Projector Calibration Method Research Articles

Efficient and accurate projector calibration method based on phase-shifting method with sinusoidal structured light

ObjectiveProjector calibration is one of the most basic and important steps in structured light three-dimensional measurement. However, such as complicated calibration process and low accuracy are still the inevitable problems in the calibration process. In this paper, a projector calibration method based on phase-shifting method with sinusoidal structured light is proposed to improve the accuracy of the calibration and simplify the operation of the calibration. MethodsFirstly, projecting a set of sinusoidal fringes onto a black-and-white circular calibration board, and collecting the images synchronously by a CCD camera; Secondly, establishing the correspondence between camera pixels and projector pixels by using the consistency of phase information which means that the absolute phase value of an arbitrary point on the calibration board is equal to the absolute phase value of this point in the camera image and virtual projector image; And then a set of virtual DMD images on the projector can be obtained by using the principle above; Finally, using this set of virtual DMD images, the parameters of the projector can be calibrated with camera calibration method based on circular calibration board. ResultsThe experimental results show that the maximum reprojection error of the projector calibrated by this method is 0.0419 pixels, and the average reprojection error is 0.0343 pixels. The equipment involved in the calibration process is simple and the experimental operation is easy. The experimental results indicated that this method has high calibration accuracy and efficiency.

An Improved Projector Calibration Method by Phase Mapping Based on Fringe Projection Profilometry.

Aiming at the problem of the low accuracy of projector calibration in a structured light system, an improved projector calibration method is proposed in this paper. One of the key ideas is to estimate the sub-pixel coordinates in the projector image plane using local random sample consensus (RANSAC). A bundle adjustment (BA) algorithm is adopted to optimize the calibration parameters to further improve the accuracy and robustness of the projector calibration. After system calibration and epipolar rectification, the mapping relationship between the pixel coordinates and the absolute phase in the projector image plane is established by using cubic polynomial fitting, and the disparity is rapidly solved by using the mapping relationship, which not only ensures the measurement accuracy, but also improves the measurement efficiency. The experimental results demonstrated that the average re-projection error after optimization is reduced to 0.03 pixels, and the proposed method is suitable for high-speed 3D reconstruction without the time-consuming homogenous point searching.

Accurate Local Homography-Based Projector Calibration Method

Accurate Local Homography-Based Projector Calibration Method

An improved projector calibration method for structured-light 3D measurement systems

In a structured-light three-dimensional measurement system, understanding the optical configuration of the projector and suppressing the eccentricity error caused by the camera perspective projection are critical to realize high-precision measurement. In this paper, we analyze the special offset optical structure in commercial projectors, where a larger diameter lens is used to ensure the quality of the projected image, and the position of the principal point has been shifted. Meanwhile, a projector calibration strategy that makes the camera’s optical axis perpendicular to the target plane is proposed to avoid the pollution of perspective projection distortion. The sub-pixel correspondence based on homography transformation is performed by relying on the phase value of the fringe, and bundle adjustment optimization is used to further improve the accuracy and robustness of projector calibration. The experimental results demonstrate that the proposed calibration method can improve the calibration accuracy by 52.52%.

High-accuracy projector calibration method for fringe projection profilometry considering perspective transformation.

Camera and projector are the key components of structured light three-dimensional (3-D) measurements, and Digital Light Processing (DLP) projector has been widely used for projecting digital structured light patterns for the measurement. The light projecting of projectors can be modeled as the inverse procedures of camera imaging, and its high-accuracy calibration is still a remaining challenge. Therefore, this paper proposes a novel projector calibration method to improve the calibration accuracy of DLP projector. By fixing the position of the camera and calibration board, this method essentially eliminates the perspective transformation error and effectively avoids the distortion of the extracted marker points. The proposed projector calibration procedures are given as follows: Firstly, the optical axis of the camera is adjusted parallel to the normal of the hollow ring calibration board, and a texture image is captured by the camera; Secondly, the horizontal and vertical fringe patterns with nine different positions and directions are projected onto the calibration board, and nine sets of projected images are taken; Finally, a one-to-one correspondence between the camera and the projector is established, and the projector is accurately calibrated using the phase equivalence. The experimental results show that the proposed projector calibration method is feasible and easy to operate, which can essentially eliminate the perspective transformation error and ensure the competitive accuracy.

Calibration Method for Trinocular Stereovision System Comprising Projector and Dual Cameras

Calibration Method for Trinocular Stereovision System Comprising Projector and Dual Cameras

Design of Authoring Tool for Static and Dynamic Projection Mapping

This study introduces the design details of a tool to create interactive projection-mapping content in a convenient manner. For the proposed tool design, a homography-based camera–projector calibration method was applied with the use of red–green–blue-depth images from a Kinect V2 sensor that did not require accurate camera calibration prerequisites. In addition, the proposed tool simultaneously achieved static projection mapping that projected the image content onto a fixed object, and dynamic projection mapping that projected the image content onto a user’s body, by tracing the moving user. To verify the effectiveness of the proposed content-creation tool, users with no programming capabilities were employed to create contents that were projected onto various objects in fixed positions and a user’s body in various poses, thereby analyzing the tool’s completeness. Moreover, the projection accuracy was analyzed at different depth positions, and the projection-mapping accuracy was verified with the use of the proposed method.

Portable System for Box Volume Measurement Based on Line-Structured Light Vision and Deep Learning.

Portable box volume measurement has always been a popular issue in the intelligent logistic industry. This work presents a portable system for box volume measurement that is based on line-structured light vision and deep learning. This system consists of a novel 2 × 2 laser line grid projector, a sensor, and software modules, with which only two laser-modulated images of boxes are required for volume measurement. For laser-modulated images, a novel end-to-end deep learning model is proposed by using an improved holistically nested edge detection network to extract edges. Furthermore, an automatic one-step calibration method for the line-structured light projector is designed for fast calibration. The experimental results show that the measuring range of our proposed system is 100–1800 mm, with errors less than ±5.0 mm. Theoretical analysis indicates that within the measuring range of the system, the measurement uncertainty of the measuring device is ±0.52 mm to ±4.0 mm, which is consistent with the experimental results. The device size is 140 mm × 35 mm × 35 mm and the weight is 110 g, thus the system is suitable for portable automatic box volume measurement.

An efficient projector calibration method for projecting virtual reality on cylindrical surfaces

In this paper, we propose a fast and easy-to-use projector calibration method needing a minimum set of input data, thus reducing the calibration time. The method is based on the Direct Linear Transformation (DLT) mathematical model, which makes it simple and fully automatic. We show the application of this method on cylindrical surfaces, as well as some real application examples. The results show that with the minimum configuration of 6 control points (CPs), the standard deviation in the projector positioning yielded by the calibration process is less than one per cent of the position values.

Out-of-Focus Projector Calibration Method with Distortion Correction on the Projection Plane in the Structured Light Three-Dimensional Measurement System

The three-dimensional measurement system with a binary defocusing technique is widely applied in diverse fields. The measurement accuracy is mainly determined by out-of-focus projector calibration accuracy. In this paper, a high-precision out-of-focus projector calibration method that is based on distortion correction on the projection plane and nonlinear optimization algorithm is proposed. To this end, the paper experimentally presents the principle that the projector has noticeable distortions outside its focus plane. In terms of this principle, the proposed method uses a high-order radial and tangential lens distortion representation on the projection plane to correct the calibration residuals caused by projection distortion. The final accuracy parameters of out-of-focus projector were obtained using a nonlinear optimization algorithm with good initial values, which were provided by coarsely calibrating the parameters of the out-of-focus projector on the focal and projection planes. Finally, the experimental results demonstrated that the proposed method can accuracy calibrate an out-of-focus projector, regardless of the amount of defocusing.

Projector calibration method based on stereo vision system

Digital projectors have been widely used in many accuracy-sensitive fields and the projector should be calibrated precisely. Different from the existing methods using a single camera and a high-accuracy diffuse planar target, the projector calibration method is proposed based on a stereo vision system and a white board. A calibration pattern with several virtual mark points is projected onto the white board at different poses and captured by the stereo vision system. A two-step optimization algorithm is proposed to calculate the intrinsic parameters with roughly coplanar points. The white board has no mark points on it and there is no need to guarantee its flatness, so it avoids using the expensive and fragile diffuse target. Finally, the experimental results have demonstrated the improvement in accuracy of the proposed method.

Sub-pixel projector calibration method for fringe projection profilometry.

Digital projectors are used as standard parts at present in fringe projection profilometry systems to project structured-light patterns onto the object surface to be measured, and the distortion of the projector lens must be calibrated and compensated accurately to satisfy the accuracy requirement of industrial applications. A novel method is proposed to determine the projector pixel coordinates of the marker points of a calibration target accurately in terms of projective transform. With the method, the projector can be calibrated with accuracy of sub-pixel level. The method is applicable for the calibration target with a chessboard pattern or a circle pattern, and the calibration result is independent on the results of camera calibration. Experimental results are shown to demonstrate the effectiveness and validity of the proposed method.

单周期条纹双四步相移投影仪的标定方法

单周期条纹双四步相移投影仪的标定方法

Novel projector calibration method for monocular structured light system based on digital image correlation

Monocular structured light system, which consists of one camera and one digital light processing (DLP) projector, is a simple and effective tool to acquire accurate three-dimensional (3D) shapes. But such high precision is only possible if the projector is accurately calibrated. Many of the existing projector calibration methods have complicated procedures or low calibration precision. In this paper, an accurate speckle-based feature points matching method is proposed to estimate the image coordinates of feature points in projection chip (digital micro-mirror device or DMD) plane. The method relies on an uncalibrated camera and makes use of affine transformation theory and two-dimensional digital image correlation (2D-DIC) method to reach sub-pixel matching precision. Considering the skew of the projection chip, a full-parameter calibration model is presented to describe the projector. A novel system is developed and series of validation tests were carried out to demonstrate the performance of the proposed method.

Flexible digital projector calibration method based on per-pixel distortion measurement and correction

When a digital projector is applied in high precision applications, the intrinsic parameters and distortion characteristics should be calibrated precisely. In this paper, a flexible full-field projector calibration method is proposed without any approximate distortion model. With planar homography theory and fringe projection technique, the projector distortion characteristic on each pixel can be measured independently and an initial distortion map is generated. The intrinsic parameters are calibrated afterwards. Then, the initial distortion map can be refined by correcting the non-perpendicularity between the optical axis and image plane. The original pattern to be projected is corrected with the refined distortion map. Thus, the calibrated projector can be regarded as an ideal projector conforming to the pinhole model. Experimental results show a nearly ideal residual map for the corrected projection pattern. In addition, the proposed calibration method is flexible without any sophisticated ancillary equipment or complicated procedure.

Accurate calibration method for camera and projector in fringe patterns measurement system.

The 3D measurement system based on fringe patterns is widely applied in diverse fields. The measurement accuracy is mainly determined by camera and projector calibration accuracy. In the existing methods, the system is calibrated by a dot calibration board with traditional image process algorithms. In this paper, an improved calibration method is proposed to increase camera and projector calibration accuracy simultaneously. To this end, first, a subpixel edge detection method is proposed to improve the detection accuracy of reference features for coarse calibration; second, an iterative compensation algorithm is developed to improve the detection accuracy of the reference feature centers for fine calibration. The experimental results demonstrate that the proposed calibration method can improve the calibration accuracy and measurement accuracy.

An accurate projector calibration method based on polynomial distortion representation.

In structure light measurement systems or 3D printing systems, the errors caused by optical distortion of a digital projector always affect the precision performance and cannot be ignored. Existing methods to calibrate the projection distortion rely on calibration plate and photogrammetry, so the calibration performance is largely affected by the quality of the plate and the imaging system. This paper proposes a new projector calibration approach that makes use of photodiodes to directly detect the light emitted from a digital projector. By analyzing the output sequence of the photoelectric module, the pixel coordinates can be accurately obtained by the curve fitting method. A polynomial distortion representation is employed to reduce the residuals of the traditional distortion representation model. Experimental results and performance evaluation show that the proposed calibration method is able to avoid most of the disadvantages in traditional methods and achieves a higher accuracy. This proposed method is also practically applicable to evaluate the geometric optical performance of other optical projection system.

Accurate projector calibration method by using an optical coaxial camera.

Digital light processing (DLP) projectors have been widely utilized to project digital structured-light patterns in 3D imaging systems. In order to obtain accurate 3D shape data, it is important to calibrate DLP projectors to obtain the internal parameters. The existing projector calibration methods have complicated procedures or low accuracy of the obtained parameters. This paper presents a novel method to accurately calibrate a DLP projector by using an optical coaxial camera. The optical coaxial geometry is realized by a plate beam splitter, so the DLP projector can be treated as a true inverse camera. A plate having discrete markers on the surface is used to calibrate the projector. The corresponding projector pixel coordinate of each marker on the plate is determined by projecting vertical and horizontal sinusoidal fringe patterns on the plate surface and calculating the absolute phase. The internal parameters of the DLP projector are obtained by the corresponding point pair between the projector pixel coordinate and the world coordinate of discrete markers. Experimental results show that the proposed method can accurately calibrate the internal parameters of a DLP projector.

Projector Calibration Method using Orthogonal Gray Code Combined with Trapezoid Phase Shift Code

In coded-structured light three dimensional measurement system, system calibration plays an important role for the measurement accuracy. Projector calibration is an important part of the system calibration. Therefore, this paper proposes a projector calibration method with simple calibration process and high accuracy. This method combines Zhang’s plane model calibration method with orthogonal Gray code + trapezoid phase shift code. During the calibration process, this paper uses Gray code pattern and trapezoid phase shift code pattern to establish the relation of projector image and camera point coordinates, the corner extraction accuracy of this paper’s method is higher than the traditional orthogonal Gray code. According to the image coordinates in the projector’s perspective, we program and calculate the projector’s internal and external parameters matrix based on the Zhang’s plane model calibration toolbox. The results show that this paper’s method is simple and flexible, the maximum relative error of the calibration parameters is 0.027%, and it meets the requirements of system calibration in engineering or industrial fields.

Coded-Structured Light System Calibration Using Orthogonal Phase Shift Coding Combined with Zhang’s Method

In coded-structured light three dimensional system, system calibration plays a vital role for the measurement accuracy. The camera calibration method is very mature, but the study about projector calibration is less. Therefore, this paper proposes a projector calibration method with simple calibration process and high accuracy. This method combines the Zhang’?s plane model calibration method with orthogonal phase shift coding. In calibration process, this paper uses phase shift coding pattern to establish the relationship of projector image and camera corner point coordinates. According to the image coordinates in the projector’?s perspective, we program and calculate the projector’?s internal and external parameters matrix based on the Zhang’?s plane model calibration toolbox. The results show that the proposed method is simple and flexible, the maximum relative error of the calibration parameters is 0.03%, and it meets the requirements of system calibration in medical or industrial fields.