Question

I followed this code project tutorial for using SharpGL with WPF to create a control which allows you to render OpenGL into it. Excellent, everything works fine.

However, I am rendering video behind the SharpGL WPF control and so need the backdrop of the render to be transparent. How can I go about this?

Code:

<Window x:Class="MyProject.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sharpGL="clr-namespace:SharpGL.WPF;assembly=SharpGL.WPF"
    Title="MainWindow" Height="600" Width="800" Loaded="Window_Loaded">
    <Canvas Height="580" Width="780" Name="CentreCanvas">
        <Image Canvas.Left="0" Canvas.Top="0" Height="565" Name="imageVideoControl" Stretch="Fill" Width="780" />
        <Image Canvas.Left="580" Canvas.Top="0" Height="150" Name="imageDepthControl" Stretch="Fill" Width="200" />
        <sharpGL:OpenGLControl OpenGLDraw="OpenGLControl_OpenGLDraw" Height="565" Width="780"></sharpGL:OpenGLControl>
    . . . 
    </Canvas>
</Window>

private void OpenGLControl_OpenGLDraw(object sender, OpenGLEventArgs args)
{
    // Get the OpenGL instance being pushed to us.
    gl = args.OpenGL;

    // Clear the colour and depth buffers.
    gl.Clear(OpenGL.GL_COLOR_BUFFER_BIT | OpenGL.GL_DEPTH_BUFFER_BIT);

    // Reset the modelview matrix.
    gl.LoadIdentity();

    // Move the geometry into a fairly central position.
    gl.Translate(1.5f, 0.0f, -6.0f);

    // Draw a quad.
    gl.Begin(OpenGL.GL_QUADS);

        // Set colour to red.
        gl.Color(1.0f, 0.0f, 0.0f);

        // Draw some quad...
        . . . 
        . . .

    gl.End();
}

Answer 1

You don’t.

OpenGL windows are not WPF windows. They are Win32 HWNDs. And HWNDs run by OpenGL cannot participate in window-to-window transparency.