A method of color phase shift fringe crosstalk error correction

Abstract

This invention relates to the field of structured light 3D measurement technology, specifically a method for correcting color phase-shifting fringe crosstalk error, comprising the following steps: Step S1: Construct a color fringe projection system. This invention corrects color crosstalk error from two dimensions: fringe intensity and phase distribution. First, the green channel is used to perform histogram specification processing on the red and blue channels to correct the fringe intensity. Then, the solved phase distribution is subjected to histogram equalization processing to correct the phase distribution. It does not require pre-calibration of the spectral response characteristics of the color fringe projection system, nor does it require training with a large-scale dataset, which greatly reduces the complexity of the correction operation. At the same time, it effectively suppresses background light intensity fluctuations, modulation depth attenuation, and phase periodic ripple errors caused by color crosstalk. It has the advantages of high speed, high accuracy, and ease of implementation, and can well meet the requirements of high-speed and high-precision 3D measurement.

Description

Technical Field

[0001] This invention relates to the field of structured light three-dimensional measurement technology, and in particular to a method for correcting crosstalk error in color phase-shifting fringes. Background Technology

[0002] In the field of structured light 3D measurement, color phase-shift profilometry typically uses three channels—red (R), green (G), and blue (B)—to encode three-step phase-shift fringes. By simultaneously acquiring multi-channel information through a single projection and imaging, and then analyzing the color fringes to reconstruct the 3D shape of the object under test, it can significantly improve the efficiency of 3D measurement.

[0003] However, in actual measurement systems, due to the nonlinear photoelectric response characteristics of digital projectors and color cameras, color crosstalk inevitably occurs between different color channels, especially adjacent color channels. This causes the intensity of the stripes in each channel to affect each other, manifesting as fluctuations in background light intensity and attenuation of modulation depth. This disrupts the ideal sinusoidal distribution of the phase-shifted stripes, resulting in periodic ripple errors in the demodulated phase, which seriously affects the accuracy of three-dimensional measurement.

[0004] Existing correction methods largely rely on pre-calibrating the spectral response characteristics of the color fringe projection system and using the calibrated color crosstalk matrix to correct the fringe intensity under different color channels. However, these methods are time-consuming and labor-intensive, making it difficult to meet the requirements of high-speed and high-precision measurements. Some researchers have constructed deep learning models to extract phase distribution information from color fringe images, directly recovering high-precision phase distribution information from the color fringe images, which can effectively suppress color crosstalk errors. However, these methods rely on large-scale data training, and their generalization and interpretability need improvement. Therefore, how to efficiently correct the color crosstalk errors in color phase-shifting profilometry is crucial for improving the accuracy of 3D measurements. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and propose a method for correcting crosstalk error in color phase shift stripes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for correcting crosstalk error in color phase-shifted fringes, comprising the following steps: Step S1: Construct a color stripe projection system, which includes a projector, a color camera, and the object to be measured. There is a preset angle between the camera and the color camera, and the object to be measured is placed within the common field of view of the projector and the color camera. Step S2: Project a colored stripe onto the surface of the object to be tested using a projector. The colored stripe is distorted and deformed due to the height change of the surface contour of the object to be tested. The modulated colored stripe of the object to be tested is captured by a camera. Step S3: Calculate the maximum and minimum values ​​of the three channels (red R, green G, and blue B) in the colored stripes one by one, and normalize the three channels (red R, green G, and blue B) in turn. Step S4: Calculate the histogram of the green G channel, and use the histogram of the green G channel to perform histogram specification processing on the red R channel and the blue B channel to suppress the problem of stripe intensity imbalance between different channels; Step S5: Calculate the full-cycle cutoff phase of the colored stripes using the three-step phase shift method, then calculate the half-cycle cutoff phase, and perform histogram equalization on the half-cycle cutoff phase to eliminate periodic crosstalk errors. Step S6: Use the geometric constraint method to perform phase unrolling on the corrected truncated phase to recover the absolute phase of the object under test, and then convert the absolute phase into a three-dimensional point cloud based on the calibration data of the color stripe projection system.

[0007] Preferably, in step S2, the color stripes modulated by the object to be tested can be represented as: In the formula These represent the red (R), green (G), and blue (B) channels of the colored stripes, respectively. Represents the pixel coordinates of the colored stripes; These represent the background intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the modulation intensities of the red (R), green (G), and blue (B) channels, respectively. This represents the truncated phase to be solved; These represent the phase shifts caused by color crosstalk in the red (R), green (G), and blue (B) channels, respectively.

[0008] Preferably, in step S3, the normalization expressions for the red (R), green (G), and blue (B) channels are as follows: ; In the formula, These represent the maximum light intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the minimum light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively. These are the normalized light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively.

[0009] Preferably, in step S3, the value of the TOP1% position of the light intensity of each channel is used as the maximum value of the corresponding channel, and the minimum value of the TOP1% position of the light intensity of each channel is used as the minimum value of the corresponding channel, so as to suppress the influence of image noise.

[0010] Preferably, in step S4, the histogram specification of the red R and blue B channels using the histogram of the green G channel is expressed as follows: In the formula This represents the histogram statistical function; This represents the histogram specification function. These are the light intensity values ​​of the red (R) and blue (B) channels after histogram specification.

[0011] Preferably, in step S5, the mathematical expression for calculating the integer-period cutoff phase of the colored stripes using the three-step phase-shift method is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire period, and its value ranges from 0 to 2π.

[0012] Preferably, in step S5, the mathematical expression for calculating the half-cycle truncation phase is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire period, and its value ranges from 0 to π.

[0013] Preferably, in step S5, the expression for histogram equalization of the half-cycle truncated phase is: In the formula This represents the histogram equalization function.

[0014] Preferably, in step S5, the half-cycle truncated phase is compared with the histogram equalization process. The corresponding integer-cycle truncation phase can be expressed as: .

[0015] Preferably, in step S6, the geometric constraint method can be found in Optics Express, 2016, 24(16):18445-18459; the calibration method of the color stripe projection system can be found in Optics and Lasers in Engineering, 2021, 143:106622.

[0016] Compared with the prior art, the advantages and positive effects of the present invention are as follows: This invention corrects color crosstalk errors from two dimensions: fringe intensity and phase distribution. First, the green channel is used to perform histogram specification processing on the red and blue channels to correct the fringe intensity. Then, the solved phase distribution is subjected to histogram equalization processing to correct the phase distribution. This eliminates the need for pre-calibrating the spectral response characteristics of the color fringe projection system and training with a large dataset, significantly reducing the complexity of the correction operation. At the same time, it effectively suppresses background light intensity fluctuations, modulation depth attenuation, and phase periodic ripple errors caused by color crosstalk. It has the advantages of high speed, high accuracy, and ease of implementation, and can well meet the requirements of high-speed and high-precision three-dimensional measurement. Attached Figure Description

[0017] Figure 1 This is a comparative schematic diagram of the colored stripes in this invention, where (a) is the original colored stripe and (b) is the colored stripe after intensity correction; Figure 2 This is a schematic diagram of the truncated phase and the absolute phase in this invention, where (a) is the original full-cycle truncated phase, (b) is the original half-cycle truncated phase, (c) is the corrected half-cycle truncated phase, (d) is the corrected full-cycle truncated phase, and (e) is the corrected absolute phase. Figure 3 This is a comparative schematic diagram of the three-dimensional morphology of the object under test in this invention, where (a) is the uncorrected three-dimensional morphology and (b) is the corrected three-dimensional morphology. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] Please see Figure 1-3 The present invention provides a technical solution: a method for correcting crosstalk error in color phase-shifted fringes, comprising the following steps: Step S1: Set up a color stripe projection system. The color stripe projection system includes a projector, a color camera and the object to be measured. There is a preset angle between the camera and the color camera. The object to be measured is placed in the common field of view of the projector and the color camera. Step S2: Project a colored stripe onto the surface of the object to be tested using a projector. The colored stripe is distorted and deformed due to the height change of the surface contour of the object to be tested. The modulated colored stripe of the object to be tested is captured by a camera. In this step, the modulated color stripes of the object under test can be represented as: In the formula These represent the red (R), green (G), and blue (B) channels of the colored stripes, respectively. Represents the pixel coordinates of the colored stripes; These represent the background intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the modulation intensities of the red (R), green (G), and blue (B) channels, respectively. This represents the truncated phase to be solved; These represent the phase shifts caused by color crosstalk in the red (R), green (G), and blue (B) channels, respectively. Step S3: Calculate the maximum and minimum values ​​of the three channels (red R, green G, and blue B) in the colored stripes one by one, and normalize the three channels (red R, green G, and blue B) in turn. In this step, the normalization expressions for the red (R), green (G), and blue (B) channels are as follows: ; In the formula, These represent the maximum light intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the minimum light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively. These are the normalized light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively. In this step, the value of the top 1% of the light intensity of each channel is used as the maximum value of the corresponding channel, and the minimum value of the top 1% of the light intensity of each channel is used as the minimum value of the corresponding channel, in order to suppress the influence of image noise. Step S4: Calculate the histogram of the green G channel, and use the histogram of the green G channel to perform histogram specification processing on the red R channel and the blue B channel to suppress the problem of stripe intensity imbalance between different channels; In this step, the histograms of the red (R) and blue (B) channels are defined using the histogram of the green (G) channel as follows: In the formula This represents the histogram statistical function; This represents the histogram specification function. These are the light intensity values ​​of the red (R) and blue (B) channels after histogram specification processing; Step S5: Calculate the full-cycle cutoff phase of the colored stripes using the three-step phase shift method, then calculate the half-cycle cutoff phase, and perform histogram equalization on the half-cycle cutoff phase to eliminate periodic crosstalk errors. In this step, the mathematical expression for calculating the integer-period cutoff phase of the colored fringes using the three-step phase-shift method is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire period, and its value ranges from 0 to 2π. In this step, the mathematical expression for calculating the half-cycle truncation phase is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire cycle, and its value ranges from 0 to π. In this step, the expression for histogram equalization of the half-cycle truncated phase is: In the formula This represents the histogram equalization function; In this step, the half-cycle truncated phase is compared with the phase after histogram equalization. The corresponding integer-cycle truncation phase can be expressed as: ; Step S6: Use the geometric constraint method to perform phase unrolling on the corrected truncated phase to recover the absolute phase of the object under test, and then convert the absolute phase into a three-dimensional point cloud based on the calibration data of the color stripe projection system. In this step, the geometric constraint method can be found in Optics Express, 2016, 24(16): 18445-18459; the calibration method of the color stripe projection system can be found in Optics and Lasers in Engineering, 2021, 143: 106622.

[0020] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for correcting crosstalk error in color phase-shifted fringes, characterized in that, Includes the following steps: Step S1: Construct a color stripe projection system, which includes a projector, a color camera, and the object to be measured. There is a preset angle between the camera and the color camera, and the object to be measured is placed within the common field of view of the projector and the color camera. Step S2: Project a colored stripe onto the surface of the object to be tested using a projector. The colored stripe is distorted and deformed due to the height change of the surface contour of the object to be tested. The modulated colored stripe of the object to be tested is captured by a camera. Step S3: Calculate the maximum and minimum values ​​of the three channels (red R, green G, and blue B) in the colored stripes one by one, and normalize the three channels (red R, green G, and blue B) in turn. Step S4: Calculate the histogram of the green G channel, and use the histogram of the green G channel to perform histogram specification processing on the red R channel and the blue B channel to suppress the problem of stripe intensity imbalance between different channels; Step S5: Calculate the full-cycle cutoff phase of the colored stripes using the three-step phase shift method, then calculate the half-cycle cutoff phase, and perform histogram equalization on the half-cycle cutoff phase to eliminate periodic crosstalk errors. Step S6: Use the geometric constraint method to perform phase unrolling on the corrected truncated phase to recover the absolute phase of the object under test, and then convert the absolute phase into a three-dimensional point cloud based on the calibration data of the color stripe projection system.

2. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S2, the color stripes modulated by the object to be tested can be represented as: In the formula These represent the red (R), green (G), and blue (B) channels of the colored stripes, respectively. Represents the pixel coordinates of the colored stripes; These represent the background intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the modulation intensities of the red (R), green (G), and blue (B) channels, respectively. This represents the truncated phase to be solved; These represent the phase shifts caused by color crosstalk in the red (R), green (G), and blue (B) channels, respectively.

3. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S3, the normalization expressions for the red (R), green (G), and blue (B) channels are as follows: ； In the formula, These represent the maximum light intensity of the red (R), green (G), and blue (B) channels, respectively. These represent the minimum light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively. These are the normalized light intensity values ​​for the red (R), green (G), and blue (B) channels, respectively.

4. The method for correcting crosstalk error in color phase-shifted fringes according to claim 3, characterized in that: In step S3, the value of the TOP1% position of the light intensity of each channel is used as the maximum value of the corresponding channel, and the minimum value of the TOP1% position of the light intensity of each channel is used as the minimum value of the corresponding channel, so as to suppress the influence of image noise.

5. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S4, the histogram specification of the red R and blue B channels using the histogram of the green G channel is expressed as follows: In the formula This represents the histogram statistical function; This represents the histogram specification function. These are the light intensity values ​​of the red (R) and blue (B) channels after histogram specification.

6. The method for correcting color phase-shifting fringe crosstalk error according to claim 1, characterized in that: In step S5, the mathematical expression for calculating the integer-period cutoff phase of the colored stripes using the three-step phase-shift method is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire period, and its value ranges from 0 to 2π.

7. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S5, the mathematical expression for calculating the half-cycle truncation phase is as follows: In the formula This represents the arctangent operation function. This represents the truncation phase of the entire period, and its value ranges from 0 to π.

8. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S5, the expression for histogram equalization of the half-cycle truncated phase is: In the formula This represents the histogram equalization function.

9. The method for correcting crosstalk error in color phase-shifted fringes according to claim 1, characterized in that: In step S5, the half-cycle truncated phase after histogram equalization is... The corresponding integer-cycle truncation phase can be expressed as: 。

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