}
  
  // this is a frame or a field which has no stored complementatry field

  // sliding window, if necessary
  if ((!img->currentPicture->idr_flag)&&(p->used_for_reference && (!img->adaptive_ref_pic_buffering_flag)))
  {
    sliding_window_memory_management(p);
  } 

  // first try to remove unused frames
  if (dpb.used_size==dpb.size)
  {
    remove_unused_frame_from_dpb();
  }
  
  // then output frames until one can be removed
  while (dpb.used_size==dpb.size)
  {
    // non-reference frames may be output directly
    if (!p->used_for_reference)
    {
      get_smallest_poc(&poc, &pos);
      if ((-1==pos) || (p->poc < poc))
      {
        direct_output(p, p_dec);
        return;
      }
    }
    // flush a frame
    output_one_frame_from_dpb();
  }
  
  // check for duplicate frame number in short term reference buffer
  if ((p->used_for_reference)&&(!p->is_long_term))
  {
    for (i=0; i<dpb.ref_frames_in_buffer; i++)
    {
      if (dpb.fs_ref[i]->frame_num == img->frame_num)
      {
        error("duplicate frame_num im short-term reference picture buffer", 500);
      }
    }

  }
  // store at end of buffer
//  printf ("store frame/field at pos %d\n",dpb.used_size);
  insert_picture_in_dpb(dpb.fs[dpb.used_size],p);
  dpb.used_size++;

  update_ref_list();
  update_ltref_list();
  dump_dpb();
}

/*!
 ************************************************************************
 * \brief
 *    Insert the picture into the DPB. A free DPB position is necessary
 *    for frames, .
 *
 * \param p
 *    Picture to be freed
 *
 ************************************************************************
 */
static void insert_picture_in_dpb(FrameStore* fs, StorablePicture* p)
{
//  printf ("insert (%s) pic with frame_num #%d, poc %d\n", (p->structure == FRAME)?"FRAME":(p->structure == TOP_FIELD)?"TOP_FIELD":"BOTTOM_FIELD", img->frame_num, p->poc);
  assert (p!=NULL);
  assert (fs!=NULL);

  // upsample a reference picture
  if (p->used_for_reference)
  {
    UnifiedOneForthPix(p);
  }

  switch (p->structure)
  {
  case FRAME: 
    fs->frame = p;
    fs->is_used = 3;
    if (p->used_for_reference)
    {
      fs->is_reference = 3;
      if (p->is_long_term)
      {
        fs->is_long_term = 3;
      }
    }
    // generate field views
    dpb_split_field(fs);
    break;
  case TOP_FIELD:
    fs->top_field = p;
    fs->is_used |= 1;
    if (p->used_for_reference)
    {
      fs->is_reference |= 1;
      if (p->is_long_term)
      {
        fs->is_long_term |= 1;
        fs->long_term_frame_idx = p->long_term_frame_idx;
      }
    }
    if (fs->is_used == 3)
    {
      // generate frame view
      dpb_combine_field(fs);
    } else
    {
      fs->poc = p->poc;
    }
    break;
  case BOTTOM_FIELD:
    fs->bottom_field = p;
    fs->is_used |= 2;
    if (p->used_for_reference)
    {
      fs->is_reference |= 2;
      if (p->is_long_term)
      {
        fs->is_long_term |= 2;
        fs->long_term_frame_idx = p->long_term_frame_idx;
      }
    }
    if (fs->is_used == 3)
    {
      // generate frame view
      dpb_combine_field(fs);
    } else
    {
      fs->poc = p->poc;
    }
    break;
  }
  fs->frame_num = img->frame_num;
  fs->is_output = 0;

}

/*!
 ************************************************************************
 * \brief
 *    Check if one of the frames/fields in frame store is used for reference
 ************************************************************************
 */
static int is_used_for_reference(FrameStore* fs)
{
  if (fs->is_reference)
  {
    return 1;
  }
  
  if (fs->is_used==3) // frame
  {
    if (fs->frame->used_for_reference)
    {
      return 1;
    }
  }

  if (fs->is_used&1) // top field
  {
    if (fs->top_field->used_for_reference)
    {
      return 1;
    }
  }

  if (fs->is_used&2) // bottom field
  {
    if (fs->bottom_field->used_for_reference)
    {
      return 1;
    }
  }
  return 0;
}


/*!
 ************************************************************************
 * \brief
 *    Check if one of the frames/fields in frame store is used for short-term reference
 ************************************************************************
 */
static int is_short_term_reference(FrameStore* fs)
{

  if (fs->is_used==3) // frame
  {
    if ((fs->frame->used_for_reference)&&(!fs->frame->is_long_term))
    {
      return 1;
    }
  }

  if (fs->is_used&1) // top field
  {
    if ((fs->top_field->used_for_reference)&&(!fs->top_field->is_long_term))
    {
      return 1;
    }
  }

  if (fs->is_used&2) // bottom field
  {
    if ((fs->bottom_field->used_for_reference)&&(!fs->top_field->is_long_term))
    {
      return 1;
    }
  }
  return 0;
}


/*!
 ************************************************************************
 * \brief
 *    Check if one of the frames/fields in frame store is used for short-term reference
 ************************************************************************
 */
static int is_long_term_reference(FrameStore* fs)
{

  if (fs->is_used==3) // frame
  {
    if ((fs->frame->used_for_reference)&&(fs->frame->is_long_term))
    {
      return 1;
    }
  }

  if (fs->is_used&1) // top field
  {
    if ((fs->top_field->used_for_reference)&&(fs->top_field->is_long_term))
    {
      return 1;
    }
  }

  if (fs->is_used&2) // bottom field
  {
    if ((fs->bottom_field->used_for_reference)&&(fs->top_field->is_long_term))
    {
      return 1;
    }
  }
  return 0;
}


/*!
 ************************************************************************
 * \brief
 *    remove one frame from DPB
 ************************************************************************
 */
static void remove_frame_from_dpb(int pos)
{
  FrameStore* fs = dpb.fs[pos];
  FrameStore* tmp;
  unsigned i;
  
//  printf ("remove frame with frame_num #%d\n", fs->frame_num);
  switch (fs->is_used)
  {
  case 3:
    free_storable_picture(fs->frame);
    free_storable_picture(fs->top_field);
    free_storable_picture(fs->bottom_field);
    fs->frame=NULL;
    fs->top_field=NULL;
    fs->bottom_field=NULL;
    break;
  case 2:
    free_storable_picture(fs->bottom_field);
    fs->bottom_field=NULL;
    break;
  case 1:
    free_storable_picture(fs->top_field);
    fs->top_field=NULL;
    break;
  case 0:
    break;
  default:
    error("invalid frame store type",500);
  }
  fs->is_used = 0;

  // move empty framestore to end of buffer
  tmp = dpb.fs[pos];

  for (i=pos; i<dpb.used_size-1;i++)
  {
    dpb.fs[i] = dpb.fs[i+1];
  }
  dpb.fs[dpb.used_size-1] = tmp;
  dpb.used_size--;
}

/*!
 ************************************************************************
 * \brief
 *    find smallest POC in the DPB.
 ************************************************************************
 */
static void get_smallest_poc(int *poc,int * pos)
{
  unsigned i;

  if (dpb.used_size<1)
  {
    error("Cannot determine smallest POC, DPB empty.",150);
  }

  *pos=-1;
  *poc = INT_MAX;
  for (i=0; i<dpb.used_size; i++)
  {
    if ((*poc>dpb.fs[i]->poc)&&(!dpb.fs[i]->is_output))
    {
      *poc = dpb.fs[i]->poc;
      *pos=i;
    }
  }
}

/*!
 ************************************************************************
 * \brief
 *    Remove a picture from DPB which is no longer needed.
 ************************************************************************
 */
static int remove_unused_frame_from_dpb()
{
  unsigned i;

  // check for frames that were already output and no longer used for reference
  for (i=0; i<dpb.used_size; i++)
  {
    if (dpb.fs[i]->is_output && (!is_used_for_reference(dpb.fs[i])))
    {
      remove_frame_from_dpb(i);
      return 1;
    }
  }
  return 0;
}

/*!
 ************************************************************************
 * \brief
 *    Output one picture stored in the DPB.
 ************************************************************************
 */
static void output_one_frame_from_dpb()
{
  int poc, pos;
  //diagnostics
  if (dpb.used_size<1)
  {
    error("Cannot output frame, DPB empty.",150);
  }

  // find smallest POC
  get_smallest_poc(&poc, &pos);

  if(pos==-1)
  {
    error("no frames for output available", 150);
  }

  // call the output function
//  printf ("output frame with frame_num #%d, poc %d (dpb. dpb.size=%d, dpb.used_size=%d)\n", dpb.fs[pos]->frame_num, dpb.fs[pos]->frame->poc, dpb.size, dpb.used_size);

  write_stored_frame(dpb.fs[pos], p_dec);

  if (dpb.last_output_poc >= poc)
  {
    error ("output POC must be in ascending order", 150);
  } 
  dpb.last_output_poc = poc;
  // free frame store and move empty store to end of buffer
  if (!is_used_for_reference(dpb.fs[pos]))
  {
    remove_frame_from_dpb(pos);
  }
}



/*!
 ************************************************************************
 * \brief
 *    All stored picture are output. Should be called to empty the buffer
 ************************************************************************
 */
void flush_dpb()
{
  unsigned i;

  //diagnostics
//  printf("Flush remaining frames from dpb. dpb.size=%d, dpb.used_size=%d\n",dpb.size,dpb.used_size);

  // mark all frames unused
  for (i=0; i<dpb.used_size; i++)
  {
    unmark_for_reference (dpb.fs[i]);
  }

  while (remove_unused_frame_from_dpb()) ;
  
  // output frames in POC order
  while (dpb.used_size)
  {
    output_one_frame_from_dpb();
  }
}

/*!
 ************************************************************************
 * \brief
 *    Extract top field from a frame
 ************************************************************************
 */
void dpb_split_field(FrameStore *fs)
{
  int i;

  fs->top_field    = alloc_storable_picture(TOP_FIELD,    fs->frame->size_x, fs->frame->size_y/2, fs->frame->size_x_cr, fs->frame->size_y_cr/2);
  fs->bottom_field = alloc_storable_picture(BOTTOM_FIELD, fs->frame->size_x, fs->frame->size_y/2, fs->frame->size_x_cr, fs->frame->size_y_cr/2);

  for (i=0; i<fs->frame->size_y/2; i++)
  {
    memcpy(fs->top_field->imgY[i], fs->frame->imgY[i*2], fs->frame->size_x);
  }

  for (i=0; i<fs->frame->size_y_cr/2; i++)
  {
    memcpy(fs->top_field->imgUV[0][i], fs->frame->imgUV[0][i*2], fs->frame->size_x_cr);
    memcpy(fs->top_field->imgUV[1][i], fs->frame->imgUV[1][i*2], fs->frame->size_x_cr);
  }

  for (i=0; i<fs->frame->size_y/2; i++)
  {
    memcpy(fs->bottom_field->imgY[i], fs->frame->imgY[i*2 + 1], fs->frame->size_x);
  }

  for (i=0; i<fs->frame->size_y_cr/2; i++)
  {
    memcpy(fs->bottom_field->imgUV[0][i], fs->frame->imgUV[0][i*2 + 1], fs->frame->size_x_cr);
    memcpy(fs->bottom_field->imgUV[1][i], fs->frame->imgUV[1][i*2 + 1], fs->frame->size_x_cr);
  }

  UnifiedOneForthPix(fs->top_field);
  UnifiedOneForthPix(fs->bottom_field);

  fs->poc = fs->top_field->poc 
          = fs->bottom_field->poc 
          =