void CLASS phase_one_flat_field (int is_float, int nc)
{
  ushort head[8];
  unsigned wide, y, x, c, rend, cend, row, col;
  float *mrow, num, mult[4];

  read_shorts (head, 8);
  wide = head[2] / head[4];
  mrow = (float *) calloc (nc*wide, sizeof *mrow);
  merror (mrow, "phase_one_flat_field()");
  for (y=0; y < head[3] / head[5]; y++) {
    for (x=0; x < wide; x++)
      for (c=0; c < nc; c+=2) {
	num = is_float ? getreal(11) : get2()/32768.0;
	if (y==0) mrow[c*wide+x] = num;
	else mrow[(c+1)*wide+x] = (num - mrow[c*wide+x]) / head[5];
      }
    if (y==0) continue;
    rend = head[1] + y*head[5];
    for (row = rend-head[5]; row < raw_height && row < rend; row++) {
      for (x=1; x < wide; x++) {
	for (c=0; c < nc; c+=2) {
	  mult[c] = mrow[c*wide+x-1];
	  mult[c+1] = (mrow[c*wide+x] - mult[c]) / head[4];
	}
	cend = head[0] + x*head[4];
	for (col = cend-head[4]; col < raw_width && col < cend; col++) {
	  c = nc > 2 ? FC(row-top_margin,col-left_margin) : 0;
	  if (!(c & 1)) {
	    c = RAW(row,col) * mult[c];
	    RAW(row,col) = LIM(c,0,65535);
	  }
	  for (c=0; c < nc; c+=2)
	    mult[c] += mult[c+1];
	}
      }
      for (x=0; x < wide; x++)
	for (c=0; c < nc; c+=2)
	  mrow[c*wide+x] += mrow[(c+1)*wide+x];
    }
  }
  free (mrow);
}