Декодирование JPEG

Это можно сделать с помощью libjpeg.

Пример (отсюда: Extracting JPEG DCT coefficients):

#include <stdio.h>
#include <jpeglib.h>
#include <iostream>
using namespace std;
JBLOCKARRAY rowPtrs[MAX_COMPONENTS];
void read(jpeg_decompress_struct srcinfo, jvirt_barray_ptr * src_coef_arrays) {
    //cout << "Started reading DCT" << endl;
    for (JDIMENSION compNum=0; compNum < srcinfo.num_components; compNum++) {
        size_t blockRowSize = (size_t) sizeof(JCOEF) * DCTSIZE2 * srcinfo.comp_info[compNum].width_in_blocks;
        for (JDIMENSION rowNum=0; rowNum < srcinfo.comp_info[compNum].height_in_blocks; rowNum++) {
            // A pointer to the virtual array of dct values
            rowPtrs[compNum] = ((&srcinfo)->mem->access_virt_barray)((j_common_ptr) &srcinfo, src_coef_arrays[compNum],rowNum, (JDIMENSION) 1, FALSE);
            // Loop through the blocks to get the dct values
            for (JDIMENSION blockNum=0; blockNum < srcinfo.comp_info[compNum].width_in_blocks; blockNum++){
                //...iterate over DCT coefficients
                for (JDIMENSION i=0; i<DCTSIZE2; i++){
                    //and print them to standard out - one per line
                    cout << rowPtrs[compNum][0][blockNum][i] << endl;
                }
            }
        }
    }
}
int main() {
  //TODO: change this to read a different file
  const char* filename = "image4.jpg";
  FILE * infile;  
  struct jpeg_decompress_struct srcinfo;
  struct jpeg_error_mgr srcerr;
  if ((infile = fopen(filename, "rb")) == NULL) {
    fprintf(stderr, "can't open %s\n", filename);
    return 0;
  }
  srcinfo.err = jpeg_std_error(&srcerr);
  jpeg_create_decompress(&srcinfo);
  jpeg_stdio_src(&srcinfo, infile);
  (void) jpeg_read_header(&srcinfo, FALSE);
  //coefficients
  jvirt_barray_ptr * src_coef_arrays = jpeg_read_coefficients(&srcinfo);
  read(srcinfo, src_coef_arrays);
  jpeg_destroy_decompress(&srcinfo);
  fclose(infile);
  return 0; 
}