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Posted 26 Mar 2007

Draw a US Flag using C# and GDI+

, 7 Dec 2007
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This article shows how to draw graphics objects using C# and GDI+

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.

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 First Prev Next
 Nice Job! Member 1252262925-May-16 23:07 Member 12522629 25-May-16 23:07
 Nice Job! Member 1252262925-May-16 23:07 Member 12522629 25-May-16 23:07
 great azimi632317-Apr-14 22:23 azimi6323 17-Apr-14 22:23
 is this... Giuseppe Tollini14-Aug-13 1:41 Giuseppe Tollini 14-Aug-13 1:41
 My vote of 5 manoj kumar choubey18-Feb-12 4:20 manoj kumar choubey 18-Feb-12 4:20
 Drawing Sine Waves vinaykskvs4-Jun-08 3:51 vinaykskvs 4-Jun-08 3:51
 How does a graphics object representing a flag relate to 'the flag' ? OrlandoCurioso7-Dec-07 13:39 OrlandoCurioso 7-Dec-07 13:39
 Easier way to compute the star points Wyld_One14-Nov-07 22:32 Wyld_One 14-Nov-07 22:32
 Help in graphics programming needed !! Lalito8019-Jun-07 5:51 Lalito80 19-Jun-07 5:51
 NICE michael_rost11-Jun-07 10:07 michael_rost 11-Jun-07 10:07
 Your book looks great - I need to order it ASAP Eric Engler2-Apr-07 11:26 Eric Engler 2-Apr-07 11:26
 Re: Your book looks great - I need to order it ASAP Jack JH Xu2-Apr-07 12:53 Jack JH Xu 2-Apr-07 12:53
 Re: Your book looks great - I need to order it ASAP Eric Engler2-Apr-07 13:16 Eric Engler 2-Apr-07 13:16
 Re: Your book looks great - I need to order it ASAP Paul Selormey22-Apr-07 19:03 Paul Selormey 22-Apr-07 19:03
 Re: Your book looks great - I need to order it ASAP Jack JH Xu23-Apr-07 10:33 Jack JH Xu 23-Apr-07 10:33
 Re: Your book looks great - I need to order it ASAP Michael Sync1-Aug-07 22:50 Michael Sync 1-Aug-07 22:50
 Re: Your book looks great - I need to order it ASAP Eric Engler6-Aug-07 6:19 Eric Engler 6-Aug-07 6:19
 More generic function for drawing a star DigitalKing26-Mar-07 20:10 DigitalKing 26-Mar-07 20:10
 Re: More generic function for drawing a star Paul Selormey26-Mar-07 21:14 Paul Selormey 26-Mar-07 21:14
 Re: More generic function for drawing a star Jack JH Xu26-Mar-07 21:54 Jack JH Xu 26-Mar-07 21:54
 Re: More generic function for drawing a star Paul Selormey26-Mar-07 22:03 Paul Selormey 26-Mar-07 22:03
 Re: More generic function for drawing a star Jack JH Xu26-Mar-07 22:43 Jack JH Xu 26-Mar-07 22:43
 Re: More generic function for drawing a star Paul Selormey26-Mar-07 23:04 Paul Selormey 26-Mar-07 23:04
 Re: More generic function for drawing a star DigitalKing26-Mar-07 22:24 DigitalKing 26-Mar-07 22:24
 Re: More generic function for drawing a star Paul Selormey26-Mar-07 23:02 Paul Selormey 26-Mar-07 23:02
 Re: More generic function for drawing a star Johann Gerell26-Mar-07 21:50 Johann Gerell 26-Mar-07 21:50
 Re: More generic function for drawing a star Paul Selormey26-Mar-07 21:56 Paul Selormey 26-Mar-07 21:56
 Re: More generic function for drawing a star DigitalKing26-Mar-07 22:15 DigitalKing 26-Mar-07 22:15
 You are missing 2 stars Gerard Nicol26-Mar-07 17:59 Gerard Nicol 26-Mar-07 17:59
 Re: You are missing 2 stars Paul Selormey26-Mar-07 18:06 Paul Selormey 26-Mar-07 18:06
 Re: You are missing 2 stars gucci26-Mar-07 21:15 gucci 26-Mar-07 21:15
 Re: You are missing 2 stars ednrgc27-Mar-07 8:31 ednrgc 27-Mar-07 8:31
 Re: You are missing 2 stars Greg Russell2-Apr-07 5:41 Greg Russell 2-Apr-07 5:41
 Re: You are missing 2 stars tec-goblin18-Jun-07 22:58 tec-goblin 18-Jun-07 22:58
 Re: You are missing 2 stars Pavel Vladov2-Aug-07 0:26 Pavel Vladov 2-Aug-07 0:26
 Re: You are missing 2 stars tec-goblin2-Aug-07 0:32 tec-goblin 2-Aug-07 0:32
 Re: You are missing 2 stars Marcus Deecke22-Jun-07 14:50 Marcus Deecke 22-Jun-07 14:50
 Nice work... Paul Selormey26-Mar-07 15:37 Paul Selormey 26-Mar-07 15:37
 Re: Nice work... Jack JH Xu26-Mar-07 22:17 Jack JH Xu 26-Mar-07 22:17
 Re: Nice work... Paul Selormey26-Mar-07 22:56 Paul Selormey 26-Mar-07 22:56
 Re: Nice work... Jack JH Xu29-Mar-07 10:35 Jack JH Xu 29-Mar-07 10:35
 Re: Nice work... Paul Selormey30-Mar-07 7:42 Paul Selormey 30-Mar-07 7:42
 Re: Nice work... [modified] Jack JH Xu30-Mar-07 8:49 Jack JH Xu 30-Mar-07 8:49
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