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PDF image JBIG2 compression in Java (Android) (lossy & lossless)

Sample Java (Android) 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.

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//---------------------------------------------------------------------------------------
// Copyright (c) 2001-2019 by PDFTron Systems Inc. All Rights Reserved.
// Consult legal.txt regarding legal and license information.
//---------------------------------------------------------------------------------------

package com.pdftron.android.pdfnetsdksamples.samples;

import com.pdftron.android.pdfnetsdksamples.OutputListener;
import com.pdftron.android.pdfnetsdksamples.PDFNetSample;
import com.pdftron.android.pdfnetsdksamples.R;
import com.pdftron.android.pdfnetsdksamples.util.Utils;
import com.pdftron.filters.Filter;
import com.pdftron.filters.FilterReader;
import com.pdftron.pdf.ColorSpace;
import com.pdftron.pdf.Image;
import com.pdftron.pdf.PDFDoc;
import com.pdftron.sdf.DictIterator;
import com.pdftron.sdf.Obj;
import com.pdftron.sdf.ObjSet;
import com.pdftron.sdf.SDFDoc;

import java.util.ArrayList;

//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.

public class JBIG2Test extends PDFNetSample {

	private static OutputListener mOutputListener;

	private static ArrayList<String> mFileList = new ArrayList<>();

    public JBIG2Test() {
        setTitle(R.string.sample_jbig_title);
        setDescription(R.string.sample_jbig_description);
    }

	@Override
	public void run(OutputListener outputListener) {
		super.run(outputListener);
		mOutputListener = outputListener;
		mFileList.clear();
		printHeader(outputListener);

        try {
            PDFDoc pdf_doc = new PDFDoc(Utils.getAssetTempFile(INPUT_PATH + "US061222892-a.pdf").getAbsolutePath());
            pdf_doc.initSecurityHandler();

            SDFDoc cos_doc = pdf_doc.getSDFDoc();
            int num_objs = (int) cos_doc.xRefSize();
            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().equals("Image"))
                        continue;

                    Image input_image = new Image(obj);
                    // Process only gray-scale images
                    if (input_image.getComponentNum() != 1)
                        continue;
                    int bpc = input_image.getBitsPerComponent();
                    if (bpc != 1)    // Recompress only 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().equals("JBIG2Decode")) continue;

                    Filter filter = obj.getDecodedStream();
                    FilterReader reader = new FilterReader(filter);

                    ObjSet hint_set = new ObjSet();
                    Obj hint = hint_set.createArray(); // A hint to image encoder to use JBIG2 compression
                    hint.pushBackName("JBIG2");
                    hint.pushBackName("Lossless");

                    Image new_image = Image.create(cos_doc, reader,
                            input_image.getImageWidth(),
                            input_image.getImageHeight(), 1, ColorSpace.createDeviceGray(), hint);

                    Obj new_img_obj = new_image.getSDFObj();
                    itr = obj.find("Decode");
                    if (itr.hasNext())
                        new_img_obj.put("Decode", itr.value());
                    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_img_obj.getObjNum());
                }
            }

            pdf_doc.save(Utils.createExternalFile("US061222892_JBIG2.pdf", mFileList).getAbsolutePath(), SDFDoc.SaveMode.REMOVE_UNUSED, null);
            pdf_doc.close();
        } catch (Exception e) {
            mOutputListener.printError(e.getStackTrace());
        }

		for (String file : mFileList) {
			addToFileList(file);
		}
		printFooter(outputListener);
	}


}
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