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Posted 26 Mar 2007

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# Draw a US Flag using C# and GDI+

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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:

C#
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:

C#
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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 FirstPrev Next
 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 Juergen Gutsch26-Mar-07 21:15 Juergen Gutsch 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 J. H. Xu26-Mar-07 22:17 Jack J. H. 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 J. H. Xu29-Mar-07 10:35 Jack J. H. 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 J. H. Xu30-Mar-07 8:49 Jack J. H. Xu 30-Mar-07 8:49
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