3 hermite cubic curves
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G43 – Geometric Modeling
Mathematical modeling of
Curves – Hermite Cubic Spline
Curves
1
Dr.C. ParamasivamE-mail: cpmech@tce.edu
Curves
1. Define the coordinate system for the development
of models based on input and geometry.
2. Develop and manipulate the curves and surfaces
using parametric equations.
3. Develop and manipulate the solid models using
Course Objectives
By the end of the course, student will be able to:
3. Develop and manipulate the solid models using
modeling techniques.
4. Implement the transformation and projection over
the geometric models.
5. Implement the neutral file formats over 2D
wireframe models.
Mathematical modeling of
Curves – Hermite Cubic Spline
Curves
By the end of the course, studentwill be able to:
Develop and manipulate thecurves and surfaces usingparametric equations.
Types of curve equations
1. Parametric equation:
Example: circle equation:
X = Rcosθ Y = Rsin θ z = 0 0 ≤ θ ≤ 2π
(the coordinates are defined with the help of the extra
parameter θ).
2. Non-parametric equation
1. Implicit form: x2+y2-R2=0 z=0.
2. Explicit form y=±√(R2-x2) z=0.
( these equations defined the x , y and z coordinates without the
assistance of extra parameters)
The parametric equation is the most popularform for representing curves and surfaces inCAD systems.
WIREFRAME MODELS
A wireframe model of an object is the
simplest, but more verbose, geometric model
that can be used to represent it
mathematically.
Curve Modeling
mathematically.
A wireframe
representation is a
3D line drawing of
an object
1. Analytical curvesThe curves which are having rigid form of equation without any flexibility to modify its original shapes after display.
Example:
1. Point
Types of curves
1. Point
2. Line, Line segment
3. Conic sections – Circle, Ellipse, parabola, hyperbola.
4. Fillet
5. Chamfer
2. Synthetic curves• The curves which are usually described by a
polynomial equation having flexibility to modify itsoriginal shape after display.
• A parameter (u) is used to control its shape anddegrees-of-curves.
Examples:
1. Hermite cubic spline curve1. Hermite cubic spline curve
2. Bezier curve
3. B-spline curve
4. Rational curve
a) Rational Hermite cubic spline
b) Rational Bezier curve
c) Rational B-spline curve
Synthetic Curves
• Analytic curves are usually not sufficient to meetgeometric design requirements of mechanical parts.
Example: Car bodies, ship hulls, airplane wings,propeller blades, shoe insoles, and bottles etc.
Need for synthetic curves
• When a curve is represented by a collection ofmeasured data points.
• When an existing curve must change to meetnew design requirements.
• When an existing curve must change to meetnew design requirements.
1. Hermite cubic spline curve
• Parametric spline curves are defined aspiecewise polynomial curves with certainorder of continuity.
• The parametric cubic spline curve connects• The parametric cubic spline curve connectstwo data (end) points and utilizes a cubicequation.
General condition required:
1. Two end points
2. Two end slopes
• The parametric equation of a cubic splinesegment is given by:
where, u – parameter
Ci - Polynomial coefficients
The scalar form of equation:
10)(3
0
≤≤∑==
uuCuPi
ii
The scalar form of equation:
X(u) = C3x u3 + C2x u2 + C1x u + C0x
y(u) = C3y u3 + C2y u2 + C1y u + C0y
z(u) = C3z u3 + C2z u2 + C1z u + C0z
The vector form of equation:
P(u) = C3u3 + C2u2 + C1u + C0
P0 – Starting point
P1 – End point
P’0 – Starting slope
P’1 – Starting slope
The matrix form of equation:
P(u) = UTC
where,
U = [u3 u2 u1 1]T
C = [C3 C2 C1 C0]T (coefficient vector)
Tangent Vector
10)('1
3
0
≤≤∑= −
=
uuiCuPi
ii
To find the coefficients Ci
Apply the known boundary conditions
P0 , P’0 at u = 0
P1 , P’1 at u = 1
Position and slope equation are:Position and slope equation are:
P(u) = C3u3 + C2u2 + C1u + C0
P’(u) = 3C3u2 + 2C2u + C1
When applying u = 0 and u = 1 on the position
and slope equations:
• P0 = C0
• P’0 = C1
• P1 = C3 + C2 + C1 + C0• P1 = C3 + C2 + C1 + C0
• P’1 = 3C3 + 2C2 + C1
After solving the above four equations bysimultaneous solution method the coefficientsare:
C0 = P0
C1 = P’0
C2 = 3(P1-P0) -2(P’0 - P’
1)C2 = 3(P1-P0) -2(P 0 - P 1)
C3 = 2(P0-P1) + P’0 + P’
1
After substituting all four coefficient, the
final blending functions are:
P(u) = (2u3 – 3u2 + 1)P0+ (-2u3 + 3u2) P1
+ (u3 – 2u2 + u)P’0 + (u3 – u2) P’
1
P’(u) = (6u3 – 6u)P0+ (-6u2 + 6u) P1 +P’(u) = (6u – 6u)P0+ (-6u + 6u) P1 +
(3u2 – 4u + 1)P’0 + (3u2 – 2u) P’
1
P(u) = UTC
P(u) = UT [MH]V
Where,
[MH] = Hermite matrix
V= Geometry (Boundary) vector
V= [P0
P1
P’0
P’1
]T
−−−
−
=
0001
0100
1233
1122
][H
M
Comparing the above two equations:
UTC = UT [MH]V
C = [MH]V
Final position and slope equation in the Final position and slope equation in the
matrix are:
P(u) = UT [MH]V
P’(u) = UT[MH]uV
Modification of resultant curve shape
1.By changing control point(s)
2.By changing end slope(s)2.By changing end slope(s)
1.By changing control point(s)
2. By changing end slope(s)
Hermite Cubic Spline curve using AutoCAD software
Characteristics of Hermite cubic spline
curve
1. The resultant shape will pass through all the
given data or control points.
2. It uses interpolation technique for curve
generation.generation.
3. The resultant curve has tangential property
with start and end slopes.
4. It has only global control.
5. The resultant curve has always cubic curve.
Limitations
1. It has only global control (or) Lack of local.
2. Always it has cubic degree.
3. It is not possible to apply, when higher
degree of curves required.degree of curves required.
Answer
Sample questions
1. Find the shape of the hermite cubic spline curve usingP0(10, 20) and P1(30, 50) with an inclination of 300 incounter clockwise direction? Plot your result on the graphsheet.
2. Plot the resultant shape of hermite cubic spline curve usingP0(0, 20) and P1(50, 50) with 300 inclination at the startingP0(0, 20) and P1(50, 50) with 30 inclination at the startingpoint and -450 inclination at the end point.
3. Derive the blending function of hermite cubic spline curve.
4. Describe the important properties of hermite cubic splinecurve.
5. Illustrate the method of modifying the resultant hermitecurve shape.
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