I am using a RenderTargetBitmap. I have a piece of code like this

using (var context = WorldImage.CreateDrawingContext()) {
    Pen pen = new Pen(Brushes.Black);
    context.DrawLine(pen, previousPoint, currentPoint);
}

The purpose of this code is two draw a curve by pairs of points. However, it doesn’t work because it clears the image before each draw. How to disable this clearing?