Chapter: Mechanical : Computer Aided Design : Geometric Modeling

Geometric Modeling

Types Of Curves Used In Geometric Modelling: • Hermite curves • Bezeir curves • B-spline curves • NURBS curves








(1) Parametric equation x, y, z coordinates are related by a parametric var iable (u or θ)


(2) Nonparametric equation x, y, z coordinates are related by a function


Example: Circle (2-D)




       Hermite curves


       Bezeir curves


       B-spline curves


       NURBS curves





Effect of tangent vector on t he curve’s shape




Two Drawbacks of Bezier Curves







NURBS curve


Advantages of B-spline curves and NURBS curve




i.           Surface Patch


ii.           Coons Patch


iii.           Bicubic Patch


iv.          Be’zier Surface


v.           B-Spline Surface


i.           Surface Patch


The patch is the fundamental building block for surfaces. The two variables u and v vary across the patch; the patch may be termed biparametric. The parametric variables often lie in the range 0 to 1. Fixing the value of one of the parametric variables results in a curve on the patch in terms of the other variable (Isoperimetric curve). Figure shows a surface with curves at intervals of u and v of 0 : 1.


ii.  Coons Patch


The sculptured surface often involve interpolation across an intersecting mesh of curves that in effect comprise a rectangular grid of patches, each bounded by four boundary curves. The linearly blended coons patch is the simplest for interpolating between such boundary curves. This patch definition technique blends for four boundary curves Ci(u) and Dj(v) and the corner points pij of the patch with the linear blending functions,





iii.           Bicubic Patch



The bi-cubic patch is used for surface descriptions defined in terms of point and tangent vector information. The general form of the expressions for a bi-cubic patch is given by:

This is a vector equation with 16 unknown parameters kij which can be found by Lagrange interpolation through 4 x 4 grid.


iv.          Be’zier Surface


The Be’zier surface formulation use a characteristic polygon


Points the Bezier surface are given by


v.           B-Spline Surfaces


The B-spline surface approximates a characteristics polygon as shown and passes through the corner points of the polygon, where its edges are tangential to the edges of the polygon


This may not happen when the control polygon is closed


A control point of the surface influences the surface only over a limited portion of the parametric space of variables u and v.


The expression for the B-spline surfaces is given by



Geometric modeling is the starting point of the product design and manufacture process. Functions of Geometric Modeling are:


Design Analysis


Evaluation of area, volume, mass and inertia properties


Interference checking in assemblies


Analysis of tolerance build-up in assemblies


Kinematic analysis of mechanisms and robots


Automatic mesh generation for finite element analysis




Automatic planar cross-sectioning


Automatic hidden lines and surface removal


Automatic production of shaded images


Automatic dimensioning


Automatic creation of exploded views of assemblies




Parts classification


Process planning


NC data generation and verification


Robot program generation


Production Engineering


Bill of materials


Material requirement


Manufacturing resource requirement




Inspection and quality control


Program generation for inspection machines


Comparison of produced parts with design







The geometric model must stay invariant with regard to its location and orientation The solid must have an interior and must not have isolated parts


The solid must be finite and occupy only a finite shape


The application of a transformation or Boolean operation must produce another solid The solid must have a finite number of surfaces which can be described


The boundary of the solid must not be ambiguous






It uses networks of interconnected lines (wires) to represent the edges of the physical objects being modeled


Also called ‘Edge-vertex’ or ‘stick-figure’ models Two types of wire frame modeling:


1.     2 ½ - D modeling


2.     3 – D modeling


3-D Wire frame models: These are


Simple and easy to create, and they require relatively little computer time and memory; however they do not give a complete description of the part. They contain little information about the surface and volume of the part and cannot distinguish the inside from the outside of part surfaces. They are visually ambiguous as the model can be interpreted in many different ways because in many wire frame models hidden lines cannot be removed. Section property and mass calculations are impossible, since the object has no faces attached to it. It has limited values a basis for manufacture and analysis


2 ½ - D Wire frame models:


Two classes of shape for which a simple wire-frame representation is often adequate are those shapes defined by projecting a plane profile along its normal or by rotating a planar profile about an axis. Such shapes are not two-dimensional, but neither do they require sophisticated three-dimensional schemes for their representation. Such representation is called 2 ½ - D.


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