Synthetic curve
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This document discusses synthetic curves used in mechanical CAD. Synthetic curves are needed to model complex curved shapes and allow manipulation by changing control point positions. There are three main types of synthetic curves: Hermite cubic splines, Bezier curves, and B-spline curves. Bezier curves use control points to influence the curve path without requiring the curve to pass through the points. The curve is defined by a polynomial equation involving blending functions. Bezier curves have tangent lines at the start and end points and maintain tangency when control points are moved.
Synthetic curve
- 1. Computer Addied Design Presented By :- Dhruv Shah TOPIC : Synthetic Curve
- 2. Synthetic Curves The analytical and interpolated curves, discussed in the previous section are insufficient to meet the requirements of mechanical parts that have complex curved shapes, such as, propeller blades, aircraft fuselage, automobile body, etc. These components contain non-analytical, synthetic curves. Design of curved boundaries and surfaces require curve representations that can be manipulated by changing data points, which will create bends and sharp turns in the shape of the curve. The curves are called synthetic curves, and the data points are called vertices or control points. If the curve passes through all the data points, it is called an inter polant (interpolated). Smoothness of the curve is the most important requirement of a synthetic curve.
- 3. Various continuity requirements at the data points can be specified to impose various degrees of smoothness of the curve. A complex curve may consist of several curve segments joined together. Smoothness of the resulting curve is assured by imposing one of the continuity requirements. A zero order continuity (C0) assures a continuous curve, first order continuity (C1) assures a continuous slope, and a second order continuity (C2) assures a continuous curvature, as shown below.
- 4. A cubic polynomial is the lowest degree polynomial that can guarantee a C2 curve. Higher order polynomials are not used in CAD, because they tend to oscillate about the control points and require large data storage. Major CAD/CAM systems provide three types of synthetic curves: Hermite Cubic Spline, Bezier Curves, and B-Spline Curves. Cubic Spline curves pass through all the data points and therefore they can be called as interpolated curves. Bezier and B-Spline curves do not pass through all the data points, instead, they pass through the vicinity of these data points. Both the cubic spline and Bezier curve have first-order continuity, where as, B-Spline curves have a second-order continuity.
- 5. Bezier Curves Equation of the Bezier curve provides an approximate polynomial that passes near the given control points and through the first and last points. In 1960s, the French engineer P. Bezier, while working for the Renault automobile manufacturer, developed a system of curves that combine the features of both interpolating and approximating polynomials. In this curve, the control points influence the path of the curve and the first two and last two control points define lines which are tangent to the beginning and the end of the curve. Several curves can be combined and blended together. In engineering, only the quadratic, cubic and quartic curves are frequently used.
- 7. Bezier ’s Polynomial Equation The curve is defined by the equation P(t) = Σ Vi Bi,n (t) where, 0 ≤ t ≤ 1 and i = 0, 1, 2, …, n Here, Vi represents the n+1 control points, and Bi,n (t) is the blending function for the Bezier representation and is given as n Bi,n (t) = i ( ti ) (1-t)n-i
- 8. The equations force the curve to lie entirely within the convex figure (or envelop) set by the extreme points of the polygon formed by the control points. The envelope represents the figure created by stretching a rubber band around all the control points. The figure below shows that the first two points and the last two points form lines that are tangent to the curve. Also, as we move point v1, the curve changes shape, such that the tangent lines always remain tangent to the curve.
- 9. Bezier ’s blending function produces an nth degree polynomial for n+1 control points and forces the Bezier curve to interpolate the first and last control points. The intermediate control points pull the curve toward them, and can be used to adjust the curve to the desired shape.