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PDF image JBIG2 compression in Ruby (lossy & lossless)

Sample Ruby 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 LICENSE.txt regarding license information.
#---------------------------------------------------------------------------------------

require '../../../PDFNetC/Lib/PDFNetRuby'
include PDFNetRuby

$stdout.sync = true

# 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

	PDFNet.Initialize
	
	pdf_doc = PDFDoc.new("../../TestFiles/US061222892-a.pdf")
	pdf_doc.InitSecurityHandler
	
	cos_doc = pdf_doc.GetSDFDoc
	num_objs = cos_doc.XRefSize
	
	i = 1
	while i < num_objs do
		obj = cos_doc.GetObj(i)
		if !obj.nil? and !obj.IsFree and obj.IsStream
			# Process only images
			itr = obj.Find("Subtype")
			if !itr.HasNext or !itr.Value.GetName == "Image"
				i = i + 1
				next
			end
			
			input_image = Image.new(obj)
			# Process only gray-scale images
			if input_image.GetComponentNum != 1
				i = i + 1
				next
			end			

			# Skip images that are already compressed using JBIG2
			itr = obj.Find("Filter")
			if itr.HasNext and itr.Value.IsName and itr.Value.GetName == "JBIG2Decode"
				i = i + 1
				next
			end
			
			filter = obj.GetDecodedStream
			reader = FilterReader.new(filter)
			
			hint_set = ObjSet.new	 # hint to image encoder to use JBIG2 compression
			hint = hint_set.CreateArray
			
			hint.PushBackName("JBIG2")
			hint.PushBackName("Lossless")
			
			new_image = Image.Create(cos_doc, reader, 
						input_image.GetImageWidth, 
						input_image.GetImageHeight, 
						1, 
						ColorSpace.CreateDeviceGray, 
						hint)
			
			new_img_obj = new_image.GetSDFObj
			itr = obj.Find("Decode")
			
			if itr.HasNext
				new_img_obj.Put("Decode", itr.Value)
			end
			itr = obj.Find("ImageMask")
			if itr.HasNext
				new_img_obj.Put("ImageMask", itr.Value)
			end
			itr = obj.Find("Mask")
			if itr.HasNext
				new_img_obj.Put("Mask", itr.Value)
			end
				
			cos_doc.Swap(i, new_img_obj.GetObjNum)
		end
		i = i + 1
	end
			
	pdf_doc.Save("../../TestFiles/Output/US061222892_JBIG2.pdf", SDFDoc::E_remove_unused)
	pdf_doc.Close
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