2 * RTP parser for H.261 payload format (RFC 4587)
3 * Copyright (c) 2014 Thomas Volkert <thomas@homer-conferencing.com>
5 * This file is part of Libav.
7 * Libav 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.
12 * Libav 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.
17 * You should have received a copy of the GNU Lesser General Public
18 * License along with Libav; if not, write to the Free Software
19 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
22 #include "libavcodec/bitstream.h"
25 #include "avio_internal.h"
26 #include "rtpdec_formats.h"
28 #define RTP_H261_PAYLOAD_HEADER_SIZE 4
30 struct PayloadContext {
37 static av_cold void h261_close_context(PayloadContext *pl_ctx)
39 /* return if context is invalid */
43 /* free buffer if it is valid */
44 ffio_free_dyn_buf(&pl_ctx->buf);
47 static int h261_handle_packet(AVFormatContext *ctx, PayloadContext *rtp_h261_ctx,
48 AVStream *st, AVPacket *pkt, uint32_t *timestamp,
49 const uint8_t *buf, int len, uint16_t seq,
52 int sbit, ebit, gobn, mbap, quant;
55 /* drop data of previous packets in case of non-continuous (lossy) packet stream */
56 if (rtp_h261_ctx->buf && rtp_h261_ctx->timestamp != *timestamp) {
57 ffio_free_dyn_buf(&rtp_h261_ctx->buf);
58 rtp_h261_ctx->endbyte_bits = 0;
61 /* sanity check for size of input packet: 1 byte payload at least */
62 if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
63 av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);
64 return AVERROR_INVALIDDATA;
68 * decode the H.261 payload header according to section 4.1 of RFC 4587:
69 * (uses 4 bytes between RTP header and H.261 stream per packet)
72 * 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
73 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 * |SBIT |EBIT |I|V| GOBN | MBAP | QUANT | HMVD | VMVD |
75 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
77 * Start bit position (SBIT): 3 bits
78 * End bit position (EBIT): 3 bits
79 * INTRA-frame encoded data (I): 1 bit
80 * Motion Vector flag (V): 1 bit
81 * GOB number (GOBN): 4 bits
82 * Macroblock address predictor (MBAP): 5 bits
83 * Quantizer (QUANT): 5 bits
84 * Horizontal motion vector data (HMVD): 5 bits
85 * Vertical motion vector data (VMVD): 5 bits
87 sbit = (buf[0] >> 5) & 0x07;
88 ebit = (buf[0] >> 2) & 0x07;
89 gobn = (buf[1] >> 4) & 0x0f;
90 mbap = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
91 quant = (buf[2] >> 2) & 0x1f;
93 /* pass the H.261 payload header and continue with the actual payload */
94 buf += RTP_H261_PAYLOAD_HEADER_SIZE;
95 len -= RTP_H261_PAYLOAD_HEADER_SIZE;
97 /* start frame buffering with new dynamic buffer */
98 if (!rtp_h261_ctx->buf) {
99 /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, quant=0 */
100 if (!gobn && !sbit && !mbap && !quant) {
101 res = avio_open_dyn_buf(&rtp_h261_ctx->buf);
104 /* update the timestamp in the frame packet with the one from the RTP packet */
105 rtp_h261_ctx->timestamp = *timestamp;
107 /* frame not started yet, need more packets */
108 return AVERROR(EAGAIN);
112 /* do the "byte merging" at the boundaries of two consecutive frame fragments */
113 if (rtp_h261_ctx->endbyte_bits || sbit) {
114 if (rtp_h261_ctx->endbyte_bits == sbit) {
115 rtp_h261_ctx->endbyte |= buf[0] & (0xff >> sbit);
116 rtp_h261_ctx->endbyte_bits = 0;
119 avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
121 /* ebit/sbit values inconsistent, assuming packet loss */
123 bitstream_init(&bc, buf, len * 8 - ebit);
124 bitstream_skip(&bc, sbit);
125 if (rtp_h261_ctx->endbyte_bits) {
126 rtp_h261_ctx->endbyte |= bitstream_read(&bc, 8 - rtp_h261_ctx->endbyte_bits);
127 avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
129 while (bitstream_bits_left(&bc) >= 8)
130 avio_w8(rtp_h261_ctx->buf, bitstream_read(&bc, 8));
131 rtp_h261_ctx->endbyte_bits = bitstream_bits_left(&bc);
132 if (rtp_h261_ctx->endbyte_bits)
133 rtp_h261_ctx->endbyte = bitstream_read(&bc, rtp_h261_ctx->endbyte_bits) <<
134 (8 - rtp_h261_ctx->endbyte_bits);
141 avio_write(rtp_h261_ctx->buf, buf, len - 1);
142 rtp_h261_ctx->endbyte_bits = 8 - ebit;
143 rtp_h261_ctx->endbyte = buf[len - 1] & (0xff << ebit);
145 avio_write(rtp_h261_ctx->buf, buf, len);
148 /* RTP marker bit means: last fragment of current frame was received;
149 otherwise, an additional fragment is needed for the current frame */
150 if (!(flags & RTP_FLAG_MARKER))
151 return AVERROR(EAGAIN);
153 /* write the completed last byte from the "byte merging" */
154 if (rtp_h261_ctx->endbyte_bits)
155 avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
156 rtp_h261_ctx->endbyte_bits = 0;
158 /* close frame buffering and create resulting A/V packet */
159 res = ff_rtp_finalize_packet(pkt, &rtp_h261_ctx->buf, st->index);
166 RTPDynamicProtocolHandler ff_h261_dynamic_handler = {
168 .codec_type = AVMEDIA_TYPE_VIDEO,
169 .codec_id = AV_CODEC_ID_H261,
170 .need_parsing = AVSTREAM_PARSE_FULL,
171 .priv_data_size = sizeof(PayloadContext),
172 .close = h261_close_context,
173 .parse_packet = h261_handle_packet,
174 .static_payload_id = 31,