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PDF image JBIG2 compression in Obj-C (lossy & lossless)

Sample Obj-C 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.
//---------------------------------------------------------------------------------------

#import <OBJC/PDFNetOBJC.h>
#import <Foundation/Foundation.h>

// 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
//
int main(int argc, char *argv[]) 
{
    @autoreleasepool {

        [PTPDFNet Initialize: 0];
        
        @try 
        {
            PTPDFDoc *pdf_doc = [[PTPDFDoc alloc] initWithFilepath: @"../../TestFiles/US061222892-a.pdf"];
            [pdf_doc InitSecurityHandler];

            PTSDFDoc *cos_doc = [pdf_doc GetSDFDoc];
            int num_objs = [cos_doc XRefSize];
        int i=1;
            for(; i<num_objs; ++i) 
            {
                PTObj * obj = [cos_doc GetObj: i];
                if(obj && ![obj IsFree] && [obj IsStream]) 
                {
                    // Process only images
                    PTDictIterator *itr = [obj Find: @"Subtype"];
                    if(![itr HasNext] || !([[[itr Value] GetName] isEqualToString:@"Image"]))
                        continue;
                    
                    PTImage *input_image = [[PTImage alloc] initWithImage_xobject: 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] isEqualToString:@"JBIG2Decode"]) continue;

                    PTFilter *filter=[obj GetDecodedStream];
                    PTFilterReader *reader = [[PTFilterReader alloc] initWithFilter: filter];


                    PTObjSet *hint_set = [[PTObjSet alloc] init]; 	// A hint to image encoder to use JBIG2 compression
                    PTObj * hint=[hint_set CreateArray];
                
                    [hint PushBackName: @"JBIG2"];
                    [hint PushBackName: @"Lossless"];

                    PTImage *new_image = [PTImage CreateWithFilterData: cos_doc image_data: reader width: [input_image GetImageWidth] height: [input_image GetImageHeight] bpc: 1 color_space: [PTColorSpace CreateDeviceGray] encoder_hints: hint];

                    PTObj * new_img_obj = [new_image GetSDFObj];
                    itr = [obj Find: @"Decode"];
                    if([itr HasNext])
                        [new_img_obj Put: @"Decode" obj: [itr Value]];
                    itr = [obj Find: @"ImageMask"];
                    if ([itr HasNext])
                        [new_img_obj Put: @"ImageMask" obj: [itr Value]];
                    itr = [obj Find: @"Mask"];
                    if ([itr HasNext])
                        [new_img_obj Put: @"Mask" obj: [itr Value]];

                    [cos_doc Swap: i obj_num2: [new_img_obj GetObjNum]];
                }
            }

            [pdf_doc SaveToFile: @"../../TestFiles/Output/US061222892_JBIG2.pdf" flags: e_ptremove_unused];
        }
        @catch(NSException *e)
        {
            NSLog(@"%@", e.reason);
            NSLog(@"Please make sure that the pathname to the test file is correct.");
        }
        
        NSLog(@"Done.");

        return 0;
    }
}
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