High-speed three-dimensional (3D) measurement is increasingly important in many fields. Phase measurement profilometry (PMP) based on the binary defocusing technique has been applied to the high-speed 3D measurement scene for its higher measurement resolution and precision, and breaking the speed limitations of projector. However, because the PMP needs three phase-shifting (3-PS) patterns, motion error is inevitable to measuring dynamic objects. In this research, we construct a complete high-speed 3-PS PMP system, and re-derive two clearer motion error models than those in Weise’s research [Conference on Computer Vision and Pattern Recognition (CVPR) (IEEE, 2007), pp. 1 [CrossRef] ]. Then, we theoretically analyze the effects of the truncation error on the model accuracy, especially when the motion error is higher. To this end, a polynominal-based motion error model by fitting coefficient matrix of pre-simulation is proposed. Meanwhile, its corresponding error compensation method based on local domain estimation of the Nelder-Mead algorithm is developed. Finally, both simulations and quantitative and qualitative experiments verify the accuracy and effectiveness of the proposed method, as well as demonstrate the proposed method has improvements compared with the Weise’s research.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fringe projection profilometry (FPP) plays an important role in some academia and applied fields, such as product inspection, reverse engineering, and computer animation, because of its advantages of being noncontact, nondestructive, full field, and of high precision [1–5]. Recently, with the development of high speed imaging sensors and digital projection technology (e.g., the digital-light-processing module developed by Texas Instruments), it is possible to reach higher level on the quality and speed [6,7]. For this reason, researchers start to expand the application domain of FPP, e.g., biomechanics, on-line inspection, human-computer interaction, robot navigation, and solid mechanics .
In general, FPP has two main kinds of representative methods: Fourier-transform profilometry (FTP) and Phase measurement profilometry (PMP). For FTP, one single fringe pattern is only need to compute phase map, so it well settles for high-speed three-dimensional (3D) reconstruction . Essentially, FTP can be viewed as a band-pass ﬁlter to accurately retrieve the phase information of the continuous surface. Unfortunately, in practice, it may produce a large reconstruction error, since the measured surface usually contain discontinuities, sharp edges, or high reflectivity. For PMP, it has the advantages of higher spatial resolution, measurement accuracy, and robustness towards ambient illumination and varying surface reﬂectivity [10–12]. Generally, the more steps used, the better accuracy and stability PMP achieves . If PMP is used for high-speed, it typically requires a minimum of three fringe patterns. Therefore, the phase distortion artifacts induced motion is apparently avoidless for reconstructing dynamic scenes. In other words, when the scanned object is moving, it violates the basic phase-shifting (PS) assumption that corresponding pixels in the three phase images depict the same surface point .
For the past few years, dynamic 3D reconstruction with PMP has become one of the hotspots in the 3D reconstruction field, and the strategy generally can be categorized into the hardware methods, the reducing frames methods and the compensation methods. (1) For the hardware methods, researchers focus on a binary defocusing technique, which allows the projector to cast binary fringe patterns, thanks to the development of projector technique. For instance, the digital-light-processing (DLP) module developed by Texas Instruments (TI) not only realizes the precise synchronization for the time modulation of digital-mirror device (DMD) technique, but also alleviates optical nonlinearity and greatly increases refresh rate (kHz  or even 10 kHz [16,17]) in the 1-bit mode . (2) The temporal phase unwrapping algorithms of PMP are utilized to recover the absolute phase . That is, the additional patterns (e.g., gray-coding patterns or multi-wavelength fringes patterns) are essential. Normally, the additional information is no contribution to the reconstruction accuracy. On the contrary, the acquisition speed is limited. Therefore, researchers start to study the way to reduce the number of patterns required per reconstruction [20–22]. It is noted that the number of patterns required per reconstruction in these proposed methods is still more than three.
As the above, the binary defocusing technique using the high-speed hardware and reducing the number of patterns required per reconstruction, to some extent, assist PMP to reconstruct dynamic scenes. However, the challenge that motion will lead to non-corresponding pixels in the three phase-shifting (3-PS) patterns still remains. To this end, (3) researchers develop the post-processing algorithms to improve the dynamic reconstruction capability. Lu et al.  developed an iterative least-squares algorithm to calculate the motion phase error of a rigid object, but the non-rigid object is not covered. Liu et al.  proposed a compensation method of motion phase error through utilizing the projector pinhole model. Although the method can apply to both rigid and non-rigid motion, the iterative process is too complicated to be practical. Zhang et al. [25,26] proposed a method that the diﬀerent strategies correspond to the diﬀerent regions (i.e., a single-frame method corresponds to a motion region and a multi-frame method corresponds to a static region). This method is goal-oriented, but it can be easily affected by ambient light noise. Li et al.  developed a hybrid method that combines FTP with PMP, whereas the accuracy is limited since the phase is unwrapped by FTP. Weise et al.  derived two closed-form expressions for motion error according to Taylor series expansion and exploited a least-square ﬁtting to estimate the motion error. However, the truncation error of the introduced Taylor expansion will affect the model accuracy, especially for higher motion error (note: this paper will discuss the influence about the truncation error). Cong et al.  proposed a Fourier-assisted PMP approach which corrects the phase error by differentiating the phase maps of two successive fringe images. Since the FTP is used to retrieve the phase, the accuracy is limited. Feng et al.  proposed a motion-compensated PMP, which is suitable for the N-step PS algorithm and compensated using the statistical nature of the fringes. The method is a novel compensation strategy, but more than 3-PS patterns are projected to conduct the absolute phase unwrapping since the bi-frequency tripolar pulse-width-modulation fringe projection  is adopted. Namely, the real-time performance may be a little limited.
Our paper mainly contains three contributions: (1) we re-derive two motion error models, which are clearer than the Weise’s ones ; (2) we theoretically analyze and verify the effects of the truncation error on the model accuracy, especially for higher motion error; (3) a polynominal-based motion error model through fitting coefficient matrix of pre-simulation is proposed and an error compensation method based on local domain estimation of Nelder-Mead algorithm is developed. And, both simulations and quantitative and qualitative experiments verify the accuracy and effectiveness of the proposed method, as well as demonstrate the proposed method has improvements compared with the Weise’s research .
This paper is organized as follows: Section 2 explains a complete high-speed 3-PS PMP system. Section 3 analyzes the motion error and describes the compensation method. Section 4 presents the simulations and the experiments. Section 5 concludes this paper.
2. High-speed phase measurement profilometry
In this section, we construct a complete high-speed 3-PS PMP system, and it mainly includes four points: (1) the 3-PS algorithm is used because it’s fast and accurate; (2) a multilevel quality-guided phase unwrapping algorithm is utilized because it only requires the minimum number of recorded images to unwrap phase; (3) the calibration method based on nonlinear phase-to-height mapping equation is introduced since the projector is defocusing; (4) the digital binary defocusing technique is introduced in order to break the speed limitations of projector.
2.1 Three-step phase-shifting algorithm
Figure 1 shows a schematic system of the 3D measurement. The projector projects fringe patterns into the measurement volume. Then, the camera captures the images distorted by surface profile. Meanwhile, these images can be processed to obtain the 3D information. In the existing approaches, PS algorithms have gained great popularity owing to its speed and accuracy . And, these algorithms aim at computing the wrapped phase, since the 3D information can be implied in the phase information. Here, for high-speed applications, we adopt the 3-PS algorithm and its fringe patterns can be mathematically expressed as31]. the data modulation r(x, y)= b(x, y)/a(x, y), which is usually used as the map for background masking, represents the quality map and can be expressed as
In Eq. (5), it is needed to note that “tan−1” is a four-quadrant inverse tangent, which returns values in the closed interval [-π, π], i.e., the wrapped phase φ(x, y) ranges [–π, +π] and contains 2π discontinuity. Thus, an unwrapping algorithm is needed to obtain a continuous phase (i.e., absolute phase or unwrapped phase).
2.2 Multilevel quality-guided phase unwrapping algorithm
The phase unwrapping is to find integer number n(x, y) in the following:3) is based on the 3-PS, we introduce Zhang’s phase unwrapping approach . The algorithm is surprisingly robust in practice without needing any other supplementary information for unwrapping phase, which has a good phase unwrapping effect for human face and so on. However, it is pointed out that the algorithm obtains only one single connected patch.
Firstly, in order to reduce phase unwrapping region, data modulation [see Eq. (4)] is utilized as a mask to remove the background. Secondly, a quality map guides the phase unwrapping path that define the quality or goodness of each phase value. The map can be defined from a maximum phase gradient map, which is determined as
In Eq. (8), W is an operator that estimates the true gradient by wrapping the differences of the wrapped phase; Δx and Δy respectively are maximum values of the partial derivatives of the phase in the x and y directions. In Eq. (7), the larger Q(x, y) is, the worse the data quality is. Thirdly, the unwrapping process starts from a high-quality point and its four neighbors are examined, unwrapped and stored in a list. Then, an iterative process of (1) removing the highest quality pixel from the list, (2) unwrapping its four neighbors and (3) inserting them in the list continues until all the pixels have been unwrapped. It is natural to view that the algorithm can be as a region-growing approach or the flood-fill method. In this way, all the high-quality region is unwrapped first, and the low-quality region are unwrapped last.
2.3 Calibration method with a binary defocusing technique
The calibration method with binary defocusing technique usually has a challenge: the projector is defocusing rather than in focus. Thus, our research refers to an accurate calibration method , which avoids the calibration of projector. In the method, a reference-plane based on nonlinear phase-to-height mapping  is adopted, and the mapping can be described as
2.4 Digital binary defocusing technique
Digital binary defocusing technique can greatly increase projector refresh rate to achieve a high measurement speed in the 1-bit mode. Essentially, the technique is that the computer generates binary patterns and the defocused optical system blurs them to become smooth ones . Mathematically, the defocusing effect can be simplified to a convolution operation,36],
Simply, g(x, y) can be approximated by a circular Gaussian function ,
3. Phase error analysis and compensation for motion
Section 2 has constructed a complete high-speed 3-PS PMP system. Three recorded fringe images are required to calculate the phase. For the static scene, the pixels among the 3-PS images are completely corresponding. However, for the dynamic ones, the correspondence among them is violated. The result of phase unwrapping will generate the phase error. In other words, the analysis and compensation of motion error is essentially to analyze and compensate the phase error.
3.1 Phase error analysis for motion
Figure 2 demonstrates a schematic analysis model of phase error analysis for motion. In the Fig. 2(a), a captured light beam l intersects the dynamic surface at point P−1, P0 and P1, and arrives at the point c on the CMOS. Or said differently, the camera captures the point P−1, P0 and P1 on the dynamic surface at three times (t−1, t0 and t1) to form the 3-PS images and in the meanwhile the point P−1, P0 and P1 have a same image point c on the CMOS. Here, two research suppositions are introduced: (1) the relation Δt = t0- t−1= t1- t0 makes sense because the camera is set at a constant frame rate, and (2) the distance |P−1P0| closely approximates |P−1P0| on account of the high-speed performance of system and meanwhile the motion in small area. As shown in the Fig. 2(a), d−1, d0 and d1 are the corresponding DMD points of P−1, P0 and P1, respectively. Therefore, from the viewpoint of geometry, the equation Δd=|d−1d0|≈|d1d0| is true. In addition, for clear analysis, a cross section on the DMD is considered, as shown in Fig. 2(b). the relation between the points (d−1, d0 and d1) and phase (φ−1, φ0 and φ1) can be build. That is, the relation Δθ= φ1-φ0≈φ0-φ−1 is existent, and there are three mappings: P−1→d−1→φ0-Δθ, P0→d0→φ0 and P1→d1→φ0+Δθ that can be given. Next, the actual captured intensities can be described as follows [note: in order to simplify representation, the (x, y) is omitted in the following equations.]:
Similarly, the distorted phase φf is given by:
Next, substituting Eq. (19) into Δφ=φf-φt, we can express Δφ as
Secondly, Eq. (20) can be abstracted as f(A), and then f(A) is Taylor expanded at A = A0, the result as shown in the following:
Next, specifying Eq. (21) and setting A0=1, we can obtain:
Finally, ignoring the truncation error in Eq. (22), we can obtain a one-rank Tailor multinomial model (hereafter referred to as one-rank model, or ORM, for short) [see Eq. (23)] and a two-rank Tailor multinomial model (hereafter referred to as two-rank model, or TRM, for short) [see Eq. (24)].
3.2 Phase error compensation for motion
In Eqs. (23) and (24), the relatively phase Δφ is caused by Δθ. Therefore, in order to eliminate Δθ, we need to adopt an optimization strategy to estimate Δθ, i.e., an objective function can be introduced,
Combining the two ways above, Eq. (25) can be transformed into
As above, combining Eq. (26) or Eq. (27) with Eq. (28) can estimate Δθ. However, as defined in Sec. 3.1, the truncation error is unavoidable since Taylor expansion is applied. As shown in Fig. 3 and Table 1, the truncation error have to be considered when Δθ ≥ 0.3. Therefore, for removing the influence of truncation error, we proposed a novel phase error model based on fitting coefficient matrix (or FCM, for short). The core of this method is that the Δφ’(φf, Δθ) is written in the following equation:
In practice, the main steps of the proposed FCM method are as following:
Step 1. The range and step size of Δθ are respectively set to [−1, 1] and 0.01. And, the corresponding Δφ-φf curve of each Δθ can be calculated by Eq. (18) [note that: the space domain of Eq. (18) needs to be converted into the phase domain]. Then, a 20th-order polynomial is used to fit the φf-Δφ curve of each Δθ.
Step 2. A 5th-order polynomial is utilized to fit the coefficients obtained by Step 1, and ultimately the coefficient matrix ck,l is obtained.
Step 3. In the measurement, Δθ of each pixel is estimated by the Nelder-Mead algorithm and the following objective formula:
4. Simulations and experiments
Both simulations and quantitative and qualitative experiments verify the effectiveness and accuracy of the proposed method.
Case 1. Estimation for a constant Δθ
Firstly, a pixel is selected, the estimated Δθ is supposed to 0.1 (i.e., φf is determined), and the initial values of h and g are respectively calculated as 0.0491 and 0. Secondly, substituting Δθ and φf into Eqs. (26), (27) and (29), the phase errors Δφ1’, Δφ2’ and Δφ’ can be obtained. Next, these phase errors are respectively substituted in Eq. (28) to construct three objective functions. Finally, combining the obtained functions above with the fminsearch function in Optimization-Tool Toolbox of MATLAB, we can get the estimation results, as listed in Table 2. From Table 2, (1) our proposed FCM works the best when TRM works better than the ORM; (2) the three models are basically the same number of iterations (one-rank model: 321; two-rank model: 326; our proposed model: 335).
Case 2. Estimation for different Δθ
For this case, most of the process looks like Case 1. The main difference is that a series of different Δθ (±0.1, ±0.2, ±0.3, ±0.4, ±0.5 and ±0.6 rad) are estimated by using ORM, TRM and our proposed FCM, respectively. In Table 3, the first column lists the actual values of Δθ; the columns from second to forth show the calculated initial values of h and g, as well as the initial value of Δθ is set to zero; the columns from fifth to seventh display the estimation values of Δθ with ORM, TRM and our proposed FCM. From Table 3, they are shown that (1) the actual values and the estimation values of Δθ with the three methods have the same variation trend; (2) for different Δθ, the estimation values of Δθ with our proposed FCM are nearest to the actual values of Δθ when TRM works better than ORM. To see the trend of Table 3 more clearly, Fig. 5 illustrates the absolute differences between the actual values and the estimated values of Δθ with the three methods. Similarly, the accuracy and stability of our proposed FCM is superior to other two methods.
According to the actual requirement for theoretical verification, an experimental system was established. The system mainly consists of a projector (TI Light Crafter DLP 4500) and a digital high-speed CMOS camera (Optronis CP80-3-m-540). The camera has a 1708×1696 pixels resolution with a frame rate of 2000 Frames/s and attaches a 16 mm focal length 10 Mega-Pixel lens (VOC 16mm10MP). The DLP module has a resolution of 912×1140 with a projection distance of 500–2000mm. In our experiment, the level of defocusing is realized by manually adjusting the focal distance of the projector. A red linear laser is used to determine the initial position of phase unwrapping. A white plate to be a reference plane is attached to a motorized linear translation stage, which has 250 mm travel. A dual channel signal generator (MingHe JDS 6600) is regarded as an external trigger to synchronize the camera and the projector. Moreover, the DLP module and the high-speed CMOS camera are set to 2000Hz in binary mode and in 8-bit gray mode, respectively. Specifically, the experimental system and main devices arrangement pictures are shown in Fig. 6. In addition, it is to note that 5×5 spatial Gaussian smoothing is applied to the resulting geometry.
Quantitative evaluation: Reconstruction of the simple motion scenes
For quantitative evaluation, a 250×250 mm2 flat surface was introduced. Here, there are three tests.
- (1) The flat surface at 19 positions of about 850 mm to 1100 mm are measured. At each position, 3-PS patterns are projected onto the flat surface, as shown in Fig. 7. And, the compensated (with FCM) and uncompensated RMS phase error values of the residuals are calculated, as shown in Fig. 9(a). The results show that our proposed FCM does not affect the reconstruction accuracy for the static scenes, and the values are relatively stable.
- (2) The 3-PS patterns (I1, I2 and I3) are respectively cased onto the flat surfaces in three positions, which is to be a group. In each group, there is a same interval between I1-I2 and I2-I3. We selected 11 intervals from 0 to 25 mm, i.e., there exist 11 groups, as shown in Fig. 8. Then, the compensated (with FCM) and uncompensated RMS phase errors values are obtained, as shown in Fig. 9(b). From the figure, we can analyze that: (a) at the interval of 0 mm, the results are not disturbed by compensation; (b) for the intervals from 2.5 mm to 22.5 mm, our method can effectively remove the effect of motion; (c) at the interval 25 mm, our method starts invalid.
- (3) Similarly, we performed a comparative experiment with our method, Weise’s method , Cong’s method  and uncompensated result. Five intervals from 0 to 10 mm are selected. Figure 9(c) compares the results. For the compensation methods, generally, all of them can effectively suppress the effect of motion, and the phase errors for motion increase with the rise of the intervals. In addition, the compensation effect of our method is better than that of the other two methods, and with the increase of the intervals, the difference of compensation effect is more obvious.
Then, a fast-moving tablet and a fluttering tissue are captured in the measured volume. We captured a comment and reconstructed the compensated (with FCM) and uncompensated results, as shown in Figs. 10 and 11, respectively. Figure 10(a) displays the test moment of the fast-moving tablet (note: the picture was captured with a mobile phone). In Fig. 10(a), the red, yellow and green dashed box represent the tablet in the captured 3-PS patterns, respectively. And, the wrapped phase is shown in the top right-hand corner. Figure 10(b) and (c) show the sections A-B in Fig. 10(a); Fig. 10(d) and (e) show the sections C-D in Fig. 10(a). For the section A-B, the pixel distance is 100 pixels from point A (272, 528) to point B (371, 528), where about 2.43 fringes are contained. Thus, the phase change from point A to point B is approximately 2.43×2π≈15.27 rad. In the Fig. 10(b), the uncompensated phase curve obviously shows wave shape; on the contrary, the compensated phase curve is smooth. In addition, based on Fig. 10(c), the compensated and uncompensated RMS height error values are respectively calculated: 0.287 mm and 0.099 mm, which demonstrated the effectiveness and accuracy of our proposed compensation method. Similarly, for the section C-D, the pixel distance is 50 pixels from point C (591, 690) to point D (640, 690), where the phase change is approximately 1.10×2π≈6.91 rad. In the Fig. 10(d), the compensated and uncompensated curves have no noticeable change since the section C-D is on the static background. Meanwhile, In the Fig. 10(e), the two RMS height error values (0.035 mm and 0.023 mm) are almost the same. Moreover, Fig. 11(b) shows the test moment of the fluttering tissue, and the dashed box represents the ROI (Region of Interest). Figure 11(a) and (c) respectively show the uncompensated and compensated results of the ROI. it can be clearly seen that our FCM was proved to be effective, i.e., the ripples are significantly reduced.
Qualitative evaluation: Reconstruction for the complex motion scenes
For qualitative evaluation, the complex motion scenes are reconstructed. Figure 12 displays a waving cloth. It can be seen that the reconstructed surfaces are relatively smooth, i.e., the motion ripples are effectively compensated. Figure 13 shows a face with expression rapidly changing. Again, less motion artifacts are visible. It is pointed out that some tiny stripes on the reconstructed surface are caused by the incomplete binary defocusing since the static areas (e.g., the forehead of face) also exist tiny stripes.
5. Conclusions and discussions
In this paper, firstly, we construct a complete high-speed PMP system, which includes 3-PS algorithm, multilevel quality-guided phase unwrapping algorithm, nonlinear polynomial calibration technique and digital binary defocusing technique. Then, we re-derive two phase error expressions (ORM and TRM), which are clearer than the ones in Ref. . For ORM and TRM, the effect of truncation error affecting accuracy is demonstrated, especially when the phase error for motion is greater than 0.3 rad. On this account, we proposed the polynominal-based motion error model based on fitting coefficient matrix of pre-simulation and developed the error compensation method based on local domain estimation of Nelder-Mead algorithm. Finally, simulation cases demonstrate our proposed FCM is superior to ORM and TRM (i.e., the models in Ref. ). In addition, the quantitative and qualitative experiments show the effectiveness and accuracy of our proposed FCM. In practical application, to some extent, the motion objects are reconstructed at high accuracy and the motion ripples are correctly compensated. Future work will consider other more advanced models and attempt to realize real-time and online compensation for motion error through utilizing the graphics processing unit (GPU) calculation.
Authors would like to thank Xing Yanfeng in SUES, whose lab supported this research.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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