Expand canvas/transparent background in BitmapImage in a WPF app

Question

This is a follow up question on Save image to file keeping aspect ration in a WPF app

I know howto scale the image, but how do I expand the canvas size, to ensure the im

Answer 1

Just some code in case others are trying to postprocess files from a form upload.

if (file.PostedFile != null)
{
    //write the new file to disk
    string cachePath = String.Format("{0}temp\\", Request.PhysicalApplicationPath);
    string photoPath = String.Format("{0}temp.png", cachePath);                
    if (!Directory.Exists(cachePath))
{
   Directory.CreateDirectory(cachePath);
}    

file.PostedFile.SaveAs(photoPath);

//resize the new file and save it to disk               
BitmapSource banana = FitImage(ReadBitmapFrame(file.PostedFile.InputStream), 640, 480);
PngBitmapEncoder encoder = new PngBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(banana));
FileStream pngStream = new FileStream(cachePath + "test.png", FileMode.Create);
encoder.Save(pngStream);
pngStream.Close();

//set a couple images on page to the newly uploaded and newly processed files
image.Src = "temp/temp.png";
image.Visible = true;
image2.Src = "temp/test.png"; 


   image2.Visible = true;
}