Computer Graphics Introduction To Curves

1. Introduction To Curves
C. P. Divate

2. Abstract
• We present and study existing digital differential analyzer (DDA) algorithms for circle generation,
including an improved two-step DDA algorithm which can be implemented solely in terms of
elementary shifts, addition, and subtraction.

3. Introduction
• Digital interpolation algorithms are widely used in machine tools with numerical control, graphics
displays and plotters, and manipulation robots. Circles and circular arcs frequently appear in
computer graphics, computer-controlled printing, and automated control;
• One of most popular methods for generation of circles and arcs is known as digital differential
analyzer (DDA).
• In this chapter a new improved two-step algorithm for DDA circle generation is presented.
• The accuracy of this method is higher than the accuracy of other known algorithms.
• Because of its simplicity (it uses only elementary shift, addition, and subtraction); this method can
also be used in numerical control, planning mechanisms, and so forth.

4. DDA Algorithms
• The general class of DDA algorithms for circles generation is based on obvious trigonometric
transformations describing rotation of vector R in the coordinate plane x and y
• Advantages of DDA algorithms include simplicity and high speed in generating circle point coordinate
xn+1, yn+1 .

5. DDA Algorithms

6. Bezier Curve-
Bezier Curve may be defined as-
•Bezier Curve is parametric curve defined by a set of control points.
•Two points are ends of the curve.
•Other points determine the shape of the curve.
The concept of Bezier curves was given by Pierre Bezier.
Bezier Curve Example-
The following curve is an example of a Bezier curve-
Here,
• This bezier curve is defined by a set of control
points b0, b1, b2 and b3.
• Points b0 and b3 are ends of the curve.
• Points b1 and b2 determine the shape of the curve.

7. Bezier Curve Properties- Few important properties of a Bezier curve are-
Property-01: Bezier curve is always contained within a polygon called as convex hull of its control points.
Property-02:
• Bezier curve generally follows the shape of its defining polygon.
• The first and last points of the curve are coincident with the first and last points of the defining polygon.
Property-03:
• The degree of the polynomial defining the curve segment is one less than the total number of control
points.
Degree = Number of Control Points – 1

8. Bezier Curve Properties- Few important properties of a Bezier curve are-
Property-04:
• The order of the polynomial defining the curve segment is equal to the total number of control points.
Order = Number of Control Points
Property-05:
• Bezier curve exhibits the variation diminishing property.
• It means the curve do not oscillate about any straight line more often than the defining polygon.

9. Bezier Curve Equation-
A bezier curve is parametrically represented by-
Here,
• t is any parameter where 0 <= t <= 1
• P(t) = Any point lying on the bezier curve
• Bi = ith control point of the bezier curve
• n = degree of the curve
• Jn,i(t) = Blending function = C(n,i)ti(1-t)n-i where C(n,i) = n! / i!(n-i)!

10. Cubic Bezier Curve-
• Cubic bezier curve is a bezier curve with degree 3.
• The total number of control points in a cubic bezier curve is 4.
Here,
• This curve is defined by 4 control points b0, b1, b2 and b3.
• The degree of this curve is 3.
• So, it is a cubic bezier curve.
Substituting n = 3 for a cubic bezier curve, we get-

11. Cubic Bezier Curve-
• Cubic bezier curve is a bezier curve with degree 3.
• The total number of control points in a cubic bezier curve is 4.
Substituting n = 3 for a cubic bezier curve, we get-
Expanding the above equation, we get-
P (t) = B0J3,0(t) + B1J3,1(t) + B2J3,2(t) + B3J3,3(t) ………..(1)
Now,

12. Cubic Bezier Curve-
Expanding the above equation, we get-
P (t) = B0J3,0(t) + B1J3,1(t) + B2J3,2(t) + B3J3,3(t) ………..(1)
Now,

13. Cubic Bezier Curve-
Expanding the above equation, we get-
P (t) = B0J3,0(t) + B1J3,1(t) + B2J3,2(t) + B3J3,3(t) ………..(1)
Now,
Using (2), (3), (4) and (5) in (1), we get-
P(t) = B0(1-t)3 + B13t(1-t)2 + B23t2(1-t) + B3t3
This is the required parametric equation for a cubic bezier curve.

14. Applications of Bezier Curve-
Bezier curves have their applications in the following fields-
1. Computer Graphics-
• Bezier curves are widely used in computer graphics to model smooth curves.
• The curve is completely contained in the convex hull of its control points.
• So, the points can be graphically displayed & used to manipulate the curve intuitively.
2. Animation-
• Bezier curves are used to outline movement in animation applications such as Adobe Flash and synfig.
• Users outline the wanted path in bezier curves.
• The application creates the needed frames for the object to move along the path.
• For 3D animation, bezier curves are often used to define 3D paths as well as 2D curves.
3. Fonts-
• True type fonts use composite bezier curves composed of quadratic bezier curves.
• Modern imaging systems like postscript, asymptote etc use composite bezier curves composed of cubic bezier
curves for drawing curved shapes.

15. 2
Koch Curve
What is Koch Curve?
The Koch snowflake (also known as the Koch curve, Koch star, or Koch island) is a
mathematical curve and one of the earliest fractal curves to have been described. It is
based on the Koch curve, which appeared in a 1904 paper titled “On a continuous curve
without tangents, constructible from elementary geometry” by the Swedish
mathematician Helge von Koch.
The progression for the area of the snowflake converges to 8/5 times the area of the
original triangle, while the progression for the snowflake’s perimeter diverges to infinity.
Consequently, the snowflake has a finite area bounded by an infinitely long line.

16. 3
Depth 1
1 line segment

17. 4
Depth 2
Contains 4 depth-1 curves
(4 line segments)

18. 5
Depth 3
Contains 4 depth-2 curves
(16 line segments)

19. 6
Depth 4
Contains 4 depth-3 curves
(64 line segments)

20. 7
Depth 5
Contains 4 depth-4 curves
(256 line segments)

21. Construction
Step1: Draw an equilateral triangle. You can draw it with a compass or
protractor, or just eyeball it if you don’t want to spend too much time
drawing the snowflake.
It’s best if the length of the sides are divisible by 3, because of the nature
of this fractal. This will become clear in the next few steps.

22. Construction
Step2: Divide each side in three equal parts. This is why it is handy to have the
sides divisible by three.

23. Construction
Step3: Draw an equilateral triangle on each middle part. Measure the length of the
middle third to know the length of the sides of these new triangles.

24. Construction
Step4: Divide each outer side into thirds. You can see the 2nd generation of
triangles covers a bit of the first. These three line segments shouldn’t
be parted in three.

25. Construction
Step5: Draw an equilateral triangle on each middle part. Note how you draw each
next generation of parts that are one 3rd of the mast one.

26. 8
Pseudocode Algorithm
Tomake a Koch Curve:
• Draw a straight line if depth is zero; otherwise
draw four smaller Koch Curves.

27. Koch Curve
The Koch curve is to divide a line into three equal segments and replace the
middle segment with two lines of the same length.
12

28. • The Koch snowflake can be constructed by starting
with an equilateral triangle, then recursively altering
each line segment as follows:
divide the line segment into fore segments of equal
length.
draw an equilateral triangle that has the middle
segment from step 1 as its base and points outward.
remove the line segment that is the base of the triangle
from step 2.
28
Koch Curve(Construction)

29. Koch Curve
Start with a line.

30. Line Segment Transformation
(x1,y1)
(x4,y4)
(x3,y3)
(x2,y2)
(x5,y5)
(x1,y1)
(x5,y5)

31. Segment Calculations

32. Calculating the Projection
l
l
y = √3/2l
x = ½l
m2 + (½ l)2 =l2
m2 + ¼ l2 =l2
√m2 = √¾ l2
m = √3/2l * 1/3 = √3l/6
1/3 length of each iteration