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  1. /*
  2.  * RTP packetization for H.261 video (RFC 4587)
  3.  * Copyright (c) 2014 Thomas Volkert <thomas@homer-conferencing.com>
  4.  *
  5.  * This file is part of FFmpeg.
  6.  *
  7.  * FFmpeg is free software; you can redistribute it and/or
  8.  * modify it under the terms of the GNU Lesser General Public
  9.  * License as published by the Free Software Foundation; either
  10.  * version 2.1 of the License, or (at your option) any later version.
  11.  *
  12.  * FFmpeg is distributed in the hope that it will be useful,
  13.  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14.  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15.  * Lesser General Public License for more details.
  16.  *
  17.  * You should have received a copy of the GNU Lesser General Public
  18.  * License along with FFmpeg; if not, write to the Free Software
  19.  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  20.  */
  21.  
  22. #include "avformat.h"
  23. #include "rtpenc.h"
  24.  
  25. #define RTP_H261_HEADER_SIZE 4
  26.  
  27. static const uint8_t *find_resync_marker_reverse(const uint8_t *av_restrict start,
  28.                                                  const uint8_t *av_restrict end)
  29. {
  30.     const uint8_t *p = end - 1;
  31.     start += 1; /* Make sure we never return the original start. */
  32.     for (; p > start; p--) {
  33.         if (p[0] == 0 && p[1] == 1)
  34.             return p;
  35.     }
  36.     return end;
  37. }
  38.  
  39. void ff_rtp_send_h261(AVFormatContext *ctx, const uint8_t *frame_buf, int frame_size)
  40. {
  41.     int cur_frame_size;
  42.     int last_packet_of_frame;
  43.     RTPMuxContext *rtp_ctx = ctx->priv_data;
  44.  
  45.     /* use the default 90 KHz time stamp */
  46.     rtp_ctx->timestamp = rtp_ctx->cur_timestamp;
  47.  
  48.     /* continue as long as not all frame data is processed */
  49.     while (frame_size > 0) {
  50.         /*
  51.          * encode the H.261 payload header according to section 4.1 of RFC 4587:
  52.          * (uses 4 bytes between RTP header and H.261 stream per packet)
  53.          *
  54.          *    0                   1                   2                   3
  55.          *    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
  56.          *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  57.          *   |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
  58.          *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  59.          *
  60.          *      Start bit position (SBIT): 3 bits
  61.          *      End bit position (EBIT): 3 bits
  62.          *      INTRA-frame encoded data (I): 1 bit
  63.          *      Motion Vector flag (V): 1 bit
  64.          *      GOB number (GOBN): 4 bits
  65.          *      Macroblock address predictor (MBAP): 5 bits
  66.          *      Quantizer (QUANT): 5 bits
  67.          *      Horizontal motion vector data (HMVD): 5 bits
  68.          *      Vertical motion vector data (VMVD): 5 bits
  69.          */
  70.         rtp_ctx->buf[0] = 1; /* sbit=0, ebit=0, i=0, v=1 */
  71.         rtp_ctx->buf[1] = 0; /* gobn=0, mbap=0 */
  72.         rtp_ctx->buf[2] = 0; /* quant=0, hmvd=5 */
  73.         rtp_ctx->buf[3] = 0; /* vmvd=0 */
  74.         if (frame_size < 2 || frame_buf[0] != 0 || frame_buf[1] != 1) {
  75.             /* A full, correct fix for this would be to make the H261 encoder
  76.              * support inserting extra GOB headers (triggered by setting e.g.
  77.              * "-ps 1"), and including information about macroblock boundaries
  78.              * (such as for h263_rfc2190). */
  79.             av_log(ctx, AV_LOG_WARNING,
  80.                    "RTP/H261 packet not cut at a GOB boundary, not signaled correctly\n");
  81.         }
  82.  
  83.         cur_frame_size = FFMIN(rtp_ctx->max_payload_size - RTP_H261_HEADER_SIZE, frame_size);
  84.  
  85.         /* look for a better place to split the frame into packets */
  86.         if (cur_frame_size < frame_size) {
  87.             const uint8_t *packet_end = find_resync_marker_reverse(frame_buf,
  88.                                                                    frame_buf + cur_frame_size);
  89.             cur_frame_size = packet_end - frame_buf;
  90.         }
  91.  
  92.         /* calculate the "marker" bit for the RTP header */
  93.         last_packet_of_frame = cur_frame_size == frame_size;
  94.  
  95.         /* complete and send RTP packet */
  96.         memcpy(&rtp_ctx->buf[RTP_H261_HEADER_SIZE], frame_buf, cur_frame_size);
  97.         ff_rtp_send_data(ctx, rtp_ctx->buf, RTP_H261_HEADER_SIZE + cur_frame_size, last_packet_of_frame);
  98.  
  99.         frame_buf  += cur_frame_size;
  100.         frame_size -= cur_frame_size;
  101.     }
  102. }
  103.