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rtpdec_h261.c
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1 /*
2  * RTP parser for H.261 payload format (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 "rtpdec_formats.h"
24 #include "libavcodec/get_bits.h"
25 
26 #define RTP_H261_PAYLOAD_HEADER_SIZE 4
27 
28 struct PayloadContext {
32  uint32_t timestamp;
33 };
34 
36 {
37  return av_mallocz(sizeof(PayloadContext));
38 }
39 
40 static void h261_free_dyn_buffer(AVIOContext **dyn_buf)
41 {
42  uint8_t *ptr_dyn_buffer;
43  avio_close_dyn_buf(*dyn_buf, &ptr_dyn_buffer);
44  av_free(ptr_dyn_buffer);
45  *dyn_buf = NULL;
46 }
47 
49 {
50  /* return if context is invalid */
51  if (!pl_ctx)
52  return;
53 
54  /* free buffer if it is valid */
55  if (pl_ctx->buf) {
56  h261_free_dyn_buffer(&pl_ctx->buf);
57  }
58 
59  /* free context */
60  av_free(pl_ctx);
61 }
62 
63 static av_cold int h261_init(AVFormatContext *ctx, int st_index,
65 {
66  //av_log(ctx, AV_LOG_DEBUG, "h261_init() for stream %d\n", st_index);
67 
68  if (st_index < 0)
69  return 0;
70 
71  ctx->streams[st_index]->need_parsing = AVSTREAM_PARSE_FULL;
72 
73  return 0;
74 }
75 
77  AVStream *st, AVPacket *pkt, uint32_t *timestamp,
78  const uint8_t *buf, int len, uint16_t seq, int flags)
79 {
80  int sbit, ebit, gobn, mbap, quant;
81  int res;
82 
83  /* drop data of previous packets in case of non-continuous (loss) packet stream */
84  if (data->buf && data->timestamp != *timestamp) {
85  h261_free_dyn_buffer(&data->buf);
86  }
87 
88  /* sanity check for size of input packet: 1 byte payload at least */
89  if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
90  av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);
91  return AVERROR_INVALIDDATA;
92  }
93 
94  /*
95  decode the H.261 payload header according to section 4.1 of RFC 4587:
96  (uses 4 bytes between RTP header and H.261 stream per packet)
97 
98  0 1 2 3
99  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
100  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
101  |SBIT |EBIT |I|V| GOBN | MBAP | QUANT | HMVD | VMVD |
102  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
103 
104  Start bit position (SBIT): 3 bits
105  End bit position (EBIT): 3 bits
106  INTRA-frame encoded data (I): 1 bit
107  Motion Vector flag (V): 1 bit
108  GOB number (GOBN): 4 bits
109  Macroblock address predictor (MBAP): 5 bits
110  Quantizer (QUANT): 5 bits
111  Horizontal motion vector data (HMVD): 5 bits
112  Vertical motion vector data (VMVD): 5 bits
113 
114  */
115  sbit = (buf[0] >> 5) & 0x07;
116  ebit = (buf[0] >> 2) & 0x07;
117  gobn = (buf[1] >> 4) & 0x0f;
118  mbap = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
119  quant = (buf[2] >> 2) & 0x1f;
120 
121  /* pass the H.261 payload header and continue with the actual payload */
124 
125  /* start frame buffering with new dynamic buffer */
126  if (!data->buf) {
127  /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, uqnat=0 */
128  if (!gobn && !sbit && !mbap && !quant){
129  res = avio_open_dyn_buf(&data->buf);
130  if (res < 0)
131  return res;
132  /* update the timestamp in the frame packet with the one from the RTP packet */
133  data->timestamp = *timestamp;
134  } else {
135  /* frame not started yet, need more packets */
136  return AVERROR(EAGAIN);
137  }
138  }
139 
140  /* do the "byte merging" at the boundaries of two consecutive frame fragments */
141  if (data->endbyte_bits || sbit) {
142  if (data->endbyte_bits == sbit) {
143  data->endbyte |= buf[0] & (0xff >> sbit);
144  data->endbyte_bits = 0;
145  buf++;
146  len--;
147  avio_w8(data->buf, data->endbyte);
148  } else {
149  /* ebit/sbit values inconsistent, assuming packet loss */
150  GetBitContext gb;
151  init_get_bits(&gb, buf, len*8 - ebit);
152  skip_bits(&gb, sbit);
153  if (data->endbyte_bits) {
154  data->endbyte |= get_bits(&gb, 8 - data->endbyte_bits);
155  avio_w8(data->buf, data->endbyte);
156  }
157  while (get_bits_left(&gb) >= 8)
158  avio_w8(data->buf, get_bits(&gb, 8));
159  data->endbyte_bits = get_bits_left(&gb);
160  if (data->endbyte_bits)
161  data->endbyte = get_bits(&gb, data->endbyte_bits) <<
162  (8 - data->endbyte_bits);
163  ebit = 0;
164  len = 0;
165  }
166  }
167  if (ebit) {
168  if (len > 0)
169  avio_write(data->buf, buf, len - 1);
170  data->endbyte_bits = 8 - ebit;
171  data->endbyte = buf[len - 1] & (0xff << ebit);
172  } else {
173  avio_write(data->buf, buf, len);
174  }
175 
176  /* RTP marker bit means: last fragment of current frame was received;
177  otherwise, an additional fragment is needed for the current frame */
178  if (!(flags & RTP_FLAG_MARKER))
179  return AVERROR(EAGAIN);
180 
181  /* write the completed last byte from the "byte merging" */
182  if (data->endbyte_bits)
183  avio_w8(data->buf, data->endbyte);
184  data->endbyte_bits = 0;
185 
186  /* close frame buffering and create resulting A/V packet */
187  res = ff_rtp_finalize_packet(pkt, &data->buf, st->index);
188  if (res < 0)
189  return res;
190 
191  return 0;
192 }
193 
195  .enc_name = "H261",
196  .codec_type = AVMEDIA_TYPE_VIDEO,
197  .codec_id = AV_CODEC_ID_H261,
198  .init = h261_init,
199  .parse_packet = ff_h261_handle_packet,
200  .alloc = h261_new_context,
201  .free = h261_free_context,
202  .static_payload_id = 31,
203 };