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PDF image JBIG2 compression in Go (lossy & lossless)

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Sample Go 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 Go PDF Library.

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//---------------------------------------------------------------------------------------
// Copyright (c) 2001-2021 by PDFTron Systems Inc. All Rights Reserved.
// Consult LICENSE.txt regarding license information.
//---------------------------------------------------------------------------------------

package main
import (
	. "pdftron"
)

import  "pdftron/Samples/LicenseKey/GO"

// 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.
//
// You can download the entire document using the following link:
//   http://www.pdftron.com/net/samplecode/data/US061222892.pdf
// Relative path to the folder containing the test files.
var inputPath = "../../TestFiles/"
var outputPath = "../../TestFiles/Output/"

func main(){
    PDFNetInitialize(PDFTronLicense.Key)
    
    pdfDoc := NewPDFDoc(inputPath + "US061222892-a.pdf")
    pdfDoc.InitSecurityHandler()
    
    cosDoc := pdfDoc.GetSDFDoc()
    numObjs := cosDoc.XRefSize()
    
    i := uint(1)
    for i < numObjs{
        obj := cosDoc.GetObj(i)
        if obj != nil && ! obj.IsFree() && obj.IsStream(){
            // Process only images
            itr := obj.Find("Subtype")
            //if not itr.HasNext() or not itr.Value().GetName() == "Image":
            if !itr.HasNext() || !(itr.Value().GetName() == "Image"){
                i = i + 1
                continue
            }
            inputImage := NewImage(obj)
            // Process only gray-scale images
            if inputImage.GetComponentNum() != 1{
                i = i + 1
                continue
            }
            // Skip images that are already compressed using JBIG2
            itr = obj.Find("Filter")
            if (itr.HasNext() && itr.Value().IsName() && itr.Value().GetName() == "JBIG2Decode"){
                i = i + 1
                continue
            }

            filter := obj.GetDecodedStream()
            reader := NewFilterReader(filter)
            
            hintSet := NewObjSet()     // hint to image encoder to use JBIG2 compression
            hint := hintSet.CreateArray()
            
            hint.PushBackName("JBIG2")
            hint.PushBackName("Lossless")
            
            newImage := (ImageCreate(cosDoc, reader, 
                                     inputImage.GetImageWidth(), 
                                     inputImage.GetImageHeight(), 
                                     1, 
                                     ColorSpaceCreateDeviceGray(), 
                                     hint))
            
            newImgObj := newImage.GetSDFObj()
            itr = obj.Find("Decode")
            
            if itr.HasNext(){
                newImgObj.Put("Decode", itr.Value())
            }
            itr = obj.Find("ImageMask")
            if itr.HasNext(){
                newImgObj.Put("ImageMask", itr.Value())
            }
            itr = obj.Find("Mask")
            if itr.HasNext(){
                newImgObj.Put("Mask", itr.Value())
            }

            cosDoc.Swap(i, newImgObj.GetObjNum())
        }
        i = i + 1
    }

    pdfDoc.Save(outputPath + "US061222892_JBIG2.pdf", uint(SDFDocE_remove_unused))
    pdfDoc.Close()                
    PDFNetTerminate()
}