readdct.cpp

#include <stdio.h>
#include <jpeglib.h>
#include <iostream>
#include <iomanip>
#include <sstream>

using namespace std;

JBLOCKARRAY rowPtrs[MAX_COMPONENTS];

void read(jpeg_decompress_struct srcinfo, jvirt_barray_ptr * src_coef_arrays, int x, int y) {
  x /= 8;
  y /= 8;
  cout << "Block at " << x << ", " << y << endl;
  for (JDIMENSION compNum=0; compNum < srcinfo.num_components; compNum++) {
    cout << endl << "Component " << compNum << endl;
	for (JDIMENSION i=0; i<DCTSIZE2; i++){
      rowPtrs[compNum] = ((&srcinfo)->mem->access_virt_barray)((j_common_ptr) &srcinfo, src_coef_arrays[compNum],x, (JDIMENSION) 1    , FALSE);
	  cout << setfill(' ') << setw(5) << rowPtrs[compNum][0][y][i];
	  if ((i+1) % 8 == 0 ) {
        cout << endl;
	  }
	}
  }
}

int main(int argc, char** argv) {
  if (argc != 4) {
    cerr << "Usage: ./getdct image.jpg x y" << endl;
    return 1;
  }
  istringstream ss(argv[2]);
  int x;
  if (!(ss >> x)) {
    cerr << "Invalid number " << argv[2] << endl;
  }
  istringstream ss2(argv[3]);
  int y;
  if (!(ss2 >> y)) {
    cerr << "Invalid number " << argv[3] << endl;
  }
 
  FILE * infile;  
  struct jpeg_decompress_struct srcinfo;
  struct jpeg_error_mgr srcerr;

  if ((infile = fopen(argv[1], "rb")) == NULL) {
    fprintf(stderr, "can't open %s\n", argv[1]);
    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, x, y);
    
  jpeg_destroy_decompress(&srcinfo);
  fclose(infile);
  return 0; 
}