I've created a simple control that loads an image, and allows a user
to "Pan" this image.  If the image is too large for the screen, the
user is able to drag the image around the screen in order to see the
parts of the image otherwise off the screen.  I'm getting an
incredible amount of flicker in the image when repainting it and I'd
love for some to look over the code below and see if there's something
I can do eliminate the flicker.

Rick

Public Class PanningImage
	Private m_Image As Bitmap
	Private m_LastPoint As Point
	Private m_StartPoint As Point
	Private m_Current As Point

	Public Property Image() As Bitmap
		Get
			Return m_Image
		End Get
		Set(ByVal value As Bitmap)
			m_Image = value
			m_Current.X = 0
			m_Current.Y = 0
			Me.Invalidate()
		End Set
	End Property

	Public Sub New()

		' This call is required by the Windows Form Designer.
		InitializeComponent()

		' Add any initialization after the InitializeComponent() call.
		m_Image = New Bitmap(20, 20)
		m_LastPoint = New Point(0, 0)
		m_Current = New Point(0, 0)
		m_StartPoint = New Point(0, 0)
	End Sub


    Protected Overrides Sub OnPaint(ByVal pe As
System.Windows.Forms.PaintEventArgs)
		Dim rect As New Rectangle
		Dim left As Integer
		Dim top As Integer

		top = m_Current.Y
		left = m_Current.X

		pe.Graphics.Clear(Me.BackColor)
		pe.Graphics.DrawImage(m_Image, left, top)


	End Sub


	Protected Overrides Sub OnPaintBackground(ByVal e As
System.Windows.Forms.PaintEventArgs)

	End Sub


	Private Sub PanningImage_MouseDown(ByVal sender As Object, ByVal e As
System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseDown
		m_StartPoint.X = e.X
		m_StartPoint.Y = e.Y
		m_LastPoint.X = m_Current.X
		m_LastPoint.Y = m_Current.Y
	End Sub


	Private Sub PanningImage_MouseMove(ByVal sender As Object, ByVal e As
System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseMove
		If e.Button = Windows.Forms.MouseButtons.Left Then

			Dim xDistance As Integer = m_StartPoint.X - e.X
			Dim yDistance As Integer = m_StartPoint.Y - e.Y
			Me.m_Current.X = m_LastPoint.X - xDistance
			Me.m_Current.Y = m_LastPoint.Y - yDistance

			If Math.Abs(xDistance) > 3 And Math.Abs(yDistance) > 3 Then
				Me.Invalidate()
			End If
		End If
	End Sub
End Class

On Mon, 16 Jul 2007 20:32:15 -0000, Rick 
wrote:


>I've created a simple control that loads an image, and allows a user
>to "Pan" this image.  If the image is too large for the screen, the
>user is able to drag the image around the screen in order to see the
>parts of the image otherwise off the screen.  I'm getting an
>incredible amount of flicker in the image when repainting it and I'd
>love for some to look over the code below and see if there's something
>I can do eliminate the flicker.


Use offscreen buffering, or double buffering as it's often called.

I've amended your OnPaint, but I don't do VB, so syntactically it'll
be wrong, but you'll get the idea:


Protected Overrides Sub OnPaint(ByVal pe As
System.Windows.Forms.PaintEventArgs)

	Dim rect As New Rectangle
	Dim left As Integer
	Dim top As Integer

	top = m_Current.Y
	left = m_Current.X

	Dim bmpOffscreen As New Bitmap (ClientSize.Width,
ClientSize.Height)
	Dim gfxOffscreen As Graphics.FromImage (bmpOffscreen)

	// Do all drawing to the offscreen version
	gfxOffscreen.Clear(Me.BackColor)
	gfxOffscreen.DrawImage(m_Image, left, top)

	// Then at the last step, copy the offscreen to the onscreen
	// in one go. Therefore the intermediate steps aren't seen
	pe.Graphics.DrawImage(bmpOffscreen, 0, 0)

	// Tidy up these properly
	gfxOffscreen.Dispose()
	bmpOffscreen.Dispose()

End Sub