static int svq1_decode_frame(AVCodecContext *avctx,
                             void *data, int *data_size,
                             AVPacket *avpkt)
{
  const uint8_t *buf = avpkt->data;
  int buf_size = avpkt->size;
  MpegEncContext *s=avctx->priv_data;
  uint8_t        *current, *previous;
  int             result, i, x, y, width, height;
  AVFrame *pict = data;
  svq1_pmv *pmv;
  /* initialize bit buffer */
  init_get_bits(&s->gb,buf,buf_size*8);
  /* decode frame header */
  s->f_code = get_bits (&s->gb, 22);
  if ((s->f_code & ~0x70) || !(s->f_code & 0x60))
    return -1;
  /* swap some header bytes (why?) */
  if (s->f_code != 0x20) {
    uint32_t *src = (uint32_t *) (buf + 4);
    for (i=0; i < 4; i++) {
      src[i] = ((src[i] << 16) | (src[i] >> 16)) ^ src[7 - i];
    }
  }
  result = svq1_decode_frame_header (&s->gb, s);
  if (result != 0)
  {
    av_dlog(s->avctx, "Error in svq1_decode_frame_header %i\n",result);
    return result;
  }
  avcodec_set_dimensions(avctx, s->width, s->height);
  //FIXME this avoids some confusion for "B frames" without 2 references
  //this should be removed after libavcodec can handle more flexible picture types & ordering
  if(s->pict_type==AV_PICTURE_TYPE_B && s->last_picture_ptr==NULL) return buf_size;
  if(  (avctx->skip_frame >= AVDISCARD_NONREF && s->pict_type==AV_PICTURE_TYPE_B)
     ||(avctx->skip_frame >= AVDISCARD_NONKEY && s->pict_type!=AV_PICTURE_TYPE_I)
     || avctx->skip_frame >= AVDISCARD_ALL)
      return buf_size;
  if(ff_MPV_frame_start(s, avctx) < 0)
      return -1;
  pmv = av_malloc((FFALIGN(s->width, 16)/8 + 3) * sizeof(*pmv));
  if (!pmv)
      return -1;
  /* decode y, u and v components */
  for (i=0; i < 3; i++) {
    int linesize;
    if (i == 0) {
      width  = FFALIGN(s->width, 16);
      height = FFALIGN(s->height, 16);
      linesize= s->linesize;
    } else {
      if(s->flags&CODEC_FLAG_GRAY) break;
      width  = FFALIGN(s->width/4, 16);
      height = FFALIGN(s->height/4, 16);
      linesize= s->uvlinesize;
    }
    current = s->current_picture.f.data[i];
    if(s->pict_type==AV_PICTURE_TYPE_B){
        previous = s->next_picture.f.data[i];
    }else{
        previous = s->last_picture.f.data[i];
    }
    if (s->pict_type == AV_PICTURE_TYPE_I) {
      /* keyframe */
      for (y=0; y < height; y+=16) {
        for (x=0; x < width; x+=16) {
          result = svq1_decode_block_intra (&s->gb, &current[x], linesize);
          if (result != 0)
          {
            av_log(s->avctx, AV_LOG_ERROR, "Error in svq1_decode_block %i (keyframe)\n",result);
            goto err;
          }
        }
        current += 16*linesize;
      }
    } else {
      /* delta frame */
      memset (pmv, 0, ((width / 8) + 3) * sizeof(svq1_pmv));
      for (y=0; y < height; y+=16) {
        for (x=0; x < width; x+=16) {
          result = svq1_decode_delta_block (s, &s->gb, &current[x], previous,
                                            linesize, pmv, x, y);
          if (result != 0)
          {
            av_dlog(s->avctx, "Error in svq1_decode_delta_block %i\n",result);
            goto err;
          }
        }
        pmv[0].x =
        pmv[0].y = 0;
        current += 16*linesize;
      }
    }
  }
  *pict = s->current_picture.f;
  ff_MPV_frame_end(s);
  *data_size=sizeof(AVFrame);
  result = buf_size;
err:
  av_free(pmv);
  return result;
}