AddWatermark.cs
//
// This code is part of GrapeCity Documents for PDF samples.
// Copyright (c) GrapeCity, Inc. All rights reserved.
//
using System;
using System.IO;
using System.Drawing;
using System.Numerics;
using GrapeCity.Documents.Pdf;
using GrapeCity.Documents.Text;

namespace GcPdfWeb.Samples
{
    // This sample shows how to add a simple text watermark-like overlay
    // to all pages of an existing PDF.
    public class AddWatermark
    {
        public int CreatePDF(Stream stream)
        {
            var doc = new GcPdfDocument();
            using (var fs = File.OpenRead(Path.Combine("Resources", "PDFs", "SlidePages.pdf")))
            {
                doc.Load(fs);
                foreach (var page in doc.Pages)
                {
                    var g = page.Graphics;

                    // Text layout used to draw the 'watermark':
                    var tl = g.CreateTextLayout();
                    tl.Append("GcPdf Demo");
                    tl.DefaultFormat.Font = Font.FromFile(Path.Combine("Resources", "Fonts", "calibrib.ttf"));
                    tl.DefaultFormat.FontSize = g.Resolution;
                    // Semi-transparent color:
                    tl.DefaultFormat.ForeColor = Color.FromArgb(128, Color.Yellow);
                    tl.DefaultFormat.GlyphAdvanceFactor = 1.5f;
                    tl.PerformLayout();

                    // Rotation angle (radians) - from left/bottom to right/top corners of the page:
                    var angle = -Math.Asin(g.CanvasSize.Width / g.CanvasSize.Height);
                    // Page center:
                    var center = new PointF(g.CanvasSize.Width / 2, g.CanvasSize.Height / 2);
                    // Additional offset from text size:
                    var delta = new PointF(
                        (float)((tl.ContentWidth * Math.Cos(angle) - tl.ContentHeight * Math.Sin(angle)) / 2),
                        (float)((tl.ContentWidth * Math.Sin(angle) + tl.ContentHeight * Math.Cos(angle)) / 2));

                    // Draw watermark text diagonally in the center of the page
                    // (matrix transforms are applied from last to first):
                    g.Transform =
                        Matrix3x2.CreateRotation((float)angle) *
                        Matrix3x2.CreateTranslation(center.X - delta.X, center.Y - delta.Y);

                    g.DrawTextLayout(tl, PointF.Empty);
                    g.Transform = Matrix3x2.Identity;
                }
                doc.Save(stream);
            }
            return doc.Pages.Count;
        }
    }
}