CAXA-ME软件中曲面造型和实体造型的融合

CAXA－ME软件存在曲面造型和Parasolid实体造型两个平台,在分析两者曲面造型模块的基础上,介绍CAXA－ME内部在这两个造型之间进行类型转换的方法,并给出主要的转换算法和接口函数,实例显示数据转换方法具有满意的效果。     

基于几何法表示的曲面实体的分割算法

分割是实体造型的重要步骤,而边界问题是影响分割算法效率和稳定性的主要因素。工程中常用的二次曲面有自封闭的特性,在相交时交线会出现退化和自交等情况,给拓扑表示和帝体重建带来困难,另外,边界重合也是实体分割时经常要遇到的问题,该文在二次曲面几何法表示的基础上,针对二次曲面相交时交线的特性,设计了合理的分割策略,提出了有效的分割算法,并对边界重合等问题做了很好的处理,同时用实例验证了本文算法的有效性。     

曲面造型中某些关键算法的实现方法

在作者建立的基于ＮＵＲＢＳ的曲面造型软件包中，对于曲面求交、消隐及光照等关键算法的实现进行了探索。给出了曲面分割求交算法和基于目标空间和图象空间的曲面线消隐算法，利用扫描线算法绘出光滑逼真的曲面光照图。这些算法实现稳定可靠，具有一定的通用性。     

建立在数据库上的实体造型,曲面造型系统—HYBRID

HYBRID是一个建立在数据库上的实体造型及曲面造型系统,该系统是在北京航天航空大学的PANDA实体造型系统的基础上开发的。HYBRID以集合运算(交、并、差)为工具,由简单物体——基本体素及二维图形,形成复杂物体。系统采用了CSG、B-rep数据结构充分地表示了体的几何信息及拓扑信息。数据库的使用使系统存储量增大,管理统一、方便,图形及非图形信息容易统一处理。曲面造型采用了非均匀有理B样条曲面插值,通过路径、截面的定义形成曲面。HYBRID适用于作为机械、建筑计算机辅助设计的支撑工具及管道的曲面设计。     

复杂空间曲面零件实体造型研究

将机械零件加工方法引入ＡｕｔｏＣＡＤ实体造型中,模仿机械加工方法,用ＡｕｔｏＬＩＳＰ编写ＡｕｔｏＣＡＤ实体造型驱动程序,实现了空间圆柱凸轮、弧面分离凸轮,蜗形凸等复杂空间曲 体图的状速,准确绘制。     

浅谈高级曲面实体造型的实现方法

随着CAD/CAM技术的不断发展,产品造型越来越趋于个性化,复杂曲面造型的出现在所难免。笔者所在公司专业生产各类耳机,使用Pro/ENGINEER、UG等软件进行三维设计已有多年。图1所示为耳机导线上用于放置驻极体传声器的话盒盖-话盒座零件装配在一起时的效果图,图2为话盒盖-话盒座零件装配后产生的爆炸效果图。     

基于NURBS曲线曲面造型理论的产品外形系统设计

为了设计一个适用于产品外形设计的系统,文中重点对曲线曲面造型模块进行了研究,在NURBS曲线曲面造型模块中融入了实体造型的功能,设计了便于扩充和维护的曲线曲面类层次结构,并讨论了曲面造型模块的应用流程,通过与数据导入导出功能的衔接实现了与其它造型软件之间的数据共享,提高了系统造型效率,缩短了产品设计周期。在内核的显示模块设计中,提出了一个包含两个显示链表的显示类结构,用以加快图形对象的显示速度。     

基于网格划分的曲面求交算法

本文综合网格逼近法和追踪法求交的优点，提出了一种曲面求交算法。该算法首先对曲面进行三角形网格划分，再用追踪法求各网格之间的交线，最后拟合成三次参数曲线。     

裁剪后二次曲面的光线跟踪

提出了一个对裁剪后二次曲面进行光线跟踪的新方法。在该方法中引进了一个易于由世界坐标求参数的辅助参数空间，通过预处理，事先将原参数域下的边界变换到新的参数空间。这样，光线与曲面的求交仍可按二次曲面的几何参数进行，求出交点后，再将交点变换到辅助参数空间，在新的参数域中判断交点是否在裁剪过的曲面上。     

Solid reconstruction using recognition of quadric surfaces from orthographic views

The reconstruction of 3D objects from 2D orthographic views is crucial for maintaining and further developing existing product designs. A B-rep oriented method for reconstructing curved objects from three orthographic views is presented by employing a hybrid wire-frame in place of an intermediate wire-frame. The Link-Relation Graph (LRG) is introduced as a multi-graph representation of orthographic views, and quadric surface features (QSFs) are defined by special basic patterns of LRG as well as aggregation rules. By hint-based pattern matching in the LRGs of three orthographic views in an order of priority, the corresponding QSFs are recognized, and the geometry and topology of quadric surfaces are recovered simultaneously. This method can handle objects with interacting quadric surfaces and avoids the combinatorial search for tracing all the quadric surfaces in an intermediate wire-frame by the existing methods. Several examples are provided.     

Quadric and cubic bitetrahedral patches

Algebraic surface patches bounded by tetrahedra are promising building blocks for low-degree implicit spline surfaces. General algebraic surface patches may have topological anomalies such as singular points and multiple sheets. The A-patch technique avoids many of these anomalies but does not ensure that the surface patches are connected and have no holes. Requiring these qualities often places overly restrictive conditions on the patch. We present a technique for building a bitetrahedral patch that is single sheeted, smooth, and singly connected in a pair of face-adjacent tetrahedra. The bitetrahedral patch technique is applicable to most low-degree algebraic surface patches, and it establishes single sheetedness, smoothness, and single connectness when existing techniques fail.     

线框模型中二次曲面的识别

定义关联两曲线间的夹角和由3条关联曲线确定二次曲面,提出适用于曲面的回路搜索最小转角法,且利用它并辅以矢量混合积验证搜索曲面上最小回路的方法．实现从线框模型中选择任意一条曲线就能够搜索到包含它的所有潜在表面回路,突破以往回路搜索方法中只能从某方向极值点开始的束缚,为线框模型表面识别中将含曲线表面与不含曲线表面分治识别奠定基础．算法可处理含有任意位置的二次曲表面的线框模型,并能较准确地判别二次曲面的类型．     

MESSAGE--具有自由曲面造型功能的立体造型系统

MESSAGE是一具有自由曲面造型功能的立体造型系统,它能够处理二次曲面、旋转曲面、广义sweeping曲面、自由曲面以及由平面和上述曲面围成的三维物体,系统以平面及双三次有理B样条曲面为基本曲面模型,提供了布尔运算、glue运算及多种局部修改运算.系统还具有一个独立的几何数据库,可以支持一些重要的工程应用.     

Direct boolean intersection between acquired and designed geometry

In this paper, a new shape modeling approach that can enable direct Boolean intersection between acquired and designed geometry without model conversion is presented. At its core is a new method that enables direct intersection and Boolean operations between designed geometry (objects bounded by NURBS and polygonal surfaces) and scanned geometry (objects represented by point cloud data).We use the moving least-squares (MLS) surface as the underlying surface representation for acquired point-sampled geometry. Based on the MLS surface definition, we derive closed formula for computing curvature of planar curves on the MLS surface. A set of intersection algorithms including line and MLS surface intersection, curvature-adaptive plane and MLS surface intersection, and polygonal mesh and MLS surface intersection are successively developed. Further, an algorithm for NURBS and MLS surface intersection is then developed. It first adaptively subdivides NURBS surfaces into polygonal mesh, and then intersects the mesh with the MLS surface. The intersection points are mapped to the NURBS surface through the Gauss-Newton method.Based on the above algorithms, a prototype system has been implemented. Through various examples from the system, we demonstrate that direct Boolean intersection between designed geometry and acquired geometry offers a useful and effective means for the shape modeling applications where point-cloud data is involved.     

Least-squares-based fitting of paraboloids

A technique for reconstructing a class of quadric surfaces from 3D data is presented. The technique is driven by a linear least-squares-based fitting mechanism. Previously, such fitting was restricted to recovery of central quadrics; here, extension of that basic mechanism to allow recovery of one commonly-occurring class of non-central quadric, the elliptic paraboloids, is described. The extension uses an indirect solution approach that involves introducing a variable to the basic mechanism that is a function of a quadric surface invariant. Results from fitting real and synthetic data are also exhibited.     

PARAMETRIC SURFACE/SURFACE INTERSECTION

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