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PDF image JBIG2 compression in Kotlin (lossy & lossless)

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Sample Kotlin 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 Kotlin PDF Library.

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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.FilterReader
import com.pdftron.pdf.ColorSpace
import com.pdftron.pdf.Image
import com.pdftron.pdf.PDFDoc
import com.pdftron.sdf.ObjSet
import com.pdftron.sdf.SDFDoc
import java.util.*

//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.

class JBIG2Test : PDFNetSample() {
    init {
        setTitle(R.string.sample_jbig_title)
        setDescription(R.string.sample_jbig_description)
    }

    override fun run(outputListener: OutputListener?) {
        super.run(outputListener)
        mOutputListener = outputListener
        mFileList.clear()
        printHeader(outputListener!!)

        try {
            val pdf_doc = PDFDoc(Utils.getAssetTempFile(PDFNetSample.INPUT_PATH + "US061222892-a.pdf")!!.absolutePath)
            pdf_doc.initSecurityHandler()

            val cos_doc = pdf_doc.sdfDoc
            val num_objs = cos_doc.xRefSize().toInt()
            for (i in 1 until num_objs) {
                val obj = cos_doc.getObj(i.toLong())
                if (obj != null && !obj.isFree && obj.isStream) {
                    // Process only images
                    var itr = obj.find("Subtype")
                    if (!itr.hasNext() || itr.value().name != "Image")
                        continue

                    val input_image = Image(obj)
                    // Process only gray-scale images
                    if (input_image.componentNum != 1)
                        continue
                    val bpc = input_image.bitsPerComponent
                    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().name != "JBIG2Decode")
                        continue

                    val filter = obj.decodedStream
                    val reader = FilterReader(filter)

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

                    val new_image = Image.create(cos_doc, reader,
                            input_image.imageWidth,
                            input_image.imageHeight, 1, ColorSpace.createDeviceGray(), hint)

                    val new_img_obj = new_image.sdfObj
                    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.toLong(), new_img_obj.objNum)
                }
            }

            pdf_doc.save(Utils.createExternalFile("US061222892_JBIG2.pdf", mFileList).absolutePath, SDFDoc.SaveMode.REMOVE_UNUSED, null)
            pdf_doc.close()
        } catch (e: Exception) {
            mOutputListener!!.printError(e.stackTrace)
        }

        for (file in mFileList) {
            addToFileList(file)
        }
        printFooter(outputListener)
    }

    companion object {

        private var mOutputListener: OutputListener? = null

        private val mFileList = ArrayList<String>()
    }

}