Weijun Liu

A computing method for accurate slice contours based on an STL model

The STL model has become a standard in rapid prototyping (RP) technology, but slice errors are high because of tessellation. To improve the precision of slice contours, a novel slicing method is presented to compute accurate contour curves based on an STL model. First, the normal vector of a triangle facet to be sliced is calculated. An interpolation curve between two vertices is constructed in a normal section to approximate the original curve in the normal section in terms of the position of the two vertices and the tangent vector at the two vertices. The exact points of the slice contour are obtained by calculating the intersection points between the interpolation curve and the slice plane. Finally, accurate smooth contours are achieved by fitting the intersection points with a cubic B-spline curve. The experimental results indicate that the presented slicing method improves the precision of cross contours.

Robust Algorithm for Detecting the Maximum Inscribed Circle

In this paper, we propose a new robust algorithm for the detection of the maximum inscribed circles (MIC) in images. We first use a vector distance transformation (VDT) strategy to create a distance field. Then, we globally search the maximal value in distance field to extract the medial axes. Finally, we give a procedure to determine the center and radius of MIC. Our experimental results show indeed that new algorithm is capable of detecting the MIC with excellent accuracy and high efficiency under various image conditions.

Geometric algorithm for point projection and inversion onto Bézier surfaces

This paper presents an accurate and efficient method for the computation of both point projection and inversion onto Bézier surfaces. First, these two problems are formulated in terms of solution of a polynomial equation with u and v variables expressed in the Bernstein basis. Then, based on subdivision of the Bézier surface and the recursive quadtree decomposition, a novel solution method is proposed. The computation of point projection is shown to be equivalent to the geometrically intuitive intersection of asurface with the u-ν plane. Finally, by comparing the distances between the test point and the candidate points, the closest point is found. Examples illustrate the feasibility of this method.

Fully automatic matching of circular markers for camera calibration

Camera calibration is one of critical steps in computer vision, also an exhaustive process because substantial human computer interactions are frequently required to deal with the matching problem. In this paper, an automatic matching method of markers for camera calibration is presented based on the local architecture characteristics of a new planar circle pattern, which can be applied to a wide range of spoiled images including those of distortion, noise, and blur. Firstly, a robust ellipse detector is developed to extract ellipses from the image. After the centroids of the ellipses are triangulated into triangle network by Delaunay method to find k annular neighborhood of each ellipse, a cost function is defined to locate orientation ellipses for the homography matrix between the image and the calibration pattern. Then the correspondences between the marker ellipses and the marker circles are established by a homography method followed by a point sets registration strategy. Finally, the method is tested with real and transformed images to show its accuracy and robustness, which is entirely unsupervised and easily embedded into the exiting algorithms.

Accurate and Efficient Algorithm for the Closest Point on a Parametric Curve

This paper presents an accurate and efficient method for computation of the closest point on parametric curves. This problem is firstly formulated in terms of solution of a polynomial equation expressed in Bernstein basis, and then based on subdivision relying on the convex hull property of Bezier curve and the recursive bintree decomposition on the parameter domain, a novel solution method is proposed. The computation of closest point is shown to be equivalent to the geometrically intuitive intersection of a curve with the parameter line. Finally, by comparing the distances between the test point and the candidate points, the closest point is found. An example illustrates the feasibility of this method.

Research on Measuring and Control System of Metal Powder Laser Shaping

The metal powder laser shaping (MPLS) has been widely used for manufacturing functional parts in recent years. In the MPLS process, a lot of process variations affect the quality and precision of the manufactured parts. The temperature field and cladding height are two key parameters that reflect the manufacturing conditions. In the temperature field measuring system, the infrared images of the molten pool are taken by a CCD camera and the image data are analyzed by the software design. The colorimetric method is applied to the temperature measurement. Image processing of the molten pool makes the temperature distribution seen clearly and extracts the characteristic parameters for feedback control system. The MPLS process builds metal parts in a layer-by-layer accumulating fashion. So it is important to manufacture every layer with a uniform height. The methods for measuring cladding height are presented. The powder flow rate and scanning velocity are effectively used as controlled parameters in the control system. Experimental results validate the good performance of the measuring and control system

Efficient Tool Path Generation for √3 Subdivision Surface

An approach of three-axis NC cutter path generation for √3 subdivision surface is proposed. A computing formula for error estimating √3 subdivision surface is derived. Using the formula, one can predict how many steps of subdivision are necessary to meet a user-specified tolerance without subdivision the meshes actually. Tool path planning is generated automatically based on cutter location surface. A collision detection and avoidance is developed for the interfere-free. The generated tool paths comply with a certain tolerance and surface finish specified on the surface. We implement our approach and present results. Our approach reduces the computational complexity and is robust efficiently

Automatic extrinsic calibration of display-camera system using an annular mirror

Our work focuses on the extrinsic calibration of a display-camera system where the camera has no direct view of the display. An annular planar mirror is used to reflect the display so that the camera can capture the reflection images. The position of the mirror can be obtained after outer circle is detected. The position parameters are determined uniquely by putting two orthogonal lines in the background and optimized by re-projecting the inner circle to the image plane. The display pixels are encoded using gray code and its imaged position in the mirror can be obtained by solving the PnP problem. Finally, the real extrinsic parameters between the camera and the display are obtained according to the mirror imaging principle. Our approach is simple and fully automatic without any manual intervention. Both simulation and real experiments validate our approach.

Facet normal vector based adaptive slicing algorithm

Layer milling with uniform layer thickness is one of the main approaches for complex surfaces roughing and semi-finishing. But uneven thick material is left near the horizontal plane in some milling experiments. And the existed slicing methods can not properly solve this problem. A new slicing algorithm based on triangular facet normal vector and facet group area threshold is given. In the new algorithm, the facet normal vector is used to locate the candidate facets near which the possible uncut regions lie. Then the candidate facets are grouped with their height values and each group area is compared with a threshold. The larger is chosen as the interested groups and their height values are recorded. At last the slicing positions are determined with the height records and the given layer thickness and computation is committed. Experiments show that the new algorithm can solve the previous problem.

Research on Algorithm of Optimized Sub-regional Scanning Path Generation for RPM

Rapid prototyping is a technology of layered manufacturing; therefore, the scanning path of certain layer is quite important. An algorithm of sub-regional scanning path generation is presented in the paper. The algorithm divides complex sectional contours into some simple non-hole sub-region, the scanning path in every sub-region is continuous and the sub-regions are scanned in turn. At the same while, the scanning path is optimized in two ways. The first one is that the joining path between the regions is optimized, which is effective on decreasing the vacant path. The second one is that the scanning path out of place at extreme points in a sub-region is optimized. The experiment shows the proposed algorithm not only improve the machine equality but also advancing the machine efficiency.