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PDF image JBIG2 compression in PHP (lossy & lossless)

Sample PHP 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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<?php
//---------------------------------------------------------------------------------------
// Copyright (c) 2001-2019 by PDFTron Systems Inc. All Rights Reserved.
// Consult LICENSE.txt regarding license information.
//---------------------------------------------------------------------------------------
include("../../../PDFNetC/Lib/PDFNetPHP.php");

// Relative path to the folder containing the test files.
$input_path = getcwd()."/../../TestFiles/";
$output_path = $input_path."Output/";

// 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 = new PDFDoc("../../TestFiles/US061222892-a.pdf") ;
	$pdf_doc->InitSecurityHandler();

	$cos_doc = $pdf_doc->GetSDFDoc();
	$num_objs = $cos_doc->XRefSize();
	for($i = 1; $i < $num_objs; ++$i) 
	{
		$obj = $cos_doc->GetObj($i);
		if($obj && !$obj->IsFree() && $obj->IsStream()) 
		{
			// Process only images
			$itr = $obj->Find("Subtype");
			if(!$itr->HasNext() || $itr->Value()->GetName() != "Image")
				continue;
			
			$input_image = new Image($obj);
			// Process only gray-scale images
			if($input_image->GetComponentNum() != 1)
				continue;
			$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() == "JBIG2Decode") continue; 

			$filter=$obj->GetDecodedStream();
			$reader = new FilterReader($filter);


			$hint_set = new ObjSet(); 	// A 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());
			$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("../../TestFiles/Output/US061222892_JBIG2.pdf", SDFDoc::e_remove_unused);
	$pdf_doc->Close();

	echo "Done.";
?>
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