppermost layer or the lowermost layer, etc., are only the direction of reference to the attached drawings. Therefore, the directional terms used are used to describe and understand the present invention, rather than to limit the present invention.

Please refer to Fig. 1, which is a preferred embodiment of the method for instantaneous topography measurement of two-dimensional coded fringe projection according to the present invention. The method includes a projection step S201, an image capturing step S202, and a conversion step S203. And an unfolding step S204. The present invention will describe in detail the relationship between the steps and the principle of operation below.

；Referring to Figures 1 and 2, in the projection step S201, a two-dimensional stripe code pattern is projected from a first viewing angle along an oblique line direction L1 onto a dynamic test object 101, so that the dynamic test object 101 is A projection stripe 102 is formed on one surface of the object, and Figure 2 is the optical configuration of the stripe projection system. The formula 1 represented by L1 is:

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In this embodiment, the method of the present invention suitable for instantaneous profile measurement of two-dimensional coded fringe projection is to propose a one-time technique for profile measurement, and irradiate the two-dimensional coded pattern on the dynamic object 101 under test. , Wherein the two-dimensional stripe coding pattern is configured to provide a plurality of additional information, and the additional information includes a quaternary number stream, a stripe sequence and a total number of stripes.

；Please refer to FIGS. 1 and 2. In the image capturing step S202, an image capturing device is used to capture the projection fringe from a second angle of view to capture an image; in this embodiment, the image The image capturing device is an image sensor array that uses monochrome photography to observe the deformed fringes of the image at the second viewing angle, wherein the first viewing angle and the second viewing angle form an acute angle. Furthermore, the gray scale obtained along an image line of the image can be expressed as:

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Where y _o is a constant value, a ( x , y ) is the background intensity, b ( x , y ) is the modulation amplitude, and d is the period obtained by the image capturing device.

Please refer to Figure 1, in the conversion step S203, the first phase extraction (phase extration) of the image is performed along the horizontal direction to obtain a first phase image, and then the Fourier transform is used to obtain a first phase image. The image is subjected to the second phase extraction along the isophase line direction of the first phase image to obtain a second phase image of the phase distribution along the isophase line direction; in this embodiment, Fourier transform is used for the image The first phase extraction is performed on the image along a horizontal direction to obtain a phase map of the phase distribution in the horizontal direction, and then Fourier transform is used to perform the first phase extraction along the isophase line direction of the first phase map for multiple image lines of the image. The second phase extraction is performed to obtain a phase map of the phase distribution in the isophase line direction (this embodiment happens to be the vertical direction).

It should be noted that Fourier transform is a well-known three-dimensional shape measurement method, because it has full-field measurement capabilities, such as non-contact detection and one-time detection, and the two-dimensional stripe code pattern can be projected onto the dynamic test object 101 , And capture the image from the second angle of view, and record the stripe distribution of the image. The phase of the image is distorted by the surface contour of the dynamic test object 101, so the three-dimensional shape can be retrieved through analysis, and in order to obtain the phase distribution of the image, a phase shift algorithm is used to evaluate the phase through arctangent calculation , Resulting in the interval (–　,　). Therefore, it is necessary to restore the continuity of the phase map through unwrapping.

；Furthermore, the formula 2 expressed by the one-dimensional Fourier transform of formula 1 on the x-axis (horizontal direction) is:

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；In addition, the formula 3 expressed by the inverse Fourier transform of the fundamental components is:

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；Which can use formula 4 to extract the phase:

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It should be noted that formula 4 uses the arctangent function to calculate the phase, resulting in the distribution containing 2　, So it needs to be expanded to produce a continuous phase distribution.

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。Furthermore, use the following formula to perform the second phase extraction in the isophase line direction of the first phase map:

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；Please refer to Figure 3, which is an example of the coding mode, showing the appearance of the two-dimensional stripe coding pattern, and the digital stream of quaternary numbers is marked below, where the transmittance of the two-dimensional stripe coding pattern can be mathematically The description is expressed as Formula 5:

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Where m is a footnote depicting a quaternary number, T _o is a fringe period, r is a quantization ratio, and the quantization ratio r is defined as Formula 6: If the quaternary number m=0, then r = 1.00; if the quaternary number m=0, then r=1.00; If the number m=1, then r =1.33; if the quaternary number m=2, then r =1.67; and if the quaternary number m=3, then r =2.00.

；The entire pattern formed by assembling all encoded fringes is expressed as formula 7:

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Where n is the stripe sequence, N is the total number of stripes, and the quaternary number m changes with the stripe sequence. As shown in Figure 3, look for the quaternary number stream under the stripe.

As shown in Figure 4a, it is a digital stream of quaternary numbers formed by assembling 64 cipher words. Any cipher word is unique among subsequent digits. Therefore, the stripe sequence can be recognized. In addition, as shown in Figure 4b, a list of stripe sequences with quaternary numbers is listed. For example, the subsequent digits 2, 3, and 0 form the password 27 (see Figure 3), and are used to mark the fringe order ranging from 27 to 29. The fringe orders represented by quaternary numbers are quaternary numbers. Replaced in a unique way, the stripe sequence can be identified by any three adjacent numbers. For a password containing 3 quaternary numbers, there are 64 permutations, that is, 4 ³ = 64. Therefore, 66 digital streams can be generated. 64 cipher words can overlap spatially by the following way: one cipher word partially overlaps another cipher word, and two numbers overlap between the two cipher words. Since it only occurs once in the entire digital stream, the corresponding 3 stripes are identified. Other passwords only appear once. Therefore, all fringe sequences can be clearly recognized.

；Referring to FIG. 1, in the expansion step S204, the phase map is expanded, and each image line corresponding to the phase map is decoded to obtain an outline of the object under test. Furthermore, fringe decoding is a task, and projection fringes can be specified using quaternary numbers, so that the order of fringes can be identified. For example, as shown in Figure 5a, it is a projected two-dimensional pattern on a plane, which shows the appearance of stripes projected on a flat surface. As shown in Figure 5b, it is the one-dimensional phase distribution along the x-axis. Formula 4 can be used to extract the phase distribution _x along the x-axis (that is, the first phase image　( x , y _o )), where isophase lines are defined as lines assembled by pixels _{with the same phase value x.} As shown in Figure 5c, it is a one-dimensional phase distribution along the isophase line　( _equal , y ), which displays the phase map (ie, the second phase map) by extracting the phase of the projected two-dimensional pattern along the direction of the isophase line　( _equal , y )). The quantization ratio r can be identified by formula 8:

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；Where T _d is the stripe period along the x-axis (a known value); therefore, quaternary numbers can be executed by referring to Equation 6, since any three subsequent digits in the digital stream are unique and do not appear elsewhere, Then you can use the list shown in Figure 4b to identify all fringe orders, and then perform phase unwrapping in the fringe order, as shown in Equation 9:

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Wherein the phase is covered (wrapped phase) obtained by the formula 4, n is the fringe order.

Furthermore, a sphere positioned on a carton can be selected as an object to be measured. As shown in Figure 6, to use coded stripes to project to the object to be measured for the appearance of image capture, formula 4 The method described in extracts the phase of the projected code fringe. Then, the phase map is obtained by extracting the phase of the projected code fringe along the isophase line, and then the quantization ratio is calculated for each fringe using Equation 8, thereby generating a three-digit digital stream for each image line. Then, the expansion is performed by searching the above-mentioned list, and the decoding process is repeated for each image line, thereby expanding the entire phase map to obtain the outline of the sphere as shown in Figure 7.

As mentioned above, the method of the present invention suitable for instantaneous topography measurement of two-dimensional coded fringe projection proposes the use of two-dimensional fringe coding pattern with Fourier transform, wherein the fringe pattern used for phase extraction can be directly used for unfolding without Phase unwrapping adopts additional predictions, so it can accurately identify fringe order to achieve phase unwrapping, and at the same time overcome poor illumination or low reflectivity, which leads to low signal-to-noise ratio, which is easy to cause errors. It also overcomes the problem of phase unwrapping errors caused by misjudgment on gray scale during fringe decoding. In addition, the design of the two-dimensional stripe coding pattern is simple and reliable. Even if the surface color or reflectivity changes rapidly with the position, the two-dimensional stripe coding pattern can distinguish the stripe sequence well, which is suitable for most of the Dynamic objects, and can measure multiple or complex color objects at the same time.

Although the present invention has been disclosed in preferred embodiments, it is not intended to limit the present invention. Anyone familiar with the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the present invention The scope of protection shall be subject to the scope of the attached patent application.

S201: Projection step S202: image acquisition step S203: Conversion step S204: unfolding steps L1: diagonal direction 101: Dynamic DUT 102: Projection fringe

FIG. 1 is a flowchart of a preferred embodiment of the method for instantaneous topography measurement of two-dimensional coded fringe projection according to the present invention. FIG. 2 is a schematic diagram of projecting a two-dimensional coded fringe pattern according to a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional coded fringe projection according to the present invention. FIG. 3 is a schematic diagram of a two-dimensional fringe encoding pattern according to a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional encoding fringe projection according to the present invention. Fig. 4a is a digital stream of quaternary numbers according to a preferred embodiment of the method for instantaneous topography measurement of two-dimensional coded fringe projection according to the present invention. FIG. 4b is a list of the quaternary number fringe sequence of a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional code fringe projection according to the present invention. FIG. 5a is a schematic diagram of the projection of a two-dimensional pattern on a plane according to a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional coded fringe projection according to the present invention. Fig. 5b is a schematic diagram of a one-dimensional phase distribution along the x-axis of a preferred embodiment of the method for instantaneous topography measurement of two-dimensional coded fringe projection according to the present invention (the first phase diagram). Figure 5c is a schematic diagram (second phase map) of the one-dimensional phase distribution along the isophase line according to a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional coded fringe projection according to the present invention. FIG. 6 is a schematic diagram of a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional coded fringe projection according to the present invention, which uses the coded fringe to project to the dynamic object to be measured for image capture. FIG. 7 is a schematic diagram of the contour of the dynamic object to be measured according to a preferred embodiment of the method for instantaneous topography measurement of the two-dimensional coded fringe projection according to the present invention.

S201: Projection step

S202: image acquisition step

S203: Conversion step

S204: unfolding steps

Claims (10)

A method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, the method includes: A projection step, projecting a two-dimensional stripe code pattern from a first viewing angle onto a dynamic object to be measured, so that a projection stripe is formed on a surface of the dynamic object to be measured; In an image capturing step, an image capturing device is used to capture the projection fringe from a second angle of view to capture an image; In a conversion step, a first phase extraction is performed on the image along a horizontal direction to obtain a first phase map, and then a first phase extraction is performed on the image along the isophase line direction of the first phase map. Secondary phase extraction to obtain a second phase map; and In an expansion step, quaternary decoding is performed on the second phase map row to identify the fringe sequence of the first phase map, and then achieve the purpose of expanding the first phase map, and then obtain the dynamics from the relationship between phase and depth An outline of the object to be measured. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein the image capturing device is an image sensor array. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the image capturing step, the gray scale obtained along an image line of the image can be expressed as:

; Where y _o is a constant value, a ( x , y ) is the background intensity, b ( x , y ) is the modulation amplitude, and d is the period obtained by the image capturing device. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein the two-dimensional coded pattern is configured to provide a plurality of additional information, and the additional information includes quaternary numbers The number stream, the sequence of stripes, and the total number of stripes. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein the first viewing angle and the second viewing angle form an acute angle. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the conversion step, Fourier transform is used to perform the first phase of the image along the horizontal direction. Extracting to obtain the first phase map of the phase distribution in the horizontal direction. As described in the first item of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the conversion step, Fourier transform is used to follow the isophase line of the first phase image to the image The second phase extraction is performed in the direction to obtain the second phase map of the phase distribution along the isophase line direction. As described in the first item of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the conversion step, the following formula is used for the first phase extraction in the horizontal direction:

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. As described in the first item of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the conversion step, the following formula is used to perform the second time of the isophase line direction of the first phase image Phase extraction:

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. As described in item 1 of the scope of patent application, the method suitable for instantaneous topography measurement of two-dimensional coded fringe projection, wherein in the projection step, the transmittance of the two-dimensional coded fringe pattern can be expressed as:

; Where m is a footnote describing a quaternary number, T _o is the fringe period, and r is the quantization ratio.

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