Sara McMains

CyberCut: an internet-based CAD/CAM system

“CyberCut TM ” is a testbed for an Internet-based CAD/CAM system. It was specifically designed to be a networked, automated system, with a seamless communication flow from a client-side designer to a server-side machining service. The creation of CyberCut required several new software modules. These include: a) a Web-based design tool in which Design-forManufacturing information and machining rules constrain the designer to manufacturable parts; b) a geometric representation called SIF-DSG, for unambiguous communication between the client-side designer and the server-side process planner; c) an automated process planning system with several sub-modules that convert an incoming design to a set of tool-paths for execution on a 3-axis CNC milling machine. Using this software-pipeline, a CyberCut service, modeled on the MOSIS service for VLSI chips, has been now been launched for limited studentuse at a group of cooperating universities.

Evaluation of drawing on 3D surfaces with haptics

We describe experiments for evaluating the speed and accuracy of a haptic drawing interface, particularly in terms of the effects of changing two haptic characteristics the magnitude of the friction force and the use of smooth, interpolated-normal force shading in combination with the effect of stereo displays and shadows. Because surface geometry affects the sense of touch, we studied the efficacy of the haptic interface in the context of varying geometric characteristics of the surface drawing task. We also found that stereo displays, in combination with haptics, can provide additional benefits for drawing in terms of reducing error and increasing speed for the drawing task. So the decision to introduce shadows in a VR system should be considered in the context of the expected task mix.

Optimized GPU evaluation of arbitrary degree NURBS curves and surfaces

This paper presents a new unified and optimized method for evaluating and displaying trimmed NURBS surfaces using the Graphics Processing Unit (GPU). Trimmed NURBS surfaces, the de facto standard in commercial mechanical CAD modeling packages, are currently being tessellated into triangles before being sent to the graphics card for display since there is no native hardware support for NURBS. Other GPU-based NURBS evaluation and display methods either approximated the NURBS patches with lower degree patches or relied on specific hard-coded programs for evaluating NURBS surfaces of different degrees. Our method uses a unified GPU fragment program to evaluate the surface point coordinates of any arbitrary degree NURBS patch directly, from the control points and knot vectors stored as textures in graphics memory. This evaluated surface is trimmed during display using a dynamically generated trim-texture calculated via alpha blending. The display also incorporates dynamic Level of Detail (LOD) for real-time interaction at different resolutions of the NURBS surfaces. Different data representations and access patterns are compared for efficiency and the optimized evaluation method is chosen. Our GPU evaluation and rendering speeds are more than 40 times faster than evaluation using the CPU.

Direct evaluation of NURBS curves and surfaces on the GPU

This paper presents a new method to evaluate and display trimmed NURBS surfaces using the Graphics Processing Unit (GPU). Trimmed NURBS surfaces, the de facto standard in commercial 3D CAD modeling packages, are currently tessellated into triangles before being sent to the graphics card for display since there is no native hardware support for NURBS. Previous GPU-based NURBS display methods relied on first approximating the NURBS patches with lower degree Bezier patches before evaluation. Our method uses a GPU fragment program to evaluate the surface point coordinates of the original NURBS patch directly, from the control points and knot vectors stored as textures in graphics memory. This evaluated surface is trimmed during display using a dynamically generated trim-texture calculated via alpha blending. The implementation incorporates dynamic Level of Detail (LOD) for real-time interaction at different resolutions of the NURBS surfaces. We obtain rendering speeds at least one order of magnitude faster than evaluation using the CPU.

A coherent sweep plane slicer for layered manufacturing

We describe the design and implementation of a coherent sweep plane slicer, built on top of a topological data structure, which “slices” a tessellated 3-D CAD model into horizontal, 2.5-D layers of uniform thickness for input to layered manufacturing processes. Previous algorithms for slicing a 3-D b-rep into the layers that form the process plan for these machines have treated each slice operation as an individual intersection with a plane, which is needlessly inefficient given the significant coherence between the finely spaced slices. An additional shortcoming of many existing slicers that we address is a lack of robustness when dealing with non-manifold geometry. Our algorithm exploits both geometric and topological inter-slice coherence to output clean slices with explicit nesting of contours.

POLYGON OFFSETTING BY COMPUTING WINDING NUMBERS

In this paper we present a simple new algorithm to offset multiple, non-overlapping polygons with arbitrary holes that makes use of winding numbers. Our algorithm constructs an intermediate “raw offset curve” as input to the tessellator routines in the OpenGL Utility library (GLU), which calculates the winding number for each connected region. By construction, the invalid loops of our raw offset curve bound areas with non-positive winding numbers and thus can be removed by using the positive winding rule implemented in the GLU tessellator. The proposed algorithm takes O((n + k)logn) time and O(n + k) space, where n is the number of vertices in the input polygon and k is the number of self-intersections in the raw offset curve. The implementation is extremely simple and reliably produces correct and logically consistent results.Copyright

A GPU-based voxelization approach to 3D Minkowski sum computation

We present a new approach for computing the voxelized Minkowski sum of two polyhedral objects using programmable Graphics Processing Units (GPUs). We first cull out surface primitives that will not contribute to the final boundary of the Minkowski sum. The remaining surface primitives are then rendered to depth textures along six orthogonal directions to generate an initial solid voxelization of the Minkowski sum. Finally we employ fast flood fill to find all the outside voxels. We generate both solid and surface voxelizations of Minkowski sums without holes and support high volumetric resolution of 10243 with low video memory cost. The whole algorithm runs on the GPU and is at least one order of magnitude faster than existing boundary representation (B-rep) based algorithms for computing Minkowski sums of objects with curved surfaces at similar accuracy. It avoids complex 3D Boolean operations and is easy to implement. The voxelized Minkowski sums can be used in a variety of applications including motion planning and penetration depth computation.

Performing Efficient NURBS Modeling Operations on the GPU

We present algorithms for evaluating and performing modeling operations on NURBS surfaces using the programmable fragment processor on the Graphics Processing Unit (GPU). We extend our GPU-based NURBS evaluator that evaluates NURBS surfaces to compute exact normals for either standard or rational B-spline surfaces for use in rendering and geometric modeling. We build on these calculations in our new GPU algorithms to perform standard modeling operations such as inverse evaluations, ray intersections, and surface-surface intersections on the GPU. Our modeling algorithms run in real time, enabling the user to sketch on the actual surface to create new features. In addition, the designer can edit the surface by interactively trimming it without the need for retessellation. Our GPU-accelerated algorithm to perform surface-surface intersection operations with NURBS surfaces can output intersection curves in the model space as well as in the parametric spaces of both the intersecting surfaces at interactive rates. We also extend our surface-surface intersection algorithm to evaluate self-intersections in NURBS surfaces.

THE EVOLUTION OF A LAYERED MANUFACTURING INTERCHANGE FORMAT

Over the last several years we have developed the Berkeley Solid Interchange Format (SIF) for layered manufacturing data exchange. By building both design software that outputs SIF as well as manufacturing software that processes the SIF input files, we gained insights into the concerns of both sides of data exchange ‐ insights which often led to major changes in successive versions of the format. In this paper, we share some of the most important lessons we learned (many of which are applicable to all geometric data exchange, not merely for layered manufacturing) and explain how they shaped SIF.

Computing the Hausdorff distance between NURBS surfaces using numerical iteration on the GPU

We present a GPU algorithm for computing the directed Hausdorff distance between two NURBS surfaces. The algorithm is based on sampling of one surface, and performing numerical iterations on the GPU to compute the minimal distance from each sample to the other surface. An error analysis for the Hausdorff distance computations is performed, based on bounds on the NURBS surfaces. We compare a CUDA implementation of our algorithm to existing methods, demonstrating that the new method addresses limitations of previous hierarchical culling methods such as the sensitivity to the position of the inputs.