How to: Copy Pixels for Reducing Flicker in Windows Forms

Code Example

When you animate a simple graphic, users can sometimes encounter flicker or other undesirable visual effects. One way to limit this problem is to use a "bitblt" process on the graphic. Bitblt is the "bit-block transfer" of the color data from an origin rectangle of pixels to a destination rectangle of pixels.

With Windows Forms, bitblt is accomplished using the CopyFromScreen method of the Graphics class. In the parameters of the method, you specify the source and destination (as points), the size of the area to be copied, and the graphics object used to draw the new shape.

In the example below, a shape is drawn on the form in its Paint event handler. Then, the CopyFromScreen method is used to duplicate the shape.

Note
Setting the form's DoubleBuffered property to true will make graphics-based code in the Paint event be double-buffered. While this will not have any discernable performance gains when using the code below, it is something to keep in mind when working with more complex graphics-manipulation code.

Example

Private Sub Form1_Paint(ByVal sender As Object, ByVal e As _
    System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint
    ' Draw a circle with a bar on top.
        e.Graphics.FillEllipse(Brushes.DarkBlue, New Rectangle _
             (10, 10, 60, 60))
        e.Graphics.FillRectangle(Brushes.Khaki, New Rectangle _
             (20, 30, 60, 10))
    ' Copy the graphic to a new location.
        e.Graphics.CopyFromScreen(New Point(10, 10), New Point _
             (100, 100), New Size(70, 70))
End Sub

private void Form1_Paint(System.Object sender,
    System.Windows.Forms.PaintEventArgs e)
        {
        e.Graphics.FillEllipse(Brushes.DarkBlue, new
            Rectangle(10,10,60,60));
        e.Graphics.FillRectangle(Brushes.Khaki, new
            Rectangle(20,30,60,10));
        e.Graphics.CopyFromScreen(new Point(10, 10), new Point(100, 100), 
            new Size(70, 70));
}

Compiling the Code

The code above is run in the form's Paint event handler so that the graphics persist when the form is redrawn. As such, do not call graphics-related methods in the Load event handler, because the drawn content will not be redrawn if the form is resized or obscured by another form.

See Also

Reference

Graphics.FillRectangle Method
Control.OnPaint Method
CopyPixelOperation

Other Resources

Graphics and Drawing in Windows Forms
Using a Pen to Draw Lines and Shapes