Freeform Surface Modelling Using NURBS SEMINAR REPORT

Bangalore Institute of Technology, Mech. Dept. Page 1

Freeform surface modelling usings NURBS

(2009-2010)

Technical Seminar Report

A technical seminar report submitted in partial fulfillment of the requirement for

the award of the degree of

BACHELOR OF ENGINEERING

IN

MECHANICAL ENGINEERING

VISVESWARAIAH TECHNOLOGICAL UNIVERSITY

BELGAUM, KARNATAKA, INDIA

MANISH O.

Report Submitted By:

(1BI06ME054)

BANGALORE INSTITUTE OF TECHNOLOGY K.R ROAD, V.V PURAM, BANGALORE – 560004


DEPARTMENT OF MECHANICAL ENGINEERING

CERTIFICATE

It is certified that the paper entitled “Freeform surface modelling using NURBS” was presented by K.A Somaiah, in partial fulfillment of the requirement for the award of the degree in Bachelor of Engineering in Mechanical Engineering of Visvesvaraya Technological University, Belgaum, Karnataka, India during the year 2009-2010. It is certified that all corrections/suggestions indicated in the internal assessment have been incorporated in the report deposited in the Department Library. The Technical Seminar Report has been approved as it satisfies the academic requirements in respect of the technical seminar prescribed for the Bachelor of Engineering Degree.

Signature of the examiners:

Signature of the H.O.D.:

1. Mr. N. Satish Dr. M. Venkatarama Reddy Lecturer, Mechanical Department, BIT HOD, Mechanical Department, BIT

2.

Manish O.

Name of candidate:

(1BI06ME054)

50


Seminar Objectives

• To understand what Freeform surface modelling is.

• To understand how complex surfaces are modelled with the use of NURBS(Non-uniform Rational B-Spline) inside a CAD software.

• To appreciate the myriad applications of Surface Modelling.

Benefits of this technology

• Reduction of time involved in virtual prototyping.

• More accurate rendering of the object in question.

• Enables the designers and engineers to express their thought more freely.

Computer Aided Designing(CAD) Revisited

• Computer-aided design (CAD) is the use of computer technology for the design of objects, real or virtual. CAD may be used to design curves and figures in two-dimensional ("2D") space; or curves, surfaces, and solids in three-dimensional ("3D") objects.

• Typical modern parametric feature based modeler and freeform surface systems are built around a number of key C (programming language) modules with their own APIs.

• A CAD system can be seen as built up from the interaction of a graphical user interface (GUI) with NURBS geometry and/or boundary representation (B-rep) data via a geometric modeling kernel.


Salient features of CAD

• Computer-Aided Design is one of the many tools used by engineers and designers and is used in many ways depending on the profession of the user and the type of software in question.

• 3D wireframe is basically an extension of 2D drafting. Each line has to be manually inserted into the drawing.

• 3D parametric solid modeling require the operator to use what is referred to as "design intent". The objects and features created are adjustable. Any future modifications will be simple, difficult, or nearly impossible, depending on how the original part was created.

Freeform Surface Modelling Computer-Aided Design is one of the many tools used by engineers and designers and is used in many ways depending on the profession of the user and the type of software in question.3D wireframe is basically an extension of 2D drafting. Each line has to be manually inserted into the drawing. 3D parametric solid modeling require the operator to use what is referred to as "design intent". The objects and features created are adjustable. Any future modifications will be simple, difficult, or nearly impossible, depending on how the original part was created.

Two main fields 1. Either creating aesthetic (Class A) surfaces that also perform a function; for example, car bodies and consumer product outer forms, AND 2. Technical surfaces for components such as gas turbine blades and other fluid dynamic engineering components.


Two basic methods for the creation of surfaces

1>> The first begins with construction curves (splines) from which the 3D surface is then swept (section along guide rail) or meshed (lofted) through. 2>> The second method is direct creation of the surface with manipulation of the surface poles/control points. From these initially created surfaces, other surfaces are constructed using either derived methods such as offset or angled extensions from surfaces; or via bridging and blending between groups of surfaces.

What are freeform surfaces?

Freeform surface, or freeform surfacing, is used in CAD and other computer graphics software to describe the skin of a 3D geometric element. Freeform surfaces do not have rigid radial dimensions, unlike regular surfaces such as planes, cylinders and conic surfaces. They are used to describe forms such as turbine blades, car bodies and boat hulls. Initially developed for the automotive and aerospace industries, freeform surfacing is now widely used in all engineering design disciplines from consumer goods products to ships. Most systems today use non-uniform rational B-spline (NURBS) mathematics to describe the surface forms; however, there are other methods such as Gorden surfaces or Coons surfaces . The forms of freeform surfaces (and curves) are not stored or defined in CAD software in terms of polynomial equations, but by their poles, degree, and number of patches (segments with spline curves). The degree of a surface determines its mathematical properties, and can be seen as representing the shape by a polynomial with variables to the power of the degree value. For example, a surface with a degree of 1 would be a flat cross section surface. A surface with degree 2 would be curved in one direction, while a degree 3 surface could (but does not necessarily) change once from concave to convex curvature. Some CAD systems use the term order instead of degree. The order of a polynomial is one greater than the degree, and gives the number of coefficients rather than the greatest exponent. The poles (or control points) of a surface define its shape.


The natural surface edges are defined by the positions of the first and last poles. The intermediate poles act like magnets drawing the surface in their direction. The surface does not, however, go through these points.The second and third poles as well as defining shape, respectively determine the start and tangent angles and the curvature. In a single patch surface (Bézier surface), there is one more pole than the degree values of the surface. Surface patches can be merged into a single NURBS surface; at these points are knot lines. The number of knots will determine the influence of the poles on either side and how smooth the transition is. The smoothness between patches, known as parametric continuity, is often referred to in terms of a C value:

• C0: just touching, could have a nick(small cut) • C1: tangent, but could have sudden change in curvature • C2: the patches are curvature continuous to one another

Two more important aspects are the U and V parameters. These are values on the surface ranging from 0 to 1, used in the mathematical definition of the surface and for defining paths on the surface. In CAD systems, surfaces are often displayed with their poles of constant U or constant V values connected together by lines; these are known as control polygons.

Modelling When defining a form, an important factor is the continuity between surfaces - how smoothly they connect to one another. One example of where surfacing excels is automotive body panels. Just blending two curved areas of the panel with different radii of curvature together, maintaining tangential continuity (meaning that the blended surface doesn't change direction suddenly, but smoothly) won't be enough. To achieve a high quality NURBS or Bezier surface, degrees of 5 or greater are generally used. Depending on the product and


production process, different levels of accuracy are used but tolerances usually range from 0.02 mm to .001 mm).

Curves in CAD and Computer Graphics Curves used in Freeform Surface Modelling can be represented by the following types

• Bézier curves • Hermite Curves • Uniform Non-rational B-Splines • Non-uniform, Non-rational B-Splines • Non-Uniform Rational B-Splines (NURBS)

NURBS and Bézier curves are ones of the most commonly used curves and the focus of this presentation.

Bézier curves Before explaining NURBS, we will stop by Bézier curve, because NURBS is a generalization of Bézier curve.

The following figure shows a simple Bézier curve (C), its control points (1), (2), (3), (4), and its control polygon (P). The control points are also called control handles.Each point on a Bézier curve (and on many other kinds of curves) is computed as a weighted sum of all control points. This means that each point is influenced by every control point. The first control point has maximum impact on the beginning of the curve, the second one reaches its maximum in the first half of the curve, etc. Each control point influences the final curve according to assigned blending function. A blending function defines the weight of the control point at each point of the curve. A value of 0 indicates that the control point is not affecting a point on the curve. If the blending function reaches 1, the curve is (usually) intersecting the control point.


Properties of blending functions define properties of a curve. Bézier curves use polynomial functions of given degree. The resulting curves have these properties:

• The curve starts in the first control point, ends in the last control point, but in general case does not cross the inner control points.

• The tangent of the curve in its ending points is controlled by the inner control points.

• The curve is always inside the convex hull of the control polygon.

Curve Degree

The previous example showed a cubic (degree 3) curve, which is one of the most often used types. The degree refers to the highest exponent in the polynomial blending functions used for Bézier curves. A Bézier curve may be of arbitrary degree. A degree 1 curve is a simple line and has two control points. A degree 2 curve is an arc and has three control points. The higher the degree, the more control points and the more complex shape is possible. But it is also more much harder to use, because each control point still influences the whole curve.

Rational Curves

Each control point in rational curve is assigned a weight. The weight defines how much does a point "attract" the curve. Only the relative weights of the control points are important, not their absolute values. A curve with all weights set to 1 will have the same shape as if all weights are set to 100. The shape only changes if weights of control points are different.

Ordinary Bézier curve is a special case or rational Bézier curve, where all weights are


equal. Rational curve gives designers additional options at the cost of a more complicated algorithm and additional data to keep track of.The rational Bézier curve adds adjustable weights to provide closer approximations to arbitrary shapes. The numerator is a weighted Bernstein-form Bézier curve and the denominator is a weighted sum of Bernstein polynomials. Rational Bézier curves can, among other uses, be used to represent segments of conic sections exactly.Given n + 1 control points Pi, the rational Bézier curve can be described by:

B-spline A B-Spline or or basis spline consists of multiple Bézier arcs and provides an unified mechanism how to define continuity in the joins.

Consider two cubic Bézier curves - that is 8 total control points (4 per curve).Lets make the last point of the first (green) curve equal to the first point of the second (violet) curve - this saves us 1 point leaving us with 7 total control points. We have replaced one control point with an external condition. The third (blue) curve and the fourth (yellow) curve share ending points just like in previous case, but and also share the same tangent direction at the junction point. There are two external conditions and only 6 control points are necessary to describe the curves.

Knot Vector • The knots are a list of degree+N-1 numbers, where N is the number of control

points. Sometimes this list of numbers is called the knot vector. In this term, the word vector does not mean 3-D direction.

• B-Splines use external conditions to put multiple pieces together while keeping the original concept of control points.

• The neighbour curves share some control points. External conditions are either implicit (uniform curves) or explicitly given by a knot vector.

• Knot vector defines how much information should be shared by neighbour curves

Blue

Red

Green Yellow


(segments).

Knot Multiplicity • The number of times a knot value is duplicated is called the knot’s multiplicity. • Knot vector is a sequence of numbers, usually from 0 to 1, for example (0, 0.5, 0.5,

0.7, 1), and it holds the information about external conditions mentioned earlier. Number of intervals defines number of segments (3 in our case: 0-0.5, 0.5-0.7, 0.7-1).

• Multiplicity of knot 0.7 is 1, while multiplicity of knot 0.5 is 2. The higher the multiplicity, the less information share the neighbour segments. When multiplicity is equal to the degree of used curves, there is a sharp edge (green and violet curves on the image).

NURBS NURBS stands for Non-Uniform Rational B-Spline. It means NURBS uses rational Bézier curves and an non-uniform explicitly given knot vector. Therefore, degree, control points, weights, and knot vector is needed to specify a NURBS curve.

It is a mathematical model commonly used in computer graphics and CAD for generating and representing curves and surfaces which offers great flexibility and precision for handling both analytic and freeform shapes.Three-dimensional NURBS surfaces can have complex, organic shapes. Control points influence the directions the surface takes. The pioneers of this development were Pierre Bézier who worked as an engineer at Renault, and Paul de Casteljau who worked at Citroën, both in France in the 1950’s.


General form of NURBS curve Using the definitions of the basis functions Ni,n, a NURBS curve takes the following form

In this, k is the number of control points and wi are the corresponding weights. The denominator is a normalizing factor that evaluates to one if all weights are one. This can be seen from the partition of unity property of the basis functions. It is customary to write this as

in which the functions

are known as the rational basis functions.

Geometric Continuity A surface under construction, e.g. the hull of a motor yacht, is usually composed of several NURBS surfaces known as patches. These patches should be fitted together in such a way that the boundaries are invisible. This is mathematically expressed by the concept of geometric continuity.


Higher-level tools exist which benefit from the ability of NURBS to create and establish geometric continuity of different levels:

• Positional continuity (G0)

holds whenever the end positions of two curves or surfaces are coincidental. The curves or surfaces may still meet at an angle, giving rise to a sharp corner or edge and causing broken highlights.

• Tangential continuity (G1)

requires the end vectors of the curves or surfaces to be parallel, ruling out sharp edges. Because highlights falling on a tangentially continuous edge are always continuous and thus look natural, this level of continuity can often be sufficient.

• Curvature continuity (G2)

further requires the end vectors to be of the same length and rate of length change. Highlights falling on a curvature-continuous edge do not display any change, causing the two surfaces to appear as one. This can be visually recognized as “perfectly smooth”. This level of continuity is very useful in the creation of models that require many bi-cubic patches composing one continuous surface

Operations on NURBS When working with NURBS in their pure form, there is one very useful operation: inserting new knot. A knot can be inserted into a NURBS curve without changing the shape of the curve. The desired side effect of this operation is an additional control point that provides finer control of the related region of the NURBS curve or surface. There are other operations with NURBS, like

• elevating degree, • removing knots, or • computing control point positions from points laying on a curve,

but they do not reach the usefulness of knot insertion.

Salient Features of NURBS

• They allow representation of geometrical shapes in a compact form. • They can be efficiently handled by the computer programs and yet allow for easy

human interaction. • NURBS surfaces are functions of two parameters mapping to a surface in three-

dimensional space. • The shape of the surface is determined by control points. • The principles can be applied to higher-dimensional objects like surfaces or

volumes. Surfaces are used when creating 3D objects, for example landscape; while volumes can be used to define a non-linear transformation.


• NURBS are nearly ubiquitous for computer-aided design (CAD), manufacturing (CAM), and engineering (CAE) and are part of numerous industry wide used standards, such as IGES, STEP, ACIS, and PHIGS.

Surface Modelling tools in use today NURBS tools are also found in various 3D modeling and animation software packages, such as

• Catia • Cobalt (Ashlar-Vellum) • Form•Z • ICEM Surf • Imageware • NX (Unigraphics) • ProEngineer • Rhinoceros 3D • SensAble Technologies • Solidworks • Alias StudioTools • FreeSHIP • GenesisIOD • OmniCAD • ProEngineer ISDX • Thinkdesign •

Other than this there are specialized NURBS modeling software packages such as • Autodesk Alias Surface and • SolidThinking.


Real world application examples of Surface Modelling

Example 1

Surfaces or revolution can roughly approximate relatively large amount of different shapes.This image demonstrates a surface created by revolving a 2D NURBS curve around Y axis. The curve itself consists of 3 pieces (knot vector: 0, 0.2, 0.6, 0.6, 0.6, 1). Join between the two upper pieces is smooth, because the multiplicity of knot 0.2 is 1 and curve degree is 3. On the other hand, knot 0.6 with multiplicity 3 causes a sharp edge.


Example 2

Surface was created by moving a 2D NURBS curve along a path defined by another 3D NURBS curve. This image shows a surface created by sweeping a 2D curve along a 3D trajectory.


Example 3

NURBS surfaces need relatively large amount of control points, which makes them hard to control.This image shows a NURBS surface and its control points. NURBS surfaces are used rather rarely in their pure form because the number of control points is usually large (4x4 in our simple case) and the surface becomes hard to control.


Example 4

The middle part of the text is magnified and the text is bent using a 2nd degree NURBS volume.This image shows a 3D text that was transformed using a Bézier (or NURBS) volume of degree 2. The text is bent and its central part is larger - that effect was caused by the non-linear transformation defined by the NURBS volume (note the control points in the center of the model).


Example 5

Highlights and reflections can reveal the perfect smoothing, which is otherwise practically impossible to achieve without NURBS surfaces that have at least G2 continuity. This same principle is used as one of the surface evaluation methods whereby a ray-traced or reflection-mapped image of a surface with white stripes reflecting on it will show even the smallest deviations on a surface or set of surfaces. This method is derived from car prototyping wherein surface quality is inspected by checking the quality of reflections of a neon-light ceiling on the car surface. This method is also known as "Zebra analysis".


Example 6

The smooth, undetailed form like shown above allows the engineer or designer to focus on crafting a seamless surface model that will machine efficiently in the 3d printers. The engineer or designer begins by modeling the Form from NURBS surfaces in Alias Studio Tools. Ruled surfaces are stitched together to avoid gaps in the surface model, which would cause the 3d print to fail.


Example 7

Modes of Modeling • Wireframe Mode • Faceted Mode • Shaded Mode


Example 8

The NURBS system of points is also being used in facial characterizing. This feature can be used for security purposes in a “facial recognition” software to give access to the individual according to his facial features. This feature is also being developed for forensic purposes.


Example 9

Topographical digitizing using surface modeling is one of the latest uses. A satellite captures the various topographical features with and sends this data to a control station. The control station then receives this data and processes it carefully with the help of NURBS enabled surface modelling algorithms. This had enabled geologists to study a piece of land at a great distance. Eg.1) A volcano studying geologist can study active volcano without facing the possible dangers of eruptions, ash clouds, etc. Eg.2) Scientists can also study the surfaces of far off planets in more detail.


Example 10

Advanced Rendering tools – • Rendering engine

Vray on 3DS max • MentalRay on Maya • Autodesk Showcase

CONCLUSION The challenge of the presentation was to simplify this complex concept for a wider audience to understand. We can see how NURBS plays a vital role in surface modelling. Freeform surfacing enables uses of CAD that were previously even hard to imagine. It increases productivity of designing and engineering firms to a whole new level.


References • “Surfaces” from mif.vu.lt/cs2/en/courses/intercg/files/surfaces.pdf. (Author

anonymous) • “Sketch- and Constraint-based Design of B-spline Surfaces” by Paul Michalik,

Dae Hyun Kim, Beat D. Bruderlin • http://www.rhino3d.com/nurbs.h • http://www.rw-designer.com/NURBStm • http://web.cs.wpi.edu/~matt/courses/cs563/talks/nurbs.html • http://www.monkeedesign.com/interviews/2009/7/21/monkee-interviews-remi-

marchand-digital-designer.html • aliasstudio.arnovillard.com/category/tutorials/ • www.carbodydesign.com/tutorials/?id=1826 • www.solidsmack.com/ • http://en.wikipedia.org/wiki/Freeform_surface • http://en.wikipedia.org/wiki/NURBS • http://en.wikipedia.org/wiki/Reflection_mapping • http://en.wikipedia.org/wiki/B-spline

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