Writeablebitmap from UIElement with transparent background (WP and Silverlight)

Question

Creating an Image from an UIElement is covered in another question. I am however trying to make an image of an UIElement that has the non-filled background kept as transparent. This is currently done in my code like this (NOT working for transparency):

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);
    }
}

The image is created is of a canvas that has one shape added inside. The shape is filled with an ImageBrush, specifically imageBrush_Final. The canvas is only partially filled by the internal shape painter that has been painted with imageBrush_Final. I would therefore like that the created writeableBitmap_Finals, is only filled with the shape and the rest is fully transparent.

Does anyone know how I can achieve this, and why is it not saved when the UIElement, i.e. canvas has set its background to Opacity to 0?

Underwent

why would you want to change the elements opacity to zero and render the element? – Doble

@ChiragShah I had to render the element for the constructor to work properly. The opacity I inserted was to try and force the black background to be transparent. But did not work. If you have another idea I can try let me know :) – Underwent

So you are not getting transparent color in your rendered image? I guess saving it As png could be the problem. Anyway, ill try the code. – Doble

@ChiragShah no I get no transparency, it is just black. But all other colors are correct, so I did not suspect any image channel issue. Great thx! – Underwent

I'm confused if you still want me to try the code. – Doble

@ChiragShah if you want to I would appreciate it. Since I do not know how to get the transparency. – Underwent

Might want to look here: #16154045 – Bismuthic