Changing DCT coefficients

Question

I decided to use libjpeg as the main library working with jpeg files. I've read libjpg.txt file. And I was pleased that library allows DCT coefficients reading/writing in a convenient way. Since writing an own decoder will take a long time.

My work is related to the lossless embedding. Currently I need to read DCT coefficients from a file then modify some of them and write changed coefficients in the same file.

Well, I found jpeg_write_coefficients() function. And I naively thought that I could apply it to a decompression object (struct jpeg_decompress_struct). But it does not work and requires a compression object.

I can't believe that such the powerful library is not able to do this. I think that most likely I'm missing something. Although I tried to be attentive. Perhaps the writing coefficients can be done more sophisticated way. But I don't know how to.

I will be very glad if you propose your ideas.

Answer 1

You should really take a look at transupp.h and sources for jpegtran that comes with the library. Anyway, here is my dirty code with comments, assembled partially from jpegtran. It lets you manipulate DCT coefficients one by one.

#include "jpeglib.h"             /* Common decls for cjpeg/djpeg applications */
#include "transupp.h"            /* Support routines for jpegtran */


struct jpeg_decompress_struct srcinfo;
struct jpeg_compress_struct dstinfo;
struct jpeg_error_mgr jsrcerr, jdsterr;

static jpeg_transform_info transformoption; /* image transformation options */
transformoption.transform = JXFORM_NONE;
transformoption.trim = FALSE;
transformoption.force_grayscale = FALSE;

jvirt_barray_ptr * src_coef_arrays;
jvirt_barray_ptr * dst_coef_arrays;

/* Initialize the JPEG decompression object with default error handling. */
srcinfo.err = jpeg_std_error(&jsrcerr);
jpeg_create_decompress(&srcinfo);
/* Initialize the JPEG compression object with default error handling. */
dstinfo.err = jpeg_std_error(&jdsterr);
jpeg_create_compress(&dstinfo);

FILE *fp;

if((fp = fopen(filePath], "rb")) == NULL) {
    //Throw an error
} else {
    //Continue
}

/* Specify data source for decompression */
jpeg_stdio_src(&srcinfo, fp);

/* Enable saving of extra markers that we want to copy */
jcopy_markers_setup(&srcinfo, JCOPYOPT_ALL);

/* Read file header */
(void) jpeg_read_header(&srcinfo, TRUE);

jtransform_request_workspace(&srcinfo, &transformoption);
src_coef_arrays = jpeg_read_coefficients(&srcinfo);
jpeg_copy_critical_parameters(&srcinfo, &dstinfo);

/* Do your DCT shenanigans here on src_coef_arrays like this (I've moved it into a separate function): */

moveDCTAround(&srcinfo, &dstinfo, 0, src_coef_arrays);

/* ..when done with DCT, do this: */
dst_coef_arrays = jtransform_adjust_parameters(&srcinfo, &dstinfo, src_coef_arrays, &transformoption);
fclose(fp);

//And write everything back
fp = fopen(filePath, "wb");

/* Specify data destination for compression */
jpeg_stdio_dest(&dstinfo, fp);

/* Start compressor (note no image data is actually written here) */
jpeg_write_coefficients(&dstinfo, dst_coef_arrays);

/* Copy to the output file any extra markers that we want to preserve */
jcopy_markers_execute(&srcinfo, &dstinfo, JCOPYOPT_ALL);

jpeg_finish_compress(&dstinfo);
jpeg_destroy_compress(&dstinfo);
(void) jpeg_finish_decompress(&srcinfo);
jpeg_destroy_decompress(&srcinfo);

fclose(fp);

And the function itself:

void moveDCTAround (j_decompress_ptr srcinfo, j_compress_ptr dstinfo, JDIMENSION x_crop_offset, jvirt_barray_ptr *src_coef_arrays)
{
    size_t block_row_size;
    JBLOCKARRAY coef_buffers[MAX_COMPONENTS];
    JBLOCKARRAY row_ptrs[MAX_COMPONENTS];

    //Allocate DCT array buffers
    for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
    {
        coef_buffers[compnum] = (dstinfo->mem->alloc_barray)((j_common_ptr) dstinfo, JPOOL_IMAGE, srcinfo->comp_info[compnum].width_in_blocks,
         srcinfo->comp_info[compnum].height_in_blocks);
    }

    //For each component,
    for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
    {
        block_row_size = (size_t) sizeof(JCOEF)*DCTSIZE2*srcinfo->comp_info[compnum].width_in_blocks;
        //...iterate over rows,
        for (JDIMENSION rownum=0; rownum<srcinfo->comp_info[compnum].height_in_blocks; rownum++)
        {
            row_ptrs[compnum] = ((dstinfo)->mem->access_virt_barray)((j_common_ptr) &dstinfo, src_coef_arrays[compnum], rownum, (JDIMENSION) 1, FALSE);
            //...and for each block in a row,
            for (JDIMENSION blocknum=0; blocknum<srcinfo->comp_info[compnum].width_in_blocks; blocknum++)
                //...iterate over DCT coefficients
                for (JDIMENSION i=0; i<DCTSIZE2; i++)
                {
                        //Manipulate your DCT coefficients here. For instance, the code here inverts the image.
                        coef_buffers[compnum][rownum][blocknum][i] = -row_ptrs[compnum][0][blocknum][i];
                }
        }
    }

    //Save the changes
    //For each component,
    for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
    {
        block_row_size = (size_t) sizeof(JCOEF)*DCTSIZE2 * srcinfo->comp_info[compnum].width_in_blocks;
        //...iterate over rows
        for (JDIMENSION rownum=0; rownum < srcinfo->comp_info[compnum].height_in_blocks; rownum++)
        {
            //Copy the whole rows
            row_ptrs[compnum] = (dstinfo->mem->access_virt_barray)((j_common_ptr) dstinfo, src_coef_arrays[compnum], rownum, (JDIMENSION) 1, TRUE);
            memcpy(row_ptrs[compnum][0][0], coef_buffers[compnum][rownum][0], block_row_size);
        }
    }

Answer 2

You can ue jpeg_write_coefficients to write your changed DCT.

The following information is avaliable in libjpeg.txt

To write the contents of a JPEG file as DCT coefficients, you must provide
the DCT coefficients stored in virtual block arrays.  You can either pass
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
allocate virtual arrays from the JPEG compression object and fill them
yourself.  In either case, jpeg_write_coefficients() is substituted for
jpeg_start_compress() and jpeg_write_scanlines().  Thus the sequence is
  * Create compression object
  * Set all compression parameters as necessary
  * Request virtual arrays if needed
  * jpeg_write_coefficients()
  * jpeg_finish_compress()
  * Destroy or re-use compression object
jpeg_write_coefficients() is passed a pointer to an array of virtual block
array descriptors; the number of arrays is equal to cinfo.num_components.

The virtual arrays need only have been requested, not realized, before
jpeg_write_coefficients() is called.  A side-effect of
jpeg_write_coefficients() is to realize any virtual arrays that have been
requested from the compression object's memory manager.  Thus, when obtaining
the virtual arrays from the compression object, you should fill the arrays
after calling jpeg_write_coefficients().  The data is actually written out
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
the file header.

When writing raw DCT coefficients, it is crucial that the JPEG quantization
tables and sampling factors match the way the data was encoded, or the
resulting file will be invalid.  For transcoding from an existing JPEG file,
we recommend using jpeg_copy_critical_parameters().  This routine initializes
all the compression parameters to default values (like jpeg_set_defaults()),
then copies the critical information from a source decompression object.
The decompression object should have just been used to read the entire
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().

jpeg_write_coefficients() marks all tables stored in the compression object
as needing to be written to the output file (thus, it acts like
jpeg_start_compress(cinfo, TRUE)).  This is for safety's sake, to avoid
emitting abbreviated JPEG files by accident.  If you really want to emit an
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
individual sent_table flags, between calling jpeg_write_coefficients() and
jpeg_finish_compress().

So to change a single dct, you can use the following simple code: To access any dct coeff, you need to change four index, cx, bx, by, bi. In my code, I used blockptr_one[bi]++; to increase one dct Coeff

#include <stdio.h>
#include <jpeglib.h>
#include <stdlib.h>
#include <iostream>
#include <string>    

int write_jpeg_file(std::string outname,jpeg_decompress_struct in_cinfo, jvirt_barray_ptr *coeffs_array ){

    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;
    FILE * infile;

    if ((infile = fopen(outname.c_str(), "wb")) == NULL) {
      fprintf(stderr, "can't open %s\n", outname.c_str());
      return 0;
    }

    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_compress(&cinfo);
    jpeg_stdio_dest(&cinfo, infile);

    j_compress_ptr cinfo_ptr = &cinfo;
    jpeg_copy_critical_parameters((j_decompress_ptr)&in_cinfo,cinfo_ptr);
    jpeg_write_coefficients(cinfo_ptr, coeffs_array);

    jpeg_finish_compress( &cinfo );
    jpeg_destroy_compress( &cinfo );
    fclose( infile );

    return 1;
}

int read_jpeg_file( std::string filename, std::string outname )
{
    struct jpeg_decompress_struct cinfo;
    struct jpeg_error_mgr jerr;
    FILE * infile;

    if ((infile = fopen(filename.c_str(), "rb")) == NULL) {
      fprintf(stderr, "can't open %s\n", filename.c_str());
      return 0;
    }

    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_decompress(&cinfo);
    jpeg_stdio_src(&cinfo, infile);
    (void) jpeg_read_header(&cinfo, TRUE);


    jvirt_barray_ptr *coeffs_array = jpeg_read_coefficients(&cinfo);

    //change one dct:
    int ci = 0; // between 0 and number of image component
    int by = 0; // between 0 and compptr_one->height_in_blocks
    int bx = 0; // between 0 and compptr_one->width_in_blocks
    int bi = 0; // between 0 and 64 (8x8)
    JBLOCKARRAY buffer_one;
    JCOEFPTR blockptr_one;
    jpeg_component_info* compptr_one;
    compptr_one = cinfo.comp_info + ci;
    buffer_one = (cinfo.mem->access_virt_barray)((j_common_ptr)&cinfo, coeffs_array[ci], by, (JDIMENSION)1, FALSE);
    blockptr_one = buffer_one[0][bx];
    blockptr_one[bi]++;

    write_jpeg_file(outname, cinfo, coeffs_array);

    jpeg_finish_decompress( &cinfo );
    jpeg_destroy_decompress( &cinfo );
    fclose( infile );

    return 1;


}

int main()
{
    std::string infilename = "you_image.jpg", outfilename = "out_image.jpg";

    /* Try opening a jpeg*/
    if( read_jpeg_file( infilename, outfilename ) > 0 )
    {
        std::cout << "It's Okay..." << std::endl;
    }
    else return -1;
    return 0;
}