GDI+ Tutorial for Beginners

原文 GDI+ Tutorial for Beginners

GDI+ is next evolution of GDI. Using GDI objects in earlier versions of Visual Studio was a pain. In Visual Studio .NET, Microsoft has taken care of most of the GDI problems and have made it easy to use.

GDI+ resides in System.Drawing.dll assembly. All GDI+ classes are reside in theSystem.Drawing, System.Text, System.Printing, System.Internal , System.Imaging,System.Drawing2D and System.Design namespaces.

The first class we must discuss is the Graphics class. After the Graphics class, I will discuss other useful GDI+ classes and structures such as Pen, Brush, and Rectangle. The final part of this tutorial are some examples in C#.

The Graphics Class

The Graphics class encapsulates GDI+ drawing surfaces. Before drawing any object (for example circle, or rectangle) we have to create a surface using Graphics class. Generally we use Paint event of a Form to get the reference of the graphics. Another way is to override OnPaint method.

Here is how you get a reference of the Graphics object:

private void form1_Paint(object sender, PaintEventArgs e) 
{ 
Graphics g = e.Graphics;
}

OR:  

protected override void OnPaint(PaintEventArgs e) 
{
Graphics g = e.Graphics;
}

Once you have the Graphics reference, you can call any of this class's members to draw various objects. Here are some of Graphics class's methods:

In .NET, GDI+ functionality resides in the System.Drawing.dll. Before you start using GDI+ classes, you must add reference to the System.Drawing.dll and import System.Drawing namespace. If you are using Visual Studio .NET for your development, you can add reference to the GDI+ library using following:

Creating this project is simple. Create a Windows application and add reference to the System.Drawing.dll using Project->Add Reference menu item. See Figure 1.

Figure 1. Adding reference to System.Drawing.dll

After that add these two lines.

using System.Drawing;
using System.Drawing.Drawing2D;

Note: If you create a Windows application using VS.NET, you only need write only one line.

using System.Drawing.Drawing2D;

After that add a Form_Paint event handler using the Properties Window. See Figure 2.

Figure 2. Adding Form_Paint event handler.

Note: You can also add Form paint event handler manually described above.

Graphics Objects

After creating a Graphics object, you can use it draw lines, fill shapes, draw text and so on. The major objects are:

The Pen Class

A pen draws a line of specified width and style. You always use Pen constructor to create a pen. The constructor initializes a new instance of the Pen class. You can initialize it with a color or brush.

Initializes a new instance of the Pen class with the specified color.

public Pen(Color);

Initializes a new instance of the Pen class with the specified Brush.

public Pen(Brush);

Initializes a new instance of the Pen class with the specified Brush and width.

public Pen(Brush, float);

Initializes a new instance of the Pen class with the specified Color and Width.

public Pen(Color, float);

Here is one example:

Pen pn = new Pen( Color.Blue ); or Pen pn = new Pen( Color.Blue, 100 );

Some of its most commonly used properties are:

The Color Structure

A Color structure represents an ARGB color. Here are ARGB properties of it:

You can call the Color members. Each color name (say Blue) is a member of the Color structure. Here is how to use a Color structure:

Pen pn = new Pen( Color.Blue );

The Font Class

The Font class defines a particular format for text such as font type, size, and style attributes. You use font constructor to create a font.

Initializes a new instance of the Font class with the specified attributes.

public Font(string, float);

Initializes a new instance of the Font class from the specified existing Font and FontStyle.

public Font(Font, FontStyle);

Where FontStyle is an enumeration and here are its members:

Here is one example:

Graphics g ; 
Font font = new Font("Times New Roman", 26);

Some of its most commonly used properties are:

The Brush Class

The Brush class  is an abstract base class and cannot be instantiated. We always use its derived classes to instantiate a brush object, such as SolidBrush, TextureBrush, RectangleGradientBrush, and LinearGradientBrush.

Here is one example:

LinearGradientBrush lBrush = new LinearGradientBrush(rect, Color.Red, Color.Yellow,LinearGradientMode.BackwardDiagonal);

OR

Brush brsh = new SolidBrush(Color.Red), 40, 40, 140, 140);

The SolidBrush class defines  a brush made up of a single color. Brushes are used to fill graphics shapes such as rectangles, ellipses, pies, polygons, and paths.

The TextureBrush encapsulates a Brush that uses an fills the interior of a shape with an image.

The LinearGradiantBrush encapsulates both two-color gradients and custom multi-color gradients.

The Rectangle Structure

public Rectangle(Point, Size); or public Rectangle(int, int, int, int);

The Rectangle structure is used to draw a rectangle on WinForms. Besides its constructor, the Rectangle structure has following members:

Its constructor initializes a new instance of the Rectangle class. Here is the definition:

public Rectangle(Point, Size); or public Rectangle(int, int, int, int);

The Point Structure

This structure is similar to the POINT structure in C++. It represents an ordered pair of x and y coordinates that define a point in a two-dimensional plane. The member x represents the x coordinates and y represents the y coordinates of the plane.

Here is how to instantiate a point structure:

Point pt1 = new Point( 30, 30); 
Point pt2 = new Point( 110, 100);

Some sample Examples:

Drawing a rectangle

You can override OnPaint event of your form to draw an rectangle. The LinearGradientBrush encapsulates a brush and linear gradient.

protected override void OnPaint(PaintEventArgs pe) 
{ 
Graphics g = pe.Graphics ; 
Rectangle rect = new Rectangle(50, 30, 100, 100); 
LinearGradientBrush lBrush = new LinearGradientBrush(rect, Color.Red, Color.Yellow,LinearGradientMode.BackwardDiagonal); 
g.FillRectangle(lBrush, rect); 
}

The output of the above code looks like Figure 3.

Figure 3. Drawing a rectangle.

Drawing an Arc

DrawArc function draws an arc. This function takes four arguments.

First is the Pen. You create a pen by using the Pen class. The Pen constructor takes at least one argument, the color or the brush of the pen. Second argument width of the pen or brush is optional.

Pen pn = new Pen( Color.Blue ); or Pen pn = new Pen( Color.Blue, 100 );

The second argument is a rectangle. You can create a rectangle by using Rectangle structure. The Rectangle constructor takes four int type arguments and they are left and right corners of the rectangle.

Rectangle rect = new Rectangle(50, 50, 200, 100); 
protected override void OnPaint(PaintEventArgs pe)
{
Graphics g = pe.Graphics ;
Pen pn = new Pen( Color.Blue );
Rectangle rect = new Rectangle(50, 50, 200, 100);
g.DrawArc( pn, rect, 12, 84 );
}

The output looks like this:

Drawing a Line

DrawLine function of the Graphics class draws a line. It takes three parameters, a pen, and two Point class parameters, starting and ending points. Point class  constructor takes x, y arguments.

protected override void OnPaint(PaintEventArgs pe) 
{ 
Graphics g = pe.Graphics ; 
Pen pn = new Pen( Color.Blue ); 
// Rectangle rect = new Rectangle(50, 50, 200, 100); 
Point pt1 = new Point( 30, 30); 
Point pt2 = new Point( 110, 100); 
g.DrawLine( pn, pt1, pt2 ); 
}

The output looks like this:

Drawing an Ellipse

An ellipse( or a circle)  can be drawn by using DrawEllipse method. This method takes only two parameters, Pen and rectangle.

protected override void OnPaint(PaintEventArgs pe) 
{ 
Graphics g = pe.Graphics ; 
Pen pn = new Pen( Color.Blue, 100 ); 
Rectangle rect = new Rectangle(50, 50, 200, 100); 
g.DrawEllipse( pn, rect ); 
}

The output looks like this:

The FillPath

Drawing bazier curves is little more complex than other objects.

protected override void OnPaint(PaintEventArgs pe)
{
Graphics g = pe.Graphics;
g.FillRectangle(new SolidBrush(Color.White), ClientRectangle);
GraphicsPath path = new GraphicsPath(new Point[] {
new Point(40, 140), new Point(275, 200),
new Point(105, 225), new Point(190, 300),
new Point(50, 350), new Point(20, 180), },
new byte[] {
(byte)PathPointType.Start,
(byte)PathPointType.Bezier,
(byte)PathPointType.Bezier,
(byte)PathPointType.Bezier,
(byte)PathPointType.Line,
(byte)PathPointType.Line,
});
PathGradientBrush pgb = new PathGradientBrush(path);
pgb.SurroundColors = new Color[] { Color.Green,Color.Yellow,Color.Red, Color.Blue,
Color.Orange, Color.White, };
g.FillPath(pgb, path);
}

The output looks like this:

Drawing Text and Strings

You can override OnPaint event of your form to draw an rectangle. The LinearGradientBrush encapsulates a brush and linear gradient.

protected override void OnPaint(PaintEventArgs pe)
{
Font fnt = new Font("Verdana", 16);
Graphics g = pe.Graphics;
g.DrawString("GDI+ World", fnt, new SolidBrush(Color.Red), 14,10);
}

The output looks like this: