How to create annotation using PdfSharp to highlight text of existing PDF

问题

I would like know whether it is possible to create an text highlight annotation in an existing PDF using PdfSharp?

In the PdfSharp documentation, I see examples of PdfTextAnnotation & PdfRubberStampAnnotation, but did not find sample code for following annotations mentioned in the documentation.

PdfLineAnnotation, PdfSquareAnnotation, PdfCircleAnnotation, 
PdfMarkupAnnotation, PdfHighlightAnnotation, PdfUnderlineAnnotation,
PdfSquigglyAnnotation, PdfSoundAnnotation, PdfMovieAnnotation.

Are these annotations yet to be implemented in PdfSharp? If someone has already implemented, please point me to the code samples.

回答1:

I also faced with such problem and I didn't find any additional annotation types at the PdfSharp library. So I looked at the article "12.5 Annotations" from the PDF Reference. For example, it says that to create a text markup annotation a developer need to specify the Subtype and QuadPoints entries. Please see the source code below.

namespace PdfSharp.Pdf.Annotations
{
    using Extensions;
    using System.Collections.Generic;

    public class PdfHighlightAnnotation : PdfMarkupAnnotation
    {
        public PdfHighlightAnnotation()
        {
            Initialize();
        }

        public PdfHighlightAnnotation(PdfDocument document)
            : base(document)
        {
            Initialize();
        }

        void Initialize()
        {
            Elements.SetName(Keys.Subtype, "/Highlight");
        }
    }

    public class PdfStrikeOutAnnotation : PdfMarkupAnnotation
    {
        public PdfStrikeOutAnnotation()
        {
            Initialize();
        }

        public PdfStrikeOutAnnotation(PdfDocument document)
            : base(document)
        {
            Initialize();
        }

        void Initialize()
        {
            Elements.SetName(Keys.Subtype, "/StrikeOut");
        }
    }

    public abstract class PdfMarkupAnnotation : PdfAnnotation
    {
        protected PdfMarkupAnnotation()
        { }

        protected PdfMarkupAnnotation(PdfDocument document)
            : base(document)
        { }

        public IEnumerable<PdfRectangle> Quadrilaterals
        {
            set {
                var points = new PdfArray();
                foreach (var r in value) {
                    points.Elements.AddRange(ToQuadPoints(r));
                }
                Elements.SetValue("/QuadPoints", points);
            }
        }

        private IEnumerable<PdfItem> ToQuadPoints(PdfRectangle r)
        {
            // Conversion from PdfRectangle coordinates
            //
            // Y ^
            //   |                     (X2 Y2)
            //   |        +-----------+
            //   |        |           |
            //   |        |           |
            //   |        +-----------+
            //   | (X1 Y1)
            //   |                              
            //   +-----------------------------> 
            //                                 X
            // to QuadPoints coordinates (x1 y1 x2 y2 x3 y3 x4 y4)
            //
            // Y ^
            //   | (x4 y4)             (x3 y3)
            //   |        +-----------+
            //   |        |           |
            //   |        |           |
            //   |        +-----------+
            //   | (x1 y1)             (x2 y2)
            //   |                              
            //   +-----------------------------> 
            //                                 X
            //
            return new List<PdfItem> { new PdfReal(r.X1), new PdfReal(r.Y1),
                                       new PdfReal(r.X2), new PdfReal(r.Y1),
                                       new PdfReal(r.X1), new PdfReal(r.Y2),
                                       new PdfReal(r.X2), new PdfReal(r.Y2)};
        }
    }
}

namespace PdfSharp.Extensions
{
    using PdfSharp.Pdf;
    using System.Collections.Generic;

    public static class ArrayElementsExtensions
    {
        public static void AddRange(this PdfArray.ArrayElements elements, IEnumerable<PdfItem> values)
        {
            foreach (var v in values) {
                elements.Add(v);
            }
        }
    }
}

来源：https://stackoverflow.com/questions/43413097/how-to-create-annotation-using-pdfsharp-to-highlight-text-of-existing-pdf