Exporting an image from a DirectX 2D Image in Windows 8 C#/XAML Using SharpDX

PART 2

I’m working on a Windows 8 app which has a drawing element to it. The original WP7 app in Silverlight was fine because a simple drawing rendered in XAML was captured using WritableBitmap.Render and written to a file stream. This feature is missing from the Windows 8 API, so it’s impossible to write a pure C#/XAML app which can export a user created image. To get round this it’s possible to use DirectX/XAML/C++ to create a XAML UI which can draw DirectX images.

DirectX/XAML/C++ offers a number of XAML containers which interface with DirectX and have different behaviours:

http://msdn.microsoft.com/en-us/library/windows/apps/hh825871.aspx

http://blogs.msdn.com/b/windowsappdev/archive/2012/03/15/combining-xaml-and-directx.aspx

I wanted to use a SurfaceImageSource, so I could have my DirectX image in line with other XAML controls and containers.

The big problem is that I don’t want to write a whole app in C++, it would take me a lot longer than C# and it seemed like a sledge hammer to crack a nut using full C++/DirectX for a simple application.

So after much research and stress about what to do, I found SharpDX and thought all my prayers had been answered! SharpDX is a full DirectX to C# interop library which allows DirectX to be brought into C#/XAML apps in the same way as C++/XAML apps.

http://sharpdx.org

The major problem was how to save the image you’ve drawn out of the DirectX image? There are a small number of examples of how to do it in C++, so after a lot of cross referencing the SharpDX source and tips from Alexandre Mutel on interfacing with a WICStream, I managed to get it working.

This save method was tested inside the EffectRenderer in the SharpDX lighting effect sample. This is the whole code so you don’t need to paste it back together in sections:

public async void Save()

{

    // Launch file picker

    Windows.Storage.Pickers.FileSavePicker picker = new Windows.Storage.Pickers.FileSavePicker();

    picker.FileTypeChoices.Add("PNG", new List<string>() { ".png" });

    var file = await picker.PickSaveFileAsync();

    if (file == null)

        return;

    // Get stream

    var fStream = await file.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite);

    var factory = new ImagingFactory();

    MemoryStream ms = new MemoryStream(100000);

    ms.Position = 0;

    // Create a WIC outputstream

    WICStream stream = new WICStream(factory, ms);

    var size = this._deviceManager.ContextDirect2D.PixelSize;

    var format = new SharpDX.Direct2D1.PixelFormat(SharpDX.DXGI.Format.B8G8R8A8_UNorm, AlphaMode.Premultiplied);

    // Initialize a Png encoder with this stream

    var wicBitmapEncoder = new SharpDX.WIC.PngBitmapEncoder(factory, BitmapEncoderGuids.Png);

    wicBitmapEncoder.Initialize(stream);

    // Create a Frame encoder

    var wicFrameEncoder = new BitmapFrameEncode(wicBitmapEncoder);

    wicFrameEncoder.Initialize();

    // Create image encoder

    ImageEncoder wicImageEncoder;

    SharpDX.WIC.ImagingFactory2 factory2 = new ImagingFactory2();

    factory2.CreateImageEncoder(this._deviceManager.DeviceDirect2D, out wicImageEncoder);

    var imgParams = new ImageParameters();

    imgParams.PixelFormat = format;

    imgParams.PixelHeight = (int)size.Height;

    imgParams.PixelWidth = (int)size.Width;

    wicImageEncoder.WriteFrame(_effectGraph.Output, wicFrameEncoder, ref imgParams);

    // Commit changes

    wicFrameEncoder.Commit();

    wicBitmapEncoder.Commit();

    byte[] buffer = new byte[ms.Length];

    ms.Position = 0;

    await ms.ReadAsync(buffer, 0, (int)ms.Length);

    var opStream = fStream.AsStreamForWrite();

    await opStream.WriteAsync(buffer, 0, buffer.Length);

    await opStream.FlushAsync();

    // Dispose

    wicFrameEncoder.Dispose();

    wicBitmapEncoder.Dispose();

    stream.Dispose();

    ms.Dispose();

    opStream.Dispose();    

    factory.Dispose();

}

The first section gets a file stream from the user to write the data to once it’s been extracted from the image, then creates an intermediary MemoryStream for the WICStream to interface with (Alexandre pointed out that the Win8 RandomAccessStream will not interface with a WICStream since it needs to perform seek operations) and finally the WICStream itself which WIC will use to write the image to.

The second section creates a Bitmap Encoder, Frame Encoder and Image Encoder and writes the image frame into the stream.

The final section and the bit which caused me a lot of problems, is getting the data from the stream. The encoders all need committing in order, but in many C++ examples the WICStream is also committed which should flush the stream to it’s destination, however it was throwing an exception:

An exception of type 'SharpDX.SharpDXException' occurred in SharpDX.DLL but was not handled in user code

Additional information: HRESULT: [0x88982F50], Module: [Unknown], ApiCode: [Unknown/Unknown], Message: The component cannot be found.

If there is a handler for this exception, the program may be safely continued.

This isn’t particularly helpful, but I was advised that the WIC stream doesn’t need flushing as the underlying stream should take care of this. At this point I could see the memory stream had a size, but when I wrote it to a buffer it was coming out all zeros. This was a simple mistake – the memory stream just needs setting back to the start and it all started magically working!

All I need to do now is work out how to use DirectX to draw my picture! I hope this helps some other people as it’s been causing me a massive headache for a number of days!