# Draw a US Flag using C# and GDI+

, 7 Dec 2007 CPOL
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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
 great azimi6323 17-Apr-14 22:23
 is this... Giuseppe Tollini 14-Aug-13 1:41
 My vote of 5 manoj kumar choubey 18-Feb-12 4:20
 Drawing Sine Waves vinaykskvs 4-Jun-08 3:51
 Easier way to compute the star points Wyld_One 14-Nov-07 22:32
 Help in graphics programming needed !! Lalito80 19-Jun-07 5:51
 NICE michael_rost 11-Jun-07 10:07
 Your book looks great - I need to order it ASAP Eric Engler 2-Apr-07 11:26
 Re: Your book looks great - I need to order it ASAP Jack JH Xu 2-Apr-07 12:53
 Re: Your book looks great - I need to order it ASAP Eric Engler 2-Apr-07 13:16
 Re: Your book looks great - I need to order it ASAP Paul Selormey 22-Apr-07 19:03
 Re: Your book looks great - I need to order it ASAP Jack JH Xu 23-Apr-07 10:33
 Re: Your book looks great - I need to order it ASAP Michael Sync 1-Aug-07 22:50
 More generic function for drawing a star DigitalKing 26-Mar-07 20:10
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 21:14
 Nice, you can get ride of the the "types" to make it a bit more efficient... ```GraphicsPath path = new GraphicsPath(); path.AddPolygon(points); return path; ```   BTW, what ratio of inner_radius to radius gives the "best" or "standard" apperance?   Best regards, Paul.     Jesus Christ is LOVE! Please tell somebody.
 Re: More generic function for drawing a star Jack JH Xu 26-Mar-07 21:54
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 22:03
 Re: More generic function for drawing a star Jack JH Xu 26-Mar-07 22:43
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 23:04
 Re: More generic function for drawing a star DigitalKing 26-Mar-07 22:24
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 23:02
 Re: More generic function for drawing a star Johann Gerell 26-Mar-07 21:50
 Re: More generic function for drawing a star Paul Selormey 26-Mar-07 21:56
 Re: More generic function for drawing a star DigitalKing 26-Mar-07 22:15
 You are missing 2 stars Gerard Nicol 26-Mar-07 17:59
 Re: You are missing 2 stars Paul Selormey 26-Mar-07 18:06
 Re: You are missing 2 stars gucci 26-Mar-07 21:15
 Re: You are missing 2 stars ednrgc 27-Mar-07 8:31
 Re: You are missing 2 stars Greg Russell 2-Apr-07 5:41
 Re: You are missing 2 stars tec-goblin 18-Jun-07 22:58
 Re: You are missing 2 stars Pavel Vladov 2-Aug-07 0:26
 Re: You are missing 2 stars tec-goblin 2-Aug-07 0:32
 Re: You are missing 2 stars Marcus Deecke 22-Jun-07 14:50
 Nice work... Paul Selormey 26-Mar-07 15:37
 Re: Nice work... Jack JH Xu 26-Mar-07 22:17
 Re: Nice work... Paul Selormey 26-Mar-07 22:56
 Re: Nice work... Jack JH Xu 29-Mar-07 10:35
 Re: Nice work... Paul Selormey 30-Mar-07 7:42
 Re: Nice work... [modified] Jack JH Xu 30-Mar-07 8:49
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