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 "libavcodec/get_bits.h"
23#include "avformat.h"
24#include "avio_internal.h"
25#include "rtpdec_formats.h"
26
27#define RTP_H261_PAYLOAD_HEADER_SIZE 4
28
29struct PayloadContext {
30    AVIOContext *buf;
31    uint8_t      endbyte;
32    int          endbyte_bits;
33    uint32_t     timestamp;
34};
35
36static av_cold void h261_close_context(PayloadContext *pl_ctx)
37{
38    /* return if context is invalid */
39    if (!pl_ctx)
40        return;
41
42    /* free buffer if it is valid */
43    ffio_free_dyn_buf(&pl_ctx->buf);
44}
45
46static int h261_handle_packet(AVFormatContext *ctx, PayloadContext *rtp_h261_ctx,
47                              AVStream *st, AVPacket *pkt, uint32_t *timestamp,
48                              const uint8_t *buf, int len, uint16_t seq,
49                              int flags)
50{
51    int sbit, ebit, gobn, mbap, quant;
52    int res;
53
54    /* drop data of previous packets in case of non-continuous (lossy) packet stream */
55    if (rtp_h261_ctx->buf && rtp_h261_ctx->timestamp != *timestamp) {
56        ffio_free_dyn_buf(&rtp_h261_ctx->buf);
57        rtp_h261_ctx->endbyte_bits = 0;
58    }
59
60    /* sanity check for size of input packet: 1 byte payload at least */
61    if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
62        av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);
63        return AVERROR_INVALIDDATA;
64    }
65
66    /*
67     * decode the H.261 payload header according to section 4.1 of RFC 4587:
68     * (uses 4 bytes between RTP header and H.261 stream per packet)
69     *
70     *    0                   1                   2                   3
71     *    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
72     *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
73     *   |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
74     *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
75     *
76     *      Start bit position (SBIT): 3 bits
77     *      End bit position (EBIT): 3 bits
78     *      INTRA-frame encoded data (I): 1 bit
79     *      Motion Vector flag (V): 1 bit
80     *      GOB number (GOBN): 4 bits
81     *      Macroblock address predictor (MBAP): 5 bits
82     *      Quantizer (QUANT): 5 bits
83     *      Horizontal motion vector data (HMVD): 5 bits
84     *      Vertical motion vector data (VMVD): 5 bits
85     */
86    sbit  =  (buf[0] >> 5) & 0x07;
87    ebit  =  (buf[0] >> 2) & 0x07;
88    gobn  =  (buf[1] >> 4) & 0x0f;
89    mbap  = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
90    quant =  (buf[2] >> 2) & 0x1f;
91
92    /* pass the H.261 payload header and continue with the actual payload */
93    buf += RTP_H261_PAYLOAD_HEADER_SIZE;
94    len -= RTP_H261_PAYLOAD_HEADER_SIZE;
95
96    /* start frame buffering with new dynamic buffer */
97    if (!rtp_h261_ctx->buf) {
98        /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, quant=0 */
99        if (!gobn && !sbit && !mbap && !quant) {
100            res = avio_open_dyn_buf(&rtp_h261_ctx->buf);
101            if (res < 0)
102                return res;
103            /* update the timestamp in the frame packet with the one from the RTP packet */
104            rtp_h261_ctx->timestamp = *timestamp;
105        } else {
106            /* frame not started yet, need more packets */
107            return AVERROR(EAGAIN);
108        }
109    }
110
111    /* do the "byte merging" at the boundaries of two consecutive frame fragments */
112    if (rtp_h261_ctx->endbyte_bits || sbit) {
113        if (rtp_h261_ctx->endbyte_bits == sbit) {
114            rtp_h261_ctx->endbyte     |= buf[0] & (0xff >> sbit);
115            rtp_h261_ctx->endbyte_bits = 0;
116            buf++;
117            len--;
118            avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
119        } else {
120            /* ebit/sbit values inconsistent, assuming packet loss */
121            GetBitContext gb;
122            res = init_get_bits(&gb, buf, len*8 - ebit);
123            if (res < 0)
124                return res;
125            skip_bits(&gb, sbit);
126            if (rtp_h261_ctx->endbyte_bits) {
127                rtp_h261_ctx->endbyte |= get_bits(&gb, 8 - rtp_h261_ctx->endbyte_bits);
128                avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
129            }
130            while (get_bits_left(&gb) >= 8)
131                avio_w8(rtp_h261_ctx->buf, get_bits(&gb, 8));
132            rtp_h261_ctx->endbyte_bits = get_bits_left(&gb);
133            if (rtp_h261_ctx->endbyte_bits)
134                rtp_h261_ctx->endbyte = get_bits(&gb, rtp_h261_ctx->endbyte_bits) <<
135                                        (8 - rtp_h261_ctx->endbyte_bits);
136            ebit = 0;
137            len  = 0;
138        }
139    }
140    if (ebit) {
141        if (len > 0)
142            avio_write(rtp_h261_ctx->buf, buf, len - 1);
143        rtp_h261_ctx->endbyte_bits = 8 - ebit;
144        rtp_h261_ctx->endbyte      = buf[len - 1] & (0xff << ebit);
145    } else {
146        avio_write(rtp_h261_ctx->buf, buf, len);
147    }
148
149    /* RTP marker bit means: last fragment of current frame was received;
150       otherwise, an additional fragment is needed for the current frame */
151    if (!(flags & RTP_FLAG_MARKER))
152        return AVERROR(EAGAIN);
153
154    /* write the completed last byte from the "byte merging" */
155    if (rtp_h261_ctx->endbyte_bits)
156        avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
157    rtp_h261_ctx->endbyte_bits = 0;
158
159    /* close frame buffering and create resulting A/V packet */
160    res = ff_rtp_finalize_packet(pkt, &rtp_h261_ctx->buf, st->index);
161    if (res < 0)
162        return res;
163
164    return 0;
165}
166
167const RTPDynamicProtocolHandler ff_h261_dynamic_handler = {
168    .enc_name          = "H261",
169    .codec_type        = AVMEDIA_TYPE_VIDEO,
170    .codec_id          = AV_CODEC_ID_H261,
171    .need_parsing      = AVSTREAM_PARSE_FULL,
172    .priv_data_size    = sizeof(PayloadContext),
173    .close             = h261_close_context,
174    .parse_packet      = h261_handle_packet,
175    .static_payload_id = 31,
176};
177