具有透明背景的UIElement的可写地图(WP和Silverlight)

Question

另一个问题UIElement涉及从a创建图像.然而,我正在尝试制作一个非填充背景保持透明的图像.这个目前在我的代码中完成(不适用于透明度):UIElement

var imageBrush_Final = new ImageBrush
{
    ImageSource = bi,
    Stretch = Stretch.None
};
FinalCanvas.Background.Opacity = 0;
FindChildShieldCanvas_FinalCanvas(FinalCanvas, imageBrush_Final);
FinalCanvas.InvalidateArrange();
FinalCanvas.UpdateLayout();
WriteableBitmap writeableBitmap_Finals = new WriteableBitmap(FinalCanvas, null);
String tempPNG = results.Username + "My_Crest.png";
using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
    using (IsolatedStorageFileStream imageStream = new IsolatedStorageFileStream(tempPNG, System.IO.FileMode.OpenOrCreate, isf))
    {
        await Cimbalino.Toolkit.Extensions.WriteableBitmapExtensions.SavePngAsync(writeableBitmap_Finals, imageStream);
    }
}

创建的图像是canvas内部添加了一个形状的图像.形状填充有ImageBrush,具体地imageBrush_Final.画布仅由涂有的内部形状画家部分填充 imageBrush_Final.因此,我希望创造的writeableBitmap_Finals,只有形状,其余是完全透明的.

有谁知道我怎么能做到这一点,为什么当它是不保存UIElement,即canvas树立了自己的后台Opacity来0？