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# Draw a US Flag using C# and GDI+

By , 7 Dec 2007
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## Introduction

Visual C# provides a powerful GDI+ class library interface that allows users to draw various graphics objects.

This article shows you how to create a US flag using C# and GDI+. The US flag contains 50 star polygons and several rectangles.

## Background

Polygon is one of the most important graphics objects we are dealing with when rendering 2D and 3D graphics or processing computational geometry. `Graphics.DrawPolygon `method draws a polygon defined by an array of point structures. Every pair of two consecutive points in the array specifies a side of the polygon.

Here, I will show you how to create a US flag object. First we need to define the coordinates of a star. As illustrated in the following figure, suppose that the center coordinates of the star are at (xc, yc), r1 is the radius of the inner circle, and r is the radius of the outer circle. The angles a = 72 degrees and ß = 36 degrees.

From this figure, we can easily determine the coordinates of points 0 to 9, as listed in the following table:

 Points x coordinates y coordinates 0 xc yc – r 1 xc + r1 sinß yc – r1 cosß 2 xc + r sina yc – r cosa 3 xc + r1 sina yc + r1 cosa 4 xc + r sinß yc + r cosß 5 xc yc + r1 6 xc – r sinß yc + r cosß 7 xc – r1 sina yc + r1 cosa 8 xc – r sina yc – r cosa 9 xc – r1 sinß yc – r1 cosß

We first implement a `DrawStar `method to draw a single star polygon at the center position (xc, yc) with a size control parameter r (the radius of the outer circle, as shown in the above figure). We then add a `DrawFlag `method that first draws seven red strips on a white rectangle background. Note that the respect ratio of the flag is maintained by setting:

`float height = 10 * width / 19;`

The method then draws the blue rectangle with proper size. Finally we put fifty stars on the blue rectangle uniformly by calling the `DrawStar `method to finish the project.

## Using the Code

The US flag is really drawn by overriding the `OnPaint `method of the `Form1 `class:

```protected override void OnPaint(PaintEventArgs e)
{
Graphics g = e.Graphics;
g.SmoothingMode = SmoothingMode.AntiAlias;
DrawFlag(g, 20, 20, this.Width - 50);
g.Dispose();
}```

Building and running this project produces the following screenshot:

This is just for fun, perhaps even useful. This project is from the examples of the new book "Practical C# Charts and Graphics", where you can find more advanced chart and graphics programming for real-world .NET applications. For more information, please visit my website.

Dr. Jack Xu has a Ph.D in theoretical physics. He has over 15 years programming experience in Basic, Fortran, C, C++, Matlab, and C#, specializing in numerical computation methods, algorithms, physical modeling, computer-aided design (CAD) development, graphics user interface, and 3D graphics. Currently, he is responsible for developing commercial CAD tools based on Microsoft .NET Framework.

United States
No Biography provided

 First PrevNext
 is this... Giuseppe Tollini 14-Aug-13 0:41
 My vote of 5 manoj kumar choubey 18-Feb-12 3:20
 Drawing Sine Waves vinaykskvs 4-Jun-08 2:51
 Easier way to compute the star points Wyld_One 14-Nov-07 21:32
 Help in graphics programming needed !! Lalito80 19-Jun-07 4:51
 NICE michael_rost 11-Jun-07 9:07
 Your book looks great - I need to order it ASAP Eric Engler 2-Apr-07 10:26
 Re: Your book looks great - I need to order it ASAP Jack JH Xu 2-Apr-07 11:53
 Re: Your book looks great - I need to order it ASAP Eric Engler 2-Apr-07 12:16
 Re: Your book looks great - I need to order it ASAP Paul Selormey 22-Apr-07 18:03
 Re: Your book looks great - I need to order it ASAP Jack JH Xu 23-Apr-07 9:33
 Hi, Paul,   Sorry to hear that. I contacted to the publisher and they told me that the first run of the book copies were sold out in few days mainly due to the bulk purchases from book resellers. Now they got second run out a week ago, so the book is available in stock. They also told me that they have completed all orders for amazon except for orders placed after April 18. They don't know why amazon takes so long to ship your book. You need to ckeck with amazon again because this is not due to the publisher for the delay. Hope you can get your book soon.   Jack   Dr. Jack Xu has a Ph.D in theoretical physics. He has over 15 years programming experience in Basic, FORTRAN, C, C++, Matlab, and C#.
 Re: Your book looks great - I need to order it ASAP Michael Sync 1-Aug-07 21:50
 More generic function for drawing a star DigitalKing 26-Mar-07 19:10
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 20:14
 Re: More generic function for drawing a star Jack JH Xu 26-Mar-07 20:54
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 21:03
 Re: More generic function for drawing a star Jack JH Xu 26-Mar-07 21:43
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 22:04
 Re: More generic function for drawing a star DigitalKing 26-Mar-07 21:24
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 22:02
 Re: More generic function for drawing a star Johann Gerell 26-Mar-07 20:50
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 20:56
 Re: More generic function for drawing a star DigitalKing 26-Mar-07 21:15
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