Overlay WinForms controls on DirectX content - wpf

Question

I'm trying to create a graphics library with graphics control. The control enables adding overlay controls. The graphics on the control uses DirectX and is part of the library, while the overlay controls are provided by end users of the library which are not graphics experts. Thus stability and convenient API are the most important issues.
For technical reasons I need to show the graphics by DirectX directly on the control and cannot host the DirectX scene using another framework as WPF (For more information see my previous question: Stereoscopic 3D on WPF).
I think the most problematic issues are:
Transparency (and semi-transparency...) within the area of the control itself.
Animations on the overlay control: That's why control rasterization (e.g. by WPF) is not an option.
My tries until now:
Following the lot of posts discussed similar issues I decided to use WS_EX_TRANSPARENT. I was surprised to see that although the Airspace issue, I can see the DirectX content under the transparent regions of the overlay control. However the overlay control was not shown except for when I resized the window - then it blinks and disappears again. Here is the code of the overlay control:
class GDIGraphicsControl : UserControl
{
private const int WS_EX_TRANSPARENT = 0x20;
protected override CreateParams CreateParams
{
get
{
CreateParams p = base.CreateParams;
p.ExStyle |= WS_EX_TRANSPARENT;
return p;
}
}
public GDIGraphicsControl()
{
this.Dock = DockStyle.Fill;
}
protected override void OnPaintBackground(PaintEventArgs e)
{
// Do nothing
}
protected override void OnPaint(PaintEventArgs e)
{
for (int i = 0; i < 10; ++i)
{
int alpha = 25 * i;
int yPos = 10 * i;
e.Graphics.FillRectangle(
new SolidBrush(Color.FromArgb(alpha, Color.Green)),
5, 5 + yPos, 100, 10);
}
e.Graphics.FillEllipse(new SolidBrush(Color.Red), 110, 5, 100, 100);
}
}
To analyze the blinking problem I tried to make things simpler. First for debugging purpose I tried to use GDI rendering instead of DirectX. I implemented it such that it will be very similar to the DirectX rendering - especially the Form.SetStyle and the Invalidate() call in every OnPaint() which I thought to be the cause to the problem. The problem do exist also in GDI-only rendering. Here is the GDI-rendering parent control code:
public partial class RenderingForm : Form
{
public RenderingForm()
{
InitializeComponent();
ControlStyles styles =
ControlStyles.AllPaintingInWmPaint |
ControlStyles.OptimizedDoubleBuffer |
ControlStyles.ResizeRedraw |
ControlStyles.UserPaint |
ControlStyles.Opaque;
this.SetStyle(styles, true);
Button button = new Button()
{
Text = "Just a button",
Left = 5,
Top = 210,
Width = 200
};
this.Controls.Add(button);
this.Controls.Add(new GDIGraphicsControl());
}
protected override void OnPaintBackground(PaintEventArgs e)
{
// Do nothing
}
protected override void OnPaint(PaintEventArgs e)
{
e.Graphics.Clear(Color.Blue);
this.Invalidate();
base.OnPaint(e);
}
}
I tried to add a simple overlay Button to check if the problem exist when no transparency is used (see in the above code). The button doesn't blink, but instead of the button I see garbage on the screen until resizing the control first time - then the button appears correctly.
I tried to invalidate the child overlay controls but it has no effect. I tried to invalidate it both in parent's OnPaint and in the child OnPaint to create the message-pump rendering effect and it has no effect. Setting the control's style as in parent rendering control didn't resolve the problem and caused the background to blink in black.
I also performed some tries using WPF but the question is long enough for now without hard Airspace issue...
Now for the questions:
Can someone explain how does GDI overlay over DirectX graphics work? Especially I don't understand it because I know that WinForms transparency is working by that the children controls render on the parent's Device Context - and DirectX has a hardware rendering context. Does it mean that the DirectX texture is copied back to software?
Why does overriding the CreateParams causes the Control to blink and how to prevent it?
What are performance cost do those transparency methods (overriding CreateParams, BackColorTransparency = True, Control.SetStyle+override OnPainBackground)? It is of high importance that the underlying graphics will be rendered efficiently but I don't care about the overlay performance (except for lightweight animation).

Answer 1

I've done a LOT of research in this area and I've come to the conclusion that Airspace is the way to go. I've collected sources from numerous projects around the web and put them into a single project that allows you to integrate WPF with XNA. I've also done a version of this that was pure DirectX, but to be honest the XNA version is a lot more straightforward and better suited to C#. I don't have enough room to post all the details here but you can download the demo from my website and probably figure out where to go from there. Good luck!