I've looked at a dozen examples (and tried more than a few) and I can't find a way to easily set up a RenderTargetBitmap or WriteableBitmap in WPF that can go on the Windows store.
Eventually I want to directly manipulate an array that I can blt on to the screen at 30 Hz or so.
This example has probably gotten me the closest:
DrawingVisual MyDrawingVisual = new DrawingVisual();
//Open its drawing context:
DrawingContext MyDC = MyDrawingVisual.RenderOpen();
// At this point you can draw
Pen p = new Pen();
p.Thickness = 5;
p.Brush = new SolidColorBrush(Colors.Green);
MyDC.DrawLine(p, new Point(1.0, 1.0), new Point(10.0, 10.0));
RenderTargetBitmap MyRenderTargetBitmap = new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Default);
MyRenderTargetBitmap.Render(MyDrawingVisual);
RenderTargetBitmap rtbm = new RenderTargetBitmap(200, 200, 96, 96, PixelFormats.Pbgra32);
rtbm.Render(MyCanvas);
The example above has 2 problems for me: it doesn't seem to draw anything to the screen, and I think MyDC uses DirectX (which I guess doesn't work with the Windows App Store).
EDIT:
This MS example is exactly what I was looking for!