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PDF image JBIG2 compression in C# (lossy & lossless)

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Sample C# code for using PDFTron SDK to recompress bitonal (black and white) images in existing PDF documents using JBIG2 compression (lossless or lossy). The sample is intended to show how to specify hint information for the image encoder and is not meant to be a generic PDF optimization tool. To demonstrate the possible compression rates, we recompressed a document containing 17 scanned pages. The original input document is ~1.4MB and is using standard CCITT Fax compression. Lossless JBIG2 compression shrunk the filesize to 641KB, while lossy JBIG2 compression shrunk it to 176KB. Learn more about our C# PDF Library.

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//
// Copyright (c) 2001-2020 by PDFTron Systems Inc. All Rights Reserved.
//

using System;
using System.IO;
using System.Threading.Tasks;
using Windows.Foundation;

using pdftron.PDF;
using pdftron.SDF;

using pdftron.Common;
using PDFNetUniversalSamples.ViewModels;
using pdftron.Filters;
using Windows.Storage.Streams;

namespace PDFNetSamples
{
    /// <summary>
    /// This sample project illustrates how to recompress bi-tonal images in an 
    /// existing PDF document using JBIG2 compression. The sample is not intended 
    /// to be a generic PDF optimization tool.
    ///
    /// Also a sample page compressed using CCITT Fax compression is located under 
    /// 'PDFNet/Samples/TestFiles' folder.
    /// </summary>
    class JBIG2Test : Sample
    {
        private string FILE_NAME = "US061222892-a.pdf";

        public JBIG2Test() :
            base("JBIG2", "This sample recompress bi-tonal images in an existing PDF document using JBIG2 compression.")
        { }

        public override IAsyncAction RunAsync()
        {
            return Task.Run(new System.Action(async () =>
           {
               WriteLine("--------------------------------");
               WriteLine("Starting JBIG2 Compression Test...");
               WriteLine("--------------------------------\n");

               PDFDoc pdf_doc = new PDFDoc(Path.Combine(InputPath, FILE_NAME));
               pdf_doc.InitSecurityHandler();

               SDFDoc cos_doc = pdf_doc.GetSDFDoc();

               int num_objs = cos_doc.XRefSize();

               // Loop through all cross reference table objects
               for (int i = 1; i < num_objs; ++i)
               {
                   Obj obj = cos_doc.GetObj(i);
                   if (obj != null && !obj.IsFree() && obj.IsStream())
                   {
                       // Process only images
                       DictIterator itr = obj.Find("Subtype");
                       if (!itr.HasNext() || itr.Value().GetName() != "Image")
                           continue;

                       pdftron.PDF.Image input_image = new pdftron.PDF.Image(obj);
                       pdftron.PDF.Image new_image = null;

                       // Process only gray-scale images
                       if (input_image.GetComponentNum() != 1)
                           continue;

                       int bpc = input_image.GetBitsPerComponent();
                       if (bpc != 1) // Recompress 1 BPC images
                           continue;

                       // Skip images that are already compressed using JBIG2
                       itr = obj.Find("Filter");
                       if (itr.HasNext() && itr.Value().IsName() &&
                           itr.Value().GetName() == "JBIG2Decode")
                           continue;

                       FilterReader reader = new FilterReader(obj.GetDecodedStream());

                       ObjSet hint_set = new ObjSet();

                       Obj hint = hint_set.CreateArray();
                       hint.PushBackName("JBIG2");
                       hint.PushBackName("Lossless");
                       hint.PushBackName("Threshold");
                       hint.PushBackNumber(0.4);
                       hint.PushBackName("SharePages");
                       hint.PushBackNumber(10000);

                       new_image = pdftron.PDF.Image.Create(
                           cos_doc,
                           reader,
                           input_image.GetImageWidth(),
                           input_image.GetImageHeight(),
                           1,
                           ColorSpace.CreateDeviceGray(),
                           hint);

                       Obj new_img_obj = new_image.GetSDFObj();

                       // Copy any important entries from the image dictionary
                       itr = obj.Find("ImageMask");
                       if (itr.HasNext()) new_img_obj.Put("ImageMask", itr.Value());

                       itr = obj.Find("Mask");
                       if (itr.HasNext()) new_img_obj.Put("Mask", itr.Value());

                       cos_doc.Swap(i, new_image.GetSDFObj().GetObjNum());
                   }
               }

               await pdf_doc.SaveAsync(Path.Combine(OutputPath, FILE_NAME), SDFDocSaveOptions.e_remove_unused).AsTask().ConfigureAwait(false);

               WriteLine("Saved " + Path.Combine(OutputPath, FILE_NAME));

               await AddFileToOutputList(Path.Combine(OutputPath, FILE_NAME));

               WriteLine("--------------------------------");
               WriteLine("Done JBIG2 Compression Test.");
               WriteLine("--------------------------------\n");

           })).AsAsyncAction();
        }
    }
}
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