## Introduction

In the mathematical subfield of numerical analysis, a B-spline is a spline function that has minimal support with respect to a given degree, smoothness, and domain partition.

The term B-spline was coined by Isaac Jacob Schoenberg and is short for basis spline.

B-splines can be evaluated in a numerically stable way by the de Boor algorithm. This Bspline code was implemented as part of my third semester project.

## Algorithm

The algorithm used for generating is for cubic Bspline.

Cubic B-spline with uniform knot-vector is the most commonly used form of B-spline. The blending function can easily be precalculated, and is equal for each segment in this case. Put in matrix-form, it is as follows:

for

## The Code

Two event handlers `pictureBox1_MouseUp `

and `pictureBox1_MouseDown `

are used.

#### MouseUp Event

`MouseUp `

event is used to print the point where the mouse is clicked and the `mousedown `

event is used to get the interpolated points and then accordingly draw the lines.

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
Graphics g = pictureBox1.CreateGraphics();
Color cl=Color.Black;
g.DrawLine (new Pen(cl, 2), e.X, e.Y, e.X+1 , e.Y+1 );
}

#### MouseDown Event

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
if (no_of_points > 3)
{
pt[0] = pt[1];pt[1] = pt[2];pt[2] = pt[3];
pt[3].setxy(e.X, e.Y);
double temp = Math.Sqrt(Math.Pow(pt[2].x - pt[1].x, 2F) +
Math.Pow(pt[2].y - pt[1].y, 2F));
int interpol = System.Convert.ToInt32(temp);
bsp(pt[0],pt[1],pt[2],pt[3],interpol);
int i;
int width=2;
Graphics g = pictureBox1.CreateGraphics();
Color cl=Color.Blue;
int x,y;
for (i = 0; i <= interpol - 1; i++)
{
x=System.Convert.ToInt32(splinex[i]);
y=System.Convert.ToInt32(spliney[i]);
g.DrawLine( new Pen(cl, width) , x - 1, y, x + 1, y);
g.DrawLine(new Pen(cl, width), x, y - 1, x, y + 1);
}
}
else
{
pt[no_of_points].setxy(e.X, e.Y);
}
no_of_points = no_of_points + 1;
}

It first checks whether the number of points are greater than four or not. If the points are greater than 4, the `bsp`

function is called and then finally the lines are printed.

#### BSP Function

public void bsp(point p1, point p2, point p3, point p4, int divisions)
{
double[] a = new double[5];
double[] b = new double[5];
a[0] = (-p1.x + 3 * p2.x - 3 * p3.x + p4.x) / 6.0;
a[1] = (3 * p1.x - 6 * p2.x + 3 * p3.x) / 6.0;
a[2] = (-3 * p1.x + 3 * p3.x) / 6.0;
a[3] = (p1.x + 4 * p2.x + p3.x) / 6.0;
b[0] = (-p1.y + 3 * p2.y - 3 * p3.y + p4.y) / 6.0;
b[1] = (3 * p1.y - 6 * p2.y + 3 * p3.y) / 6.0;
b[2] = (-3 * p1.y + 3 * p3.y) / 6.0;
b[3] = (p1.y + 4 * p2.y + p3.y) / 6.0;
splinex[0] = a[3];
spliney[0] = b[3];
int i;
for (i = 1; i <= divisions - 1; i++)
{
float t = System.Convert.ToSingle(i) / System.Convert.ToSingle(divisions);
splinex[i] = (a[2] + t * (a[1] + t * a[0]))*t+a[3] ;
spliney[i] = (b[2] + t * (b[1] + t * b[0]))*t+b[3] ;
}
}

The `bsp `

function calculates the values by solving the matrix given above.

## Conclusion

Thus we can draw Bspline curves by defining our control points by clicking on the picturebox control. The Bsplines have a wide application in the graphics field.

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