How to Convert RenderTargetBitmap to WriteableBitmap

var bitmap = new RenderTargetBitmap();
await bitmap.RenderAsync(Cam); //Cam is your Gird
var pixelBuffer = await bitmap.GetPixelsAsync();
byte[] pixels = pixelBuffer.ToArray();
var wb = new WriteableBitmap((int) bitmap.PixelWidth, (int) bitmap.PixelHeight);
using(Stream stream = wb.PixelBuffer.AsStream())
{
    await stream.WriteAsync(pixels, 0, pixels.Length);
}