Researches On Three-Dimensional Structure Reconstruction And Pose Measurements Based On Line Feature For Space Targets

In this dissertation, the methods and algorithms based on line feature are studied, by using which 3D structure of non-cooperative targets is recovered and relative pose between separated cooperative targets and the space platform is measured. The background of the works is to observe space targets on the space platform with the images of targets and scenes captured by cameras.It's useful to calibrate camera parameters on-line for the camera parameters tend to be disturbed by the unpredictable factors in the harsh environment. In the meantime, there are plenty of line features in the most of the space targets, so the main contents of the works in the dissertation are done as follows by using the characteristics of line sufficiently.1 There is little 3D information in non-cooperative targets so that 3D structure could not be reconstructed with the methods of cooperative targets simply.The algorithms used to measure the cooperative targets can not be simply used to measure the non-cooperative targets for they can only provide limited features.There are correlations of parallel, vertical and intersection between the straight lines, so the geometric model of camera is described with the central perspective equations based on the straight line feature and geometric constraint of the straight lines in the scene is used. The methods of camera self-calibration and 3D structure reconstruction from non-cooperative targets images, including single image, image pairs and sequential images, based on line feature are introduced in detail. This is the emphasis of the article.(1) An algorithm is proposed from uncalibrated single image to calibrate the camera inside parameters, based on the projective invariance that parallel lines should be intersect at one point and the similarity invariance that the vertical lines remain vertical. Then, the method to reconstruct the plane is introduced when the inside parameters of camera is calibrated, based on the affine invariance that parallel lines remain parallel and the similarity invariance that the circular points are fixed points.The effect about the choice of the camera parameters on the precision of vanishing point is analyzed quantitatively and how to choose the location and the angle of camera parameters reasonably to get the vanishing point of high precision is discussed. A method to correct the fish-eye aberration distortion, which was based on the projective invariance that straight line remains straight, was introduced to the situation of aberration distortion being deteriorated in close-range measuring.(2) An algorithm is put forward from uncalibrated image pairs to calibrate the parameters of camera and reconstruct the 3D structure, based on the projective invariance that cross-ratio should be constant and epipolar line constraint.(3) An algorithm is presented from uncalibrated image sequence to calibrate the parameters of camera and reconstruct the 3D structure, based on the similarity invariance that vertical lines remain vertical and bundle adjustment.In order to study non-cooperative targets, the simulation experiments are conducted. The algorithm of image pairs is applied when the space platform and the non-cooperative target are generally approaching. The algorithm of image sequence is used when the space platform flies around the non-cooperative target.2 The relative pose measurement methods using the system of fixed main camera and accessorial camera are applied.The images of the space separating targets and the reference calibrated target on the space platform are captured in real-time by the main camera and accessorial camera respectively. Relative pose between the cooperative targets and the space platform is computed provided that the lines of the images captured by the cameras are extracted and the location of 3D lines in cooperative target is known accurately.3 As steps of image pre-processing, the straight line feature extraction and the match algorithm are studied.(1) A two-step algorithm of the straight line extraction based on the Hough transform with the gradient's maximum direction and 1D Gaussian templet fitting is proposed. It not only reasonably uses the existed classical algorithms, but also improves the accuracy of the straight line extraction.(2) A straight-line-segment match method based on the epipolar line constraint and RANSAC algorithm is applied with regard to the demand of the 3D reconstruction, which effectively solved the matching problem of sheltering straight line segment in the image.The algorithms presented in this dissertation are all based on line feature. They have effectively solved the problems of camera self-calibration, space targets' 3D structure and pose measurements. Their precision and efficiency are verified by a lot of digital and ground synthetic experiments.