create watermark in powerpoint interop c#

Question

I have been trying to create watermark in powerpoint I have a code below where I could add picture now how do I create transparency for picture so it looks like watermark

private void watermark_Click(object sender, RibbonControlEventArgs e)
{
    PowerPoint.Application ppApp = Globals.ThisAddIn.Application;
    PowerPoint.SlideRange ppslr = ppApp.ActiveWindow.Selection.SlideRange;
    //ppApp.ActivePresentation.Slides.InsertFromFile("NepaSlide.pptx",2, 1,1);
    //PowerPoint.ShapeRange ppShR = ppApp.ActiveWindow.Selection.ShapeRange;
    int count= ppslr.Shapes.Count;

    PowerPoint.Shape shape = ppslr.Shapes[count];

    ppslr.Shapes.AddPicture("N-symbol.png",
            Microsoft.Office.Core.MsoTriState.msoFalse,
            Microsoft.Office.Core.MsoTriState.msoTrue,
            shape.Left, shape.Top, shape.Width, shape.Height);     
}

Answer 1

I know this is an old question but I didn't find a solution so I wrote the code myself.

public void AddWaterMarkToPowerPoint(string filePath)
        {
            string waterMarkText = "Top secret";

            PowerPoint.Application ppApp = new PowerPoint.Application();

            PowerPoint.Presentations pres = ppApp.Presentations;


            PowerPoint.Presentation pptPresentation = pres.Open(filePath, MsoTriState.msoTrue, MsoTriState.msoTrue, MsoTriState.msoFalse);


            for (int i = 1; i <= pptPresentation.Slides.Count; i++)
            {
                var test = pptPresentation.Slides[i].CustomLayout.Shapes.AddTextbox(MsoTextOrientation.msoTextOrientationHorizontal, 200, 200, 600, 100);

                test.TextFrame.TextRange.Text = waterMarkText;
                test.Rotation = -45;
                test.TextFrame.TextRange.Font.Color.RGB = Color.LightGray.ToArgb();
                test.TextFrame.TextRange.Font.Size = 48;

            }

            pptPresentation.SaveAs(filePath);

            pptPresentation.Close();



        }

This code adds text to every slide in presentation. PowerPoint doesn't have the ability to add waterMark so we have to manufacture one by adding a light grey text.