How to edit a WritableBitmap.BackBuffer in non UI thread?

Question

My application runs CPU-heavy algorythms to edit an Image placed at a WPF window. I need the edition to be done in a background thread. However trying to edit the BackBuffer

Answer 1

I implemented the following, based on this answer:

In the view model, there is a property like this, that is bound to the Image source in XAML:

private WriteableBitmap cameraImage;
private IntPtr cameraBitmapPtr;
public WriteableBitmap CameraImage
{
    get { return cameraImage; }
    set
    {
        cameraImage = value;
        cameraBitmapPtr = cameraImage.BackBuffer;
        NotifyPropertyChanged();
    }
}

Using a property means that if the WritableBitmap changes, e.g. because of resolution, it would be updated in the View and also a new IntPtr will be constructed.

The image is constructed when appropriate:

CameraImage = new WriteableBitmap(2448, 2048, 0, 0, PixelFormats.Bgr24, null);

In the update thread, a new image is copied in, e.g. using:

[DllImport("kernel32.dll", EntryPoint = "RtlMoveMemory")]
public static extern void CopyMemory(IntPtr Destination, IntPtr Source, uint Length);

you would do

CopyMemory(cameraImagePtr, newImagePtr, 2448 * 2048 * 3);

There might be a better function for this...

In the same thread, after the copy:

parent.Dispatcher.Invoke(new Action(() =>
{
    cameraImage.Lock();
    cameraImage.AddDirtyRect(new Int32Rect(0, 0, cameraImage.PixelWidth, cameraImage.PixelHeight));
    cameraImage.Unlock();
}), DispatcherPriority.Render);

where parent is the Control / Window with the Image.