Stripchart Scrolling
Written by Mike James   
Thursday, 03 September 2009
Article Index
Stripchart Scrolling
ScrollWindowEx

Using ScrollWindowEx

One solution to the problem is to use a direct API call to ScrollWindowEx to scroll the picturebox's display directly.

This has the disadvantage that you have go outside of a managed environment but it is relatively safe. There are some real problems, however. The ScrollWindowEx call scrolls the display bitmap which isn't persistent and in fact if you cause the picturebox to redraw itself at any point you will loose the scrolling and revert to either its background image or its initial state.

This non-persistent effect is one reason why many programmers think that ScrollWindowEx doesn't work - because they never see it for long enough! So we are going to have to be careful not to trigger a redraw of the picturebox and we can't use a bitmap to draw on.

Add a new button to the form and add to the static MyExtensions class:

public static void Scroll(
this PictureBox BP, int dx, int dy)
{
 ScrollWindowEx(BP.Handle,
dx, dy, null, null,
IntPtr.Zero, null, 2);
}

This time the extension method has been added directly to the PictureBox. We also need the definition of the API call and the RECT structure it uses:

[DllImport("user32.dll")]
private static extern int
ScrollWindowEx(
System.IntPtr hWnd,
int dx,
int dy,
[MarshalAs(UnmanagedType.LPStruct)]
RECT prcScroll,
[MarshalAs(UnmanagedType.LPStruct)]
RECT prcClip,
System.IntPtr hrgnUpdate,
[MarshalAs(UnmanagedType.LPStruct)]
RECT prcUpdate,
System.UInt32 flags);

[StructLayout(LayoutKind.Sequential)]
public class RECT
{
public Int32 left;
public Int32 top;
public Int32 right;
public Int32 bottom;
}

Don't forget to add:

using System.Runtime.InteropServices;

The second button's click handler starts off in roughly the same way:

private void button2_Click(
object sender, EventArgs e)
{
Graphics G =
pictureBox1.CreateGraphics();
for (int x = 0; x < 10000; x++)
{
int y = (int)(Math.Sin((double)x / 50) *
 pictureBox1.ClientSize.Height / 2 +
pictureBox1.ClientSize.Height / 2);
G.DrawLine(Pens.Blue, 1, y, 1, y + 1);

Notice that now we don't use a bitmap but draw directly onto the picturebox's Graphics object.  All that remains is to use the scroll method:

  pictureBox1.Scroll(1, 0);
Application.DoEvents();
}

Finally we need to clean up the Graphics object created:

 G.Dispose();
}

It may only be a single Graphics object but not cleaning up still means a memory leak.

Now if you try it out and compare the two plots you will discover that the API based one works much faster. The difference is a factor of more than 100 and very worth the effort.  The only problem is that the API scroll isn't persistent so when the display is completed covering up the final plot erases it.

One final question remains - would this be better done/easier to do using WPF? But this is another story.

If you would like the code for this project then register and click on CodeBin.

<ASIN:9812791035>

<ASIN:0979372518>

<ASIN:0735623945>



Last Updated ( Thursday, 03 September 2009 )
 
 

   
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