transparent controls issue

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From: ds (dave_at_petcominc.com)
Date: 04/20/04 

Date: Tue, 20 Apr 2004 12:36:10 -0500

 have 2 layers of related graphics that I want to display. The bottom layer
is relatively expensive to render and stays static. The top layer displays
some user-orchestrated animation, directed via the mouse- similar to a
drag-and-drop operation.

I've tried laying one control over the other. The top control is
transparent. When I call Invalidate() in the TopControl, BottomControl
repaints also. Basically, I'm hoping there is a way to continually update
the top layer without having to redraw the bottom layer.

Any suggestions would be appreciated.

public class TopControl : UserControl
TopControl()
{
  SetStyle(ControlStyles.SupportsTranparentBackColor,true);
  BackColor = Color.Transparent;
}
protected override void OnPaint(PaintEventArgs e)
{
Console.WriteLine("top paint");
  // some drawing code
}
protected override void OnMouseUp(MouseEventArgs e)
{
// collect some rendering data...
Invalidate();
}
}

public class BottomControl : UserControl
{
BottomControl()
{
}
protected override OnPaint(PaintEventArgs e)
{
Console.WriteLine("bottom paint");
// expensive drawing code
}
}

Main()
{
...
   Form f = new Form();
   f.ResizeRedraw = true;

  BottomControl bc = new BottomControl();
  bc.Dock = DockStyle.Fill;
  bc.Parent = this;

  TopControl tc = new TopControl();
  tc.Dock = DockStyle.Fill;
  tc.Parent = bc;

  f.Show();
...
}

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