RenderTargetBitmap and Viewport3D - Quality Issues

Question

I'm wanting to export a 3D scene from a Viewport3D to a bitmap.

The obvious way to do this would be to use RenderTargetBitmap -- however when I this the quality of the exported bitmap is significantly lower than the on-screen image. Looking around on the internet, it seems that RenderTargetBitmap doesn't take advantage of hardware rendering. Which means that the rendering is done at Tier 0. Which means no mip-mapping etc, hence the reduced quality of the exported image.

Does anyone know how to export a bitmap of a Viewport3D at on-screen quality?

Clarification

Though the example given below doesn't show this, I need to eventually export the bitmap of the Viewport3D to a file. As I understand the only way to do this is to get the image into something that derives from BitmapSource. Cplotts below shows that increasing the quality of the export using RenderTargetBitmap improves the image, but as the rendering is still done in software, it is prohibitively slow.

Is there a way to export a rendered 3D scene to a file, using hardware rendering? Surely that should be possible?

You can see the problem with this xaml:

<Window x:Class="RenderTargetBitmapProblem.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="400" Width="500">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Viewport3D Name="viewport3D">
            <Viewport3D.Camera>
                <PerspectiveCamera Position="0,0,3"/>
            </Viewport3D.Camera>
            <ModelVisual3D>
                <ModelVisual3D.Content>
                    <AmbientLight Color="White"/>
                </ModelVisual3D.Content>
            </ModelVisual3D>
            <ModelVisual3D>
                <ModelVisual3D.Content>
                    <GeometryModel3D>
                        <GeometryModel3D.Geometry>
                            <MeshGeometry3D Positions="-1,-10,0  1,-10,0  -1,20,0  1,20,0"
                                            TextureCoordinates="0,1 0,0 1,1 1,0"
                                            TriangleIndices="0,1,2 1,3,2"/>
                        </GeometryModel3D.Geometry>
                        <GeometryModel3D.Material>
                            <DiffuseMaterial>
                                <DiffuseMaterial.Brush>
                                    <ImageBrush ImageSource="http://www.wyrmcorp.com/galleries/illusions/Hermann%20Grid.png"
                                                TileMode="Tile" Viewport="0,0,0.25,0.25"/>
                                </DiffuseMaterial.Brush>
                            </DiffuseMaterial>
                        </GeometryModel3D.Material>
                    </GeometryModel3D>
                </ModelVisual3D.Content>
                <ModelVisual3D.Transform>
                    <RotateTransform3D>
                        <RotateTransform3D.Rotation>
                            <AxisAngleRotation3D Axis="1,0,0" Angle="-82"/>
                        </RotateTransform3D.Rotation>
                    </RotateTransform3D>
                </ModelVisual3D.Transform>
            </ModelVisual3D>
        </Viewport3D>
        <Image Name="rtbImage" Visibility="Collapsed"/>
        <Button Grid.Row="1" Click="Button_Click">RenderTargetBitmap!</Button>
    </Grid>
</Window>

And this code:

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        RenderTargetBitmap bmp = new RenderTargetBitmap((int)viewport3D.ActualWidth, 
            (int)viewport3D.ActualHeight, 96, 96, PixelFormats.Default);
        bmp.Render(viewport3D);
        rtbImage.Source = bmp;
        viewport3D.Visibility = Visibility.Collapsed;
        rtbImage.Visibility = Visibility.Visible;
    }

Answer 1

There is no setting on RenderTargetBitmap to tell it to render using hardware, so you will have to fall back to using Win32 or DirectX. I would recommend using the DirectX technique given in this article. The following code from the article and shows how it can be done (this is C++ code):

extern IDirect3DDevice9* g_pd3dDevice;
Void CaptureScreen()
{
    IDirect3DSurface9* pSurface;
    g_pd3dDevice->CreateOffscreenPlainSurface(ScreenWidth, ScreenHeight,
        D3DFMT_A8R8G8B8, D3DPOOL_SCRATCH, &pSurface, NULL);
    g_pd3dDevice->GetFrontBufferData(0, pSurface);
    D3DXSaveSurfaceToFile("Desktop.bmp",D3DXIFF_BMP,pSurface,NULL,NULL);
    pSurface->Release(); 
}

You can create the Direct3D device corresponding to the place where the WPF content is being rendered as follows:

1. Calling Visual.PointToScreen on a point within your onscreen image
2. Calling MonitorFromPoint in User32.dll to get the hMonitor
3. Calling Direct3DCreate9 in d3d9.dll to get a pD3D
4. Calling pD3D->GetAdapterCount() to count adapters
5. Iterating from 0 to count-1 and calling pD3D->GetAdapterMonitor() and comparing with the previously retrieved hMonitor to determine the adapter index
6. Calling pD3D->CreateDevice() to create the device itself

I would probably do most of this in a separate library coded in C++/CLR because that approach is familiar to me, but you may find it easy to translate it to pure C# and managed code using using SlimDX. I haven't tried that yet.