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The Graphics Device Interface (GDI) is a library of classes that allows an application developer to draw graphics on the screen and work with images, such as bitmaps and metafiles. Most GDI classes are part of the System.Drawing namespace in the .NET Framework. In Visual Studio 2005, GDI has been replaced by an enhance version, called GDI+.

GDI+ allows you to draw vector graphics. A vector graphic is a line or shape drawn on a coordinate system. The first step in drawing a graphic is to retrieve a reference to your canvas. The Paint event of a form or control includes an argument named e, which stores a Graphics object. You will use this object as your canvas.

Before you can draw a shape, you must create a pen. A pen defines the color and line width that will be used to draw the shape. You can create a pen by declaring a variable and instantiating an object of the Pen class. For example, to create a pen that draws a 4-pixel red line, you would use the following code:

Dim redPen As New Pen(Color.Red, 4)

You can then use that pen to draw lines and shapes that are not filled. For example, the following code draws a vertical line from point 30, 20 to point 30, 40.

e.Graphics.DrawLine(redPen, 30, 20, 30, 40)

To draw a rectangle outlined in red, you would use the following code:

e.Graphics.DrawRectangle( redPen, 30, 20, 60, 40)

If you want to draw a filled shape, you will need to use a brush instead of a pen. You can choose from a SolidBrush, which draws in a solid color, a TextureBrush, which draws using a texture defined by a bitmap file, or a LinearGradientBrush, which draws using a defined color gradient.

This is just a brief introduction to GDI and GDI+. GDI+ is a large library with a number of very powerful classes you can use to create and manipulate graphics in your application.

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