func: int ff_h261_handle_packet(AVFormatContext *ctx, PayloadContext *data,
  
                            AVStream *st, AVPacket *pkt, uint32_t *timestamp,
  
                            const uint8_t *buf, int len, uint16_t seq, int flags)
  
  {
  
      int sbit, ebit, gobn, mbap, quant;
  
      int res;
  
  
  
      //av_log(ctx, AV_LOG_DEBUG, "got h261 RTP packet with time: %u\n", timestamp);
  
  
  
      /* drop data of previous packets in case of non-continuous (loss) packet stream */
  
      if (data->buf && data->timestamp != *timestamp) {
  
          h261_free_dyn_buffer(&data->buf);
  
      }
  
  
  
      /* sanity check for size of input packet */
  
      if (len < 5 /* 4 bytes header and 1 byte payload at least */) {
  
          av_log(ctx, AV_LOG_ERROR, "Too short H.261 RTP packet\n");
  
          return AVERROR_INVALIDDATA;
  
      }
  
  
  
      /*
  
        decode the H.261 payload header according to section 4.1 of RFC 4587:
  
        (uses 4 bytes between RTP header and H.261 stream per packet)
  
  
  
           0                   1                   2                   3
  
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  
          |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
  
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  
  
  
             Start bit position (SBIT): 3 bits
  
             End bit position (EBIT): 3 bits
  
             INTRA-frame encoded data (I): 1 bit
  
             Motion Vector flag (V): 1 bit
  
             GOB number (GOBN): 4 bits
  
             Macroblock address predictor (MBAP): 5 bits
  
             Quantizer (QUANT): 5 bits
  
             Horizontal motion vector data (HMVD): 5 bits
  
             Vertical motion vector data (VMVD): 5 bits
  
  
  
      */
  
      sbit  =  (buf[0] >> 5) & 0x07;
  
      ebit  =  (buf[0] >> 2) & 0x07;
  
      gobn  =  (buf[1] >> 4) & 0x0f;
  
      mbap  = ((buf[1] << 1) & 0x1e) | ((buf[1] >> 7) & 0x01);
  
      quant =  (buf[1] >> 4) & 0x0f;
  
  
  
      /* pass the H.261 payload header and continue with the actual payload */
  
      buf += RTP_H261_PAYLOAD_HEADER_SIZE;
  
      len -= RTP_H261_PAYLOAD_HEADER_SIZE;
  
  
  
      /* start frame buffering with new dynamic buffer */
  
      if (!data->buf) {
  
          /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, uqnat=0 */
  
          if (!gobn  && !sbit && !mbap && !quant){
  
              res = avio_open_dyn_buf(&data->buf);
  
              if (res < 0)
  
                  return res;
  
              /* update the timestamp in the frame packet with the one from the RTP packet */
  
              data->timestamp = *timestamp;
  
          } else {
  
              /* frame not started yet, need more packets */
  
              return AVERROR(EAGAIN);
  
          }
  
      }
  
  
  
      /* do the "byte merging" at the boundaries of two consecutive frame fragments */
  
      if (data->endbyte_bits || sbit) {
  
          if (data->endbyte_bits == sbit) {
  
              data->endbyte |= buf[0] & (0xff >> sbit);
  
              data->endbyte_bits = 0;
  
              buf++;
  
              len--;
  
              avio_w8(data->buf, data->endbyte);
  
          } else {
  
              /* ebit/sbit values inconsistent, assuming packet loss */
  
              GetBitContext gb;
  
              init_get_bits(&gb, buf, len*8 - ebit);
  
              skip_bits(&gb, sbit);
  
              if (data->endbyte_bits) {
  
                  data->endbyte |= get_bits(&gb, 8 - data->endbyte_bits);
  
                  avio_w8(data->buf, data->endbyte);
  
              }
  
              while (get_bits_left(&gb) >= 8)
  
                  avio_w8(data->buf, get_bits(&gb, 8));
  
              data->endbyte_bits = get_bits_left(&gb);
  
              if (data->endbyte_bits)
  
                  data->endbyte = get_bits(&gb, data->endbyte_bits) <<
  
                                  (8 - data->endbyte_bits);
  
              ebit = 0;
  
              len = 0;
  
          }
  
      }
  
      if (ebit) {
  
          if (len > 0)
  
              avio_write(data->buf, buf, len - 1);
  
          data->endbyte_bits = 8 - ebit;
  
          data->endbyte = buf[len - 1] & (0xff << ebit);
  
      } else {
  
          avio_write(data->buf, buf, len);
  
      }
  
  
  
      /* RTP marker bit means: last fragment of current frame was received;
  
         otherwise, an additional fragment is needed for the current frame */
  
      if (!(flags & RTP_FLAG_MARKER))
  
          return AVERROR(EAGAIN);
  
  
  
      /* write the completed last byte from the "byte merging" */
  
      if (data->endbyte_bits)
  
          avio_w8(data->buf, data->endbyte);
  
      data->endbyte_bits = 0;
  
  
  
      /* close frame buffering and create resulting A/V packet */
  
      res = ff_rtp_finalize_packet(pkt, &data->buf, st->index);
  
      if (res < 0)
  
          return res;
  
  
  
      return 0;
  
  }
  