Reg saving rotated image from image control in WPF

Hello,

I have rotated image by some angle by applying transform to image control as shown below.

RotateTransform rt = newRotateTransform(angleNew);
image1.RenderTransform = rt;
image1 is image control.
How can I save this rotated image? Tried below code, but, it is saving original image without rotation.
var encoder = new PngBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create((BitmapSource)image1.source));
using (FileStream stream = new FileStream("D:\\TestImage.png", FileMode.Create))
      encoder.Save(stream);
             

Harish