rtp.c

MPEG-4编解码的实现(包括MPEG4视音频编解码)

 * \date
 *    October 24, 2001
 *
 * \author
 *    Stephan Wenger   [e-mail]
 *****************************************************************************/

int RTPSequenceHeader (FILE *out)
{
  RTPpacket_t *p;

  assert (out != NULL);

  // Set RTP structure elements and alloca() memory foor the buffers
  p = (RTPpacket_t *) alloca (sizeof (RTPpacket_t));
  p->packet=alloca (MAXRTPPACKETSIZE);
  p->payload=alloca (MAXRTPPACKETSIZE);
  p->v=2;
  p->p=0;
  p->x=0;
  p->cc=0;
  p->m=1;
  p->pt=H26LPAYLOADTYPE;
  p->seq=CurrentRTPSequenceNumber++;
  p->timestamp=CurrentRTPTimestamp;
  p->ssrc=H26LSSRC;

  // Set First Byte of RTP payload, see VCEG-N72r1
  p->payload[0] = 
    6 |               // Parameter Update Packet
    0 |               // no know error
    0;                // reserved

  // Generate the Parameter update SDP syntax, donm't overwrite First Byte
  p->paylen = 1 + ComposeHeaderPacketPayload (&p->payload[1]);

  // Generate complete RTP packet
  if (ComposeRTPPacket (p) < 0)
  {
    printf ("Cannot compose RTP packet, exit\n");
    exit (-1);
  }

  if (WriteRTPPacket (p, out) < 0)
  {
    printf ("Cannot write %d bytes of RTP packet to outfile, exit\n", p->packlen);
    exit (-1);
  }
  return (p->packlen);
}



/*!
 *****************************************************************************
 *
 * \brief 
 *    int RTPSliceHeader () write the Slice header for the RTP NAL      
 *
 * \return
 *    Number of bits used by the slice header
 *
 * \para Parameters
 *    none
 *
 * \para Side effects
 *    Slice header as per VCEG-N72r2 is written into the appropriate partition
 *    bit buffer
 *
 * \para Limitations/Shortcomings/Tweaks
 *    The current code does not support the change of picture parameters within
 *    one coded sequence, hence there is only one parameter set necessary.  This
 *    is hard coded to zero.
 *    As per Email deiscussions on 10/24/01 we decided to include the SliceID
 *    into both the Full Slice pakcet and Partition A packet
 *   
 * \date
 *    October 24, 2001
 *
 * \author
 *    Stephan Wenger   [e-mail]
 *****************************************************************************/


int RTPSliceHeader()
{
  int dP_nr = assignSE2partition[input->partition_mode][SE_HEADER];
  Bitstream *currStream = ((img->currentSlice)->partArr[dP_nr]).bitstream;
  DataPartition *partition = &((img->currentSlice)->partArr[dP_nr]);
  SyntaxElement sym;

  int len = 0;
  int RTPSliceType;

  assert (input->of_mode == PAR_OF_RTP);
  sym.type = SE_HEADER;         // This will be true for all symbols generated here
  sym.mapping = n_linfo2;       // Mapping rule: Simple code number to len/info


  SYMTRACESTRING("RTP-SH: Parameter Set");
  sym.value1 = 0;
  len += writeSyntaxElement_UVLC (&sym, partition);

  SYMTRACESTRING("RTP-SH: Picture ID");
  sym.value1 = img->currentSlice->picture_id;
// printf ("Put PictureId %d\n", img->currentSlice->picture_id);

  len += writeSyntaxElement_UVLC (&sym, partition);

  /*! StW:
  // Note: we currently do not support mixed slice types in the encoder, hence
  // we use the picture type here (although there is no such thing as a picture
  // type any more)
  //
  // This needs to be double checked against the document.  Email inquiery on
  // 10/24 evening: is there a need for distinguishing the multpred P from regular
  // P?  if so, this has to be revisited! 
  //
  // StW bug fix: write Slice type according to document
  */

  switch (img->type)
  {
    case INTER_IMG:
      if (img->types==SP_IMG)
        RTPSliceType = 2;
      else
        RTPSliceType = 0;
      break;
    case INTRA_IMG:
      RTPSliceType = 3;
      break;
    case B_IMG:
      RTPSliceType = 1;
      break;
    case SP_IMG:
      // That's what I would expect to be the case. But img->type contains INTER_IMG 
      // here (see handling above)
      //! \todo make one variable from img->type and img->types
      RTPSliceType = 2;
      break;
    default:
      printf ("Panic: unknown picture type %d, exit\n", img->type);
      exit (-1);
  }


  SYMTRACESTRING("RTP-SH: Slice Type");
  sym.value1 = RTPSliceType;
  len += writeSyntaxElement_UVLC (&sym, partition);

  SYMTRACESTRING("RTP-SH: FirstMBInSliceX");
  sym.value1 = img->mb_x;
  len += writeSyntaxElement_UVLC (&sym, partition);

  SYMTRACESTRING("RTP-SH: FirstMBinSlinceY");
  sym.value1 = img->mb_y;
  len += writeSyntaxElement_UVLC (&sym, partition);

  SYMTRACESTRING("RTP-SH: InitialQP");
  sym.value1 = 31-img->qp;
  len += writeSyntaxElement_UVLC (&sym, partition);

  if (img->types==SP_IMG)
  {
    SYMTRACESTRING("RTP-SH: SP InitialQP");
    sym.value1 = 31-img->qpsp;
    len += writeSyntaxElement_UVLC (&sym, partition);
  }


  /*! StW:
  // Note: VCEG-N72 says that the slice type UVLC is not present in single
  // slice packets.  Generally, the NAL allows to mix Single Slice and Partitioning,
  // however, the software does not.  Hence it is sufficient here to express
  // the dependency by checkling the input parameter
  */

  if (input->partition_mode == PAR_DP_3)
  {
    SYMTRACESTRING("RTP-SH: Slice ID");
    sym.value1 = img->current_slice_nr;
    len += writeSyntaxElement_UVLC (&sym, partition);
  }

  // After this, and when in CABAC mode, terminateSlice() may insert one more
  // UVLC codeword for the number of coded MBs

  return len;
}


/*!
 *****************************************************************************
 *
 * \brief 
 *    int RTPWriteBits (int marker) write the Slice header for the RTP NAL      
 *
 * \return
 *    Number of bytes written to output file
 *
 * \para Parameters
 *    marker: markber bit,
 *
 * \para Side effects
 *    Packet written, RTPSequenceNumber and RTPTimestamp updated
 *   
 * \date
 *    October 24, 2001
 *
 * \author
 *    Stephan Wenger   [e-mail]
 *****************************************************************************/


int RTPWriteBits (int Marker, int PacketType, void * bitstream, 
                    int BitStreamLenInByte, FILE *out)
{
  RTPpacket_t *p;

  assert (out != NULL);
  assert (BitStreamLenInByte < 65000);
  assert (bitstream != NULL);
  assert (PacketType < 4);

  // Set RTP structure elements and alloca() memory foor the buffers
  p = (RTPpacket_t *) alloca (sizeof (RTPpacket_t));
  p->packet=alloca (MAXRTPPACKETSIZE);
  p->payload=alloca (MAXRTPPACKETSIZE);
  p->v=2;
  p->p=0;
  p->x=0;
  p->cc=0;
  p->m=Marker&1;
  p->pt=H26LPAYLOADTYPE;
  p->seq=CurrentRTPSequenceNumber++;
  p->timestamp=CurrentRTPTimestamp;
  p->ssrc=H26LSSRC;

  p->payload[0] = PacketType;

  p->paylen = 1 + BitStreamLenInByte;


  memcpy (&p->payload[1], bitstream, BitStreamLenInByte);

  // Generate complete RTP packet
  if (ComposeRTPPacket (p) < 0)
  {
    printf ("Cannot compose RTP packet, exit\n");
    exit (-1);
  }

  if (WriteRTPPacket (p, out) < 0)
  {
    printf ("Cannot write %d bytes of RTP packet to outfile, exit\n", p->packlen);
    exit (-1);
  }
  return (p->packlen);

}


static int oldtr = -1;

void RTPUpdateTimestamp (int tr)
{
  int delta;

  if (oldtr == -1)            // First invocation
  {
    CurrentRTPTimestamp = 0;  //! This is a violation of the security req. of
                              //! RTP (random timestamp), but easier to debug
    oldtr = 0;
    return;
  }

  /*! The following code assumes a wrap around of TR at 256, and
      needs to be changed as soon as this is no more true.
      
      The support for B frames is a bit tricky, because it is not easy to distinguish
      between a natural wrap-around of the tr, and the intentional going back of the
      tr because of a B frame.  It is solved here by a heuristic means: It is assumed that
      B frames are never "older" than 10 tr ticks.  Everything higher than 10 is considered
      a wrap around.
      I'm a bit at loss whether this is a fundamental problem of B frames.  Does a decoder
      have to change its TR interpretation depenent  on the picture type?  Horrible -- we
      would have a fundamental problem when mixing slices.  This needs careful review.
  */

  delta = tr - oldtr;

  if (delta < -10)        // wrap-around
    delta+=256;

  CurrentRTPTimestamp += delta * RTP_TR_TIMESTAMP_MULT;
  oldtr = tr;
}