rtpdec_h261.c 6.51 KB
Newer Older
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
/*
 * RTP parser for H.261 payload format (RFC 4587)
 * Copyright (c) 2014 Thomas Volkert <thomas@homer-conferencing.com>
 *
 * This file is part of Libav.
 *
 * Libav is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * Libav is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with Libav; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include "libavcodec/get_bits.h"
#include "avformat.h"
24
#include "avio_internal.h"
25 26 27 28 29 30 31 32 33 34 35
#include "rtpdec_formats.h"

#define RTP_H261_PAYLOAD_HEADER_SIZE 4

struct PayloadContext {
    AVIOContext *buf;
    uint8_t      endbyte;
    int          endbyte_bits;
    uint32_t     timestamp;
};

36
static av_cold void h261_close_context(PayloadContext *pl_ctx)
37 38 39 40 41 42
{
    /* return if context is invalid */
    if (!pl_ctx)
        return;

    /* free buffer if it is valid */
43
    ffio_free_dyn_buf(&pl_ctx->buf);
44 45 46 47 48 49 50 51 52 53 54 55
}

static int h261_handle_packet(AVFormatContext *ctx, PayloadContext *rtp_h261_ctx,
                              AVStream *st, AVPacket *pkt, uint32_t *timestamp,
                              const uint8_t *buf, int len, uint16_t seq,
                              int flags)
{
    int sbit, ebit, gobn, mbap, quant;
    int res;

    /* drop data of previous packets in case of non-continuous (lossy) packet stream */
    if (rtp_h261_ctx->buf && rtp_h261_ctx->timestamp != *timestamp) {
56
        ffio_free_dyn_buf(&rtp_h261_ctx->buf);
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168
        rtp_h261_ctx->endbyte_bits = 0;
    }

    /* sanity check for size of input packet: 1 byte payload at least */
    if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
        av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);
        return AVERROR_INVALIDDATA;
    }

    /*
     * decode the H.261 payload header according to section 4.1 of RFC 4587:
     * (uses 4 bytes between RTP header and H.261 stream per packet)
     *
     *    0                   1                   2                   3
     *    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
     *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     *   |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
     *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     *
     *      Start bit position (SBIT): 3 bits
     *      End bit position (EBIT): 3 bits
     *      INTRA-frame encoded data (I): 1 bit
     *      Motion Vector flag (V): 1 bit
     *      GOB number (GOBN): 4 bits
     *      Macroblock address predictor (MBAP): 5 bits
     *      Quantizer (QUANT): 5 bits
     *      Horizontal motion vector data (HMVD): 5 bits
     *      Vertical motion vector data (VMVD): 5 bits
     */
    sbit  =  (buf[0] >> 5) & 0x07;
    ebit  =  (buf[0] >> 2) & 0x07;
    gobn  =  (buf[1] >> 4) & 0x0f;
    mbap  = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
    quant =  (buf[2] >> 2) & 0x1f;

    /* pass the H.261 payload header and continue with the actual payload */
    buf += RTP_H261_PAYLOAD_HEADER_SIZE;
    len -= RTP_H261_PAYLOAD_HEADER_SIZE;

    /* start frame buffering with new dynamic buffer */
    if (!rtp_h261_ctx->buf) {
        /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, quant=0 */
        if (!gobn && !sbit && !mbap && !quant) {
            res = avio_open_dyn_buf(&rtp_h261_ctx->buf);
            if (res < 0)
                return res;
            /* update the timestamp in the frame packet with the one from the RTP packet */
            rtp_h261_ctx->timestamp = *timestamp;
        } else {
            /* frame not started yet, need more packets */
            return AVERROR(EAGAIN);
        }
    }

    /* do the "byte merging" at the boundaries of two consecutive frame fragments */
    if (rtp_h261_ctx->endbyte_bits || sbit) {
        if (rtp_h261_ctx->endbyte_bits == sbit) {
            rtp_h261_ctx->endbyte     |= buf[0] & (0xff >> sbit);
            rtp_h261_ctx->endbyte_bits = 0;
            buf++;
            len--;
            avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
        } else {
            /* ebit/sbit values inconsistent, assuming packet loss */
            GetBitContext gb;
            init_get_bits(&gb, buf, len*8 - ebit);
            skip_bits(&gb, sbit);
            if (rtp_h261_ctx->endbyte_bits) {
                rtp_h261_ctx->endbyte |= get_bits(&gb, 8 - rtp_h261_ctx->endbyte_bits);
                avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
            }
            while (get_bits_left(&gb) >= 8)
                avio_w8(rtp_h261_ctx->buf, get_bits(&gb, 8));
            rtp_h261_ctx->endbyte_bits = get_bits_left(&gb);
            if (rtp_h261_ctx->endbyte_bits)
                rtp_h261_ctx->endbyte = get_bits(&gb, rtp_h261_ctx->endbyte_bits) <<
                                        (8 - rtp_h261_ctx->endbyte_bits);
            ebit = 0;
            len  = 0;
        }
    }
    if (ebit) {
        if (len > 0)
            avio_write(rtp_h261_ctx->buf, buf, len - 1);
        rtp_h261_ctx->endbyte_bits = 8 - ebit;
        rtp_h261_ctx->endbyte      = buf[len - 1] & (0xff << ebit);
    } else {
        avio_write(rtp_h261_ctx->buf, buf, len);
    }

    /* RTP marker bit means: last fragment of current frame was received;
       otherwise, an additional fragment is needed for the current frame */
    if (!(flags & RTP_FLAG_MARKER))
        return AVERROR(EAGAIN);

    /* write the completed last byte from the "byte merging" */
    if (rtp_h261_ctx->endbyte_bits)
        avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
    rtp_h261_ctx->endbyte_bits = 0;

    /* close frame buffering and create resulting A/V packet */
    res = ff_rtp_finalize_packet(pkt, &rtp_h261_ctx->buf, st->index);
    if (res < 0)
        return res;

    return 0;
}

RTPDynamicProtocolHandler ff_h261_dynamic_handler = {
    .enc_name          = "H261",
    .codec_type        = AVMEDIA_TYPE_VIDEO,
    .codec_id          = AV_CODEC_ID_H261,
169
    .need_parsing      = AVSTREAM_PARSE_FULL,
170
    .priv_data_size    = sizeof(PayloadContext),
171
    .close             = h261_close_context,
172 173 174
    .parse_packet      = h261_handle_packet,
    .static_payload_id = 31,
};