Double Buffering Windows Forms

 Now contains code in Visual Basic. 

As much as we would like it not to be the case, graphics can be slow enough to watch as the screen is refreshed. Our eyes, sensitive to movement and particularly to edge detection so that we don't walk off of cliffs or run into trees, often pick up the redraw cycle of computer graphics which, at best is mildly annoying or at worst can cause headaches, eyestrain and in susceptible people, even fits.

Much of this effect is caused by the sequential re-drawing of many graphical elements such as a chart with many lines or a game with many moving items. In a perfect world, the sequential redraw would be completely hidden from the user and the completed graphic presented in its entirety. This is essentially the technique used in double buffering.

Windows Forms provides an automatic method of double buffering that can be used by simply setting a few styles in your form or control. For most applications, this is enough but in certain cases, more control over the process is desirable so manually double buffering a control is also possible.

First, take a look at the standard and built in method of double buffering. This is accomplished by setting the styles:

• ControlStyles.AllPaintingInWmPaint
• ControlStyles.UserPaint
• ControlStyles.DoubleBuffer

When these are all set true, the draw process is modified so that instead of your Paint handler being passed a Graphics for the screen, it is passed a Graphics for an in-memory bitmap. When you draw to this Graphics object, you are drawing on an invisible image. At the end of the draw cycle, this bitmap is copied to the main window automatically and the actual pixels you see are all changed in a fraction of a second instead of one at a time as the draw cycle progresses.

To set up automatic double buffering for a Form, you would use the following line of code in the constructor, after the InitializeComponent method call.

C#

this.SetStyle(

  ControlStyles.AllPaintingInWmPaint |

  ControlStyles.UserPaint |

  ControlStyles.DoubleBuffer,true);

VB

me.SetStyle(

  ControlStyles.AllPaintingInWmPaint OR _

  ControlStyles.UserPaint OR _

  ControlStyles.DoubleBuffer,true)

Manual double buffering can be useful if you don't want the system so make assumptions for you such as whether the background is opaque or transparent or perhaps if you want to create a more complex buffering system. There are a few simple rules that you need to follow to get manual double buffering right.

First, don�t create a new back-buffer every draw cycle. Only create or destroy the bitmap when the window's client size changes. Second, only create a bitmap of the size you need. Clearing pixels takes time and so if there are more pixels than you need, you're just wasting processor cycles. Lastly, use the simplest draw method to copy the bitmap to the screen. DrawImageUnscaled is the way to go here.

Before the big demo, listing 1 shows all the salient points of a manual double buffered application.

    private Bitmap _backBuffer;

   

    protected override void OnPaint(PaintEventArgs e)

    {

      if(_backBuffer==null)

      {

        _backBuffer=new Bitmap(this.ClientSize.Width,this.ClientSize.Height);

      }

      Graphics g=Graphics.FromImage(_backBuffer);

     

      //Paint your graphics on g here

     

      g.Dispose();

 

      //Copy the back buffer to the screen

 

      e.Graphics.DrawImageUnscaled(_backBuffer,0,0);

 

      //base.OnPaint (e); //optional but not recommended

    }

 

    protected override void OnPaintBackground(PaintEventArgs pevent)

    {

      //Don't allow the background to paint

    }

 

    protected override void OnSizeChanged(EventArgs e)

    {

      if(_backBuffer!=null)

      {

        _backBuffer.Dispose();

        _backBuffer=null;

      }

      base.OnSizeChanged (e);

    }

 

VB

   Private _backBuffer As Bitmap

 

 

    Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)

      If _backBuffer Is Nothing Then

        _backBuffer = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height)

      End If

 

      Dim g As Graphics = Graphics.FromImage(_backBuffer)

 

      'Paint on the Graphics object here

 

      g.Dispose()

 

      'Copy the back buffer to the screen

      e.Graphics.DrawImageUnscaled(_backBuffer, 0, 0)

    End Sub 'OnPaint

 

    'base.OnPaint (e); //optional but not recommended

 

    Protected Overrides Sub OnPaintBackground(ByVal pevent As PaintEventArgs)

    End Sub 'OnPaintBackground

 

    'Don't allow the background to paint

 

    Protected Overrides Sub OnSizeChanged(ByVal e As EventArgs)

      If Not (_backBuffer Is Nothing) Then

        _backBuffer.Dispose()

        _backBuffer = Nothing

      End If

      MyBase.OnSizeChanged(e)

    End Sub 'OnSizeChanged

 

 

The following listing shows a practical double buffered application. A checkbox enables you to choose whether double buffering is used or if the graphics are drawn directly to the screen for the sake of comparison.

C#

using System;

using System.Drawing;

using System.Drawing.Drawing2D;

using System.Collections;

using System.ComponentModel;

using System.Windows.Forms;

using System.Data;

 

namespace DoubleBuffer

{

  /// <summary>

  /// Summary description for Form1.

  /// </summary>

  public class Form1 : System.Windows.Forms.Form

  {

 

    private System.Windows.Forms.Timer timer1;

    private System.ComponentModel.IContainer components;

    private System.Windows.Forms.CheckBox checkBox1;

 

    float _angle;

    bool _doBuffer;

 

    public Form1()

    {

      //

      // Required for Windows Form Designer support

      //

      InitializeComponent();

 

      //

      // TODO: Add any constructor code after InitializeComponent call

      //

    }

 

    /// <summary>

    /// Clean up any resources being used.

    /// </summary>

    protected override void Dispose( bool disposing )

    {

      if( disposing )

      {

        if (components != null)

        {

          components.Dispose();

        }

      }

      base.Dispose( disposing );

    }

 

    #region Windows Form Designer generated code

    /// <summary>

    /// Required method for Designer support - do not modify

    /// the contents of this method with the code editor.

    /// </summary>

    private void InitializeComponent()

    {

      this.components = new System.ComponentModel.Container();

      this.timer1 = new System.Windows.Forms.Timer(this.components);

      this.checkBox1 = new System.Windows.Forms.CheckBox();

      this.SuspendLayout();

      //

      // timer1

      //

      this.timer1.Enabled = true;

      this.timer1.Tick += new System.EventHandler(this.timer1_Tick);

      //

      // checkBox1

      //

      this.checkBox1.Location = new System.Drawing.Point(8, 8);

      this.checkBox1.Name = "checkBox1";

      this.checkBox1.TabIndex = 0;

      this.checkBox1.Text = "Double Buffer";

      this.checkBox1.CheckedChanged += new System.EventHandler(this.checkBox1_CheckedChanged);

      //

      // Form1

      //

      this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);

      this.ClientSize = new System.Drawing.Size(292, 273);

      this.Controls.Add(this.checkBox1);

      this.Name = "Form1";

      this.Text = "Form1";

      this.ResumeLayout(false);

 

    }

    #endregion

 

    /// <summary>

    /// The main entry point for the application.

    /// </summary>

    [STAThread]

    static void Main()

    {

      Application.Run(new Form1());

    }

 

    private void timer1_Tick(object sender, System.EventArgs e)

    {

      _angle+=3;

      if(_angle>359)

        _angle=0;

      Invalidate();

    }

 

    private Bitmap _backBuffer;

    

    protected override void OnPaint(PaintEventArgs e)

    {

      if(_backBuffer==null)

      {

        _backBuffer=new Bitmap(this.ClientSize.Width,this.ClientSize.Height);

      }

      

      Graphics g=null;

      if(_doBuffer)

        g=Graphics.FromImage(_backBuffer);

      else

        g=e.Graphics;

 

            g.Clear(Color.White);      

 

      g.SmoothingMode=SmoothingMode.AntiAlias;

 

      Matrix mx=new Matrix();

      mx.Rotate(_angle,MatrixOrder.Append);

      mx.Translate(this.ClientSize.Width/2,this.ClientSize.Height/2,MatrixOrder.Append);

      g.Transform=mx;

      g.FillRectangle(Brushes.Red,-100,-100,200,200);

 

      mx=new Matrix();

      mx.Rotate(-_angle,MatrixOrder.Append);

      mx.Translate(this.ClientSize.Width/2,this.ClientSize.Height/2,MatrixOrder.Append);

      g.Transform=mx;

      g.FillRectangle(Brushes.Green,-75,-75,149,149);

    

      mx=new Matrix();

      mx.Rotate(_angle*2,MatrixOrder.Append);

      mx.Translate(this.ClientSize.Width/2,this.ClientSize.Height/2,MatrixOrder.Append);

      g.Transform=mx;

      g.FillRectangle(Brushes.Blue,-50,-50,100,100);

 

      if(_doBuffer)

      {

        g.Dispose();

 

        //Copy the back buffer to the screen

 

        e.Graphics.DrawImageUnscaled(_backBuffer,0,0);

      }

 

      //base.OnPaint (e); //optional but not recommended

    }

 

    protected override void OnPaintBackground(PaintEventArgs pevent)

    {

      //Don't allow the background to paint

    }

 

    protected override void OnSizeChanged(EventArgs e)

    {

      if(_backBuffer!=null)

      {

        _backBuffer.Dispose();

        _backBuffer=null;

      }

      base.OnSizeChanged (e);

    }

 

    private void checkBox1_CheckedChanged(object sender, System.EventArgs e)

    {

      _doBuffer=this.checkBox1.Checked;

    }

  }

}

VB

 

Imports System

Imports System.Drawing

Imports System.Drawing.Drawing2D

Imports System.Collections

Imports System.ComponentModel

Imports System.Windows.Forms

Imports System.Data

 

 

Namespace DoubleBuffer

   '/ <summary>

   '/ Summary description for Form1.

   '/ </summary>

  

   Public Class Form1

    Inherits System.Windows.Forms.Form

    

    Private WithEvents timer1 As System.Windows.Forms.Timer

    Private components As System.ComponentModel.IContainer

    Private WithEvents checkBox1 As System.Windows.Forms.CheckBox

    

    Private _angle As Single

    Private _doBuffer As Boolean

    

    

    Public Sub New()

     '

     ' Required for Windows Form Designer support

     '

     InitializeComponent()

    End Sub 'New

     

    '

    ' TODO: Add any constructor code after InitializeComponent call

    '

    

    '/ <summary>

    '/ Clean up any resources being used.

    '/ </summary>

    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)

      If disposing Then

        If Not (components Is Nothing) Then

          components.Dispose()

        End If

      End If

      MyBase.Dispose(disposing)

    End Sub 'Dispose

 

#Region "Windows Form Designer generated code"

 

    '/ <summary>

    '/ Required method for Designer support - do not modify

    '/ the contents of this method with the code editor.

    '/ </summary>

    Private Sub InitializeComponent()

      Me.components = New System.ComponentModel.Container

      Me.timer1 = New System.Windows.Forms.Timer(Me.components)

      Me.checkBox1 = New System.Windows.Forms.CheckBox

      Me.SuspendLayout()

      '

      ' timer1

      '

      Me.timer1.Enabled = True

      '

      ' checkBox1

      '

      Me.checkBox1.Location = New System.Drawing.Point(8, 8)

      Me.checkBox1.Name = "checkBox1"

      Me.checkBox1.TabIndex = 0

      Me.checkBox1.Text = "Double Buffer"

      '

      ' Form1

      '

      Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)

      Me.ClientSize = New System.Drawing.Size(292, 273)

      Me.Controls.Add(checkBox1)

      Me.Name = "Form1"

      Me.Text = "Form1"

      Me.ResumeLayout(False)

    End Sub 'InitializeComponent

#End Region

 

 

    '/ <summary>

    '/ The main entry point for the application.

    '/ </summary>

    <STAThread()> _

    Shared Sub Main()

      Application.Run(New Form1)

    End Sub 'Main

 

 

    Private Sub timer1_Tick(ByVal sender As Object, ByVal e As System.EventArgs)Handles timer1.Tick

      _angle += 3

      If _angle > 359 Then

        _angle = 0

      End If

      Invalidate()

    End Sub 'timer1_Tick

 

    Private _backBuffer As Bitmap

 

 

    Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)

      If _backBuffer Is Nothing Then

        _backBuffer = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height)

      End If

 

      Dim g As Graphics = Nothing

      If _doBuffer Then

        g = Graphics.FromImage(_backBuffer)

      Else

        g = e.Graphics

      End If

      g.Clear(Color.White)

 

      g.SmoothingMode = SmoothingMode.AntiAlias

 

      Dim mx As New Matrix

      mx.Rotate(_angle, MatrixOrder.Append)

      mx.Translate(Me.ClientSize.Width / 2, Me.ClientSize.Height / 2, MatrixOrder.Append)

      g.Transform = mx

      g.FillRectangle(Brushes.Red, -100, -100, 200, 200)

 

      mx = New Matrix

      mx.Rotate(-_angle, MatrixOrder.Append)

      mx.Translate(Me.ClientSize.Width / 2, Me.ClientSize.Height / 2, MatrixOrder.Append)

      g.Transform = mx

      g.FillRectangle(Brushes.Green, -75, -75, 149, 149)

 

      mx = New Matrix

      mx.Rotate(_angle * 2, MatrixOrder.Append)

      mx.Translate(Me.ClientSize.Width / 2, Me.ClientSize.Height / 2, MatrixOrder.Append)

      g.Transform = mx

      g.FillRectangle(Brushes.Blue, -50, -50, 100, 100)

 

      If _doBuffer Then

        g.Dispose()

 

        'Copy the back buffer to the screen

        e.Graphics.DrawImageUnscaled(_backBuffer, 0, 0)

      End If

    End Sub 'OnPaint

 

    'base.OnPaint (e); //optional but not recommended

 

    Protected Overrides Sub OnPaintBackground(ByVal pevent As PaintEventArgs)

    End Sub 'OnPaintBackground

 

    'Don't allow the background to paint

 

    Protected Overrides Sub OnSizeChanged(ByVal e As EventArgs)

      If Not (_backBuffer Is Nothing) Then

        _backBuffer.Dispose()

        _backBuffer = Nothing

      End If

      MyBase.OnSizeChanged(e)

    End Sub 'OnSizeChanged

 

 

    Private Sub checkBox1_CheckedChanged(ByVal sender As Object, ByVal e AsSystem.EventArgs) Handles checkBox1.CheckedChanged

      _doBuffer = Me.checkBox1.Checked

    End Sub 'checkBox1_CheckedChanged

  End Class 'Form1

End Namespace 'DoubleBuffer

 

 

When running the program looks like this�

 

Figure 1: Double buffering in action.

原文转载地址:http://bobpowell.net/doublebuffer.aspx