Special issue-Optical 3D imaging and sensing
For achieving automatic, high-precision, high-density, and photorealistic 3D imaging and measurement of large-scale complex objects, the techniques about multi-node 3D sensor measurement network were described based on 3D sensing with the fringe structured light. It mainly involved the analysis of two key technologies (fringe analysis and phase reconstruction, system calibration and 3D reconstruction) of single 3D sensing, construction and optimization of multi-node 3D sensor measurement network, calibration of multi-node 3D sensor measurement network, matching and fusion of 3D depth data and texture data. Some experimental prototypes and experimental results were given.
Optical three-dimensional (3D) measurement technology is widely used in different fields due to its advantages of non-contact, non-destructive and rapid measurement. The existing technologies of Fringe Projection Measurement and Fringe Reflection Measurement are designed for the measurement of diffusely reflective and specularly reflective surfaces respectively. However, there are many complex surfaces which have both diffused and specular reflective surfaces together in aerospace and advanced manufacturing. In this paper, a method based on structured light projection and reflection was proposed to realize rapid measurement of complex surfaces. First, a Digital Light Projector (DLP) projected blue sinusoidal fringe onto the tested surface, and red fringes displayed by a screen were reflected by its specular part simultaneously. Second, deformed fringe patterns modulated by the measured surface were captured by a color Charge Coupled Device (CCD) camera. Then, deformed fringes of different reflection surface were extracted from different color channels of the camera and then absolute phase information could be calculated. Finally, after system calibration to build up the relationship between phase and depth, 3D shape data of the measured complex object was obtained. Experimental results show that the proposed method can not only measure complex surfaces effectively, but obtain 3D shape of isolated diffused and specular surfaces simultaneously.
A method for three-dimensional (3D) reconstruction from four-dimensional (4D) light fields was presented. The 4D light field image recorded the direction and intensity of all rays passing through the scene and contained useful information to estimate scene depth. Point 3D coordinates were obtained by calculating relative positional relationships between rays emitted from one point in the scene. However, it was very difficult in practice to determine these light rays from 4D light field data. The proposed method used fringe projection to mark object surfaces. Light ray information can then be accurately and quickly determined from the phase marker and 3D data calculated. The 4D light field matrix was light ray phase rather of intensity as in the conventional method and can record rays with various directions. Thus, shadow, occlusion, and surface specular reflection problems can be addressed. Feasibility and accuracy of the proposed method were verified experimentally.
In a Computational Ghost Imaging (CGI) system, the axial depth of the target can be obtained by estimating the degree of blur of the reconstructed image. However, this method is easy to be affected by background noise and requires a long working distance for the image quality evaluation function, so this method needs more samplings and the practicability is reduced. To solve this problem, a target depth estimated algorithm with adapted focusing window was proposed. Firstly the local search interval was divided according to the global characteristics of the evaluation function, and then the actual axial depth of the target was searched iteratively in a given region. In iterations, the use of adaptive window decreased the area of background and contained the whole target. Experiments show that the proposed method greatly reduces the necessary working distance, increases the robustness of this method, reduces the effect of background noise on the evaluation function, and achieves the depth of target under low samplings. This work promotes the development of depth estimation method based on computational ghost imaging system.
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