flickering when clearing and repainting graphics object

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Hi,
i'm trying to draw a filled ellipse on the screen according to a sensor signal which is read in via RS232. Currently I use a ReceivedData Handler to call my draw-method in which i first clear the form and then draw the ellipse according to the current signal value. This results in exessive flickering.
From reading through posts concerning flicker effects, I'm not sure if this way is how I should do it.
Any suggestions of how this should be done?

Thanks a lot in advance!

Michael

Posted 13-Dec-11 3:20am

Member 8404471

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Solution 2

The advice by Griff may be not enough to avoid flicker in all situations; and this technique is not always needed.

In many or even in most cases you need to setup double buffering. Consider you have a control class. You really need to create a class as you need the method System.Windows.Forms.Control.SetStyle, which is protected, see http://msdn.microsoft.com/en-us/library/system.windows.forms.control.setstyle.aspx[^].

You will need to add the styles System.Windows.Forms.ControlStyles.AllPaintingInWmPaint.OptimizedDoubleBuffer | System.Windows.Forms.ControlStyles., see http://msdn.microsoft.com/en-us/library/system.windows.forms.controlstyles.aspx[^].

However, if your control is Form, you can simply its property DoubleBuffered, see http://msdn.microsoft.com/en-us/library/system.windows.forms.control.doublebuffered.aspx[^].

—SA

Posted 13-Dec-11 5:23am

Sergey Alexandrovich Kryukov

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Member 8404471 14-Dec-11 4:04am

Thanks a lot, but I tried it now with and without using the DoubleBuffer Option, but it doesn't make a difference in my case.

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OriginalGriff · Accepted Answer · 2011-12-13T03:30:00

Don't do the clear!
Instead of drawing on your form, create a simple Panel class:

C#

public class MyPanel : Panel
    {
    protected override void OnPaintBackground(PaintEventArgs e)
        {
        // Prevent background clearing
        }
    }

The only purpose of this is to disable background clearing.
Add the MyPanel to your form.
Now draw your ellipse on the panel, in the Paint event.
What I would do is to create a region which is the elipse:

C#

GraphicsPath path = new GraphicsPath();
Rectangle rect = new Rectangle(0, 0, 100, 100);
path.AddEllipse(rect);
Region myClearBit = new Region();
myClearBit.MakeEmpty();
myClearBit.Union(path);
e.Graphics.ExcludeClip(myClearBit);
e.Graphics.FillRectangle(Brushes.Green,e.Graphics.ClipBounds);
e.Graphics.SetClip(myClearBit, CombineMode.Replace);
e.Graphics.FillPath(Brushes.Red, path);

This will draw a red circle on a green background, and shouldn't flicker as it re-sizes.