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ffmpeg 源代码简单分析 : av_read_frame()

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 abcijkxyz
发布于 2016/08/06 11:59
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FFmpeg的库函数源代码分析文章列表:

【架构图】

FFmpeg源代码结构图 - 解码

FFmpeg源代码结构图 - 编码

【通用】

FFmpeg 源代码简单分析:av_register_all()

FFmpeg 源代码简单分析:avcodec_register_all()

FFmpeg 源代码简单分析:内存的分配和释放(av_malloc()av_free()等)

FFmpeg 源代码简单分析:常见结构体的初始化和销毁(AVFormatContextAVFrame等)

FFmpeg 源代码简单分析:avio_open2()

FFmpeg 源代码简单分析:av_find_decoder()av_find_encoder()

FFmpeg 源代码简单分析:avcodec_open2()

FFmpeg 源代码简单分析:avcodec_close()

【解码】

图解FFMPEG打开媒体的函数avformat_open_input

FFmpeg 源代码简单分析:avformat_open_input()

FFmpeg 源代码简单分析:avformat_find_stream_info()

FFmpeg 源代码简单分析:av_read_frame()

FFmpeg 源代码简单分析:avcodec_decode_video2()

FFmpeg 源代码简单分析:avformat_close_input()

【编码】

FFmpeg 源代码简单分析:avformat_alloc_output_context2()

FFmpeg 源代码简单分析:avformat_write_header()

FFmpeg 源代码简单分析:avcodec_encode_video()

FFmpeg 源代码简单分析:av_write_frame()

FFmpeg 源代码简单分析:av_write_trailer()

【其它】

FFmpeg源代码简单分析:日志输出系统(av_log()等)

FFmpeg源代码简单分析:结构体成员管理系统-AVClass

FFmpeg源代码简单分析:结构体成员管理系统-AVOption

FFmpeg源代码简单分析:libswscalesws_getContext()

FFmpeg源代码简单分析:libswscalesws_scale()

FFmpeg源代码简单分析:libavdeviceavdevice_register_all()

FFmpeg源代码简单分析:libavdevicegdigrab

【脚本】

FFmpeg源代码简单分析:makefile

FFmpeg源代码简单分析:configure

【H.264】

FFmpegH.264解码器源代码简单分析:概述

=====================================================


ffmpeg中的av_read_frame()的作用是读取码流中的音频若干帧或者视频一帧。例如,解码视频的时候,每解码一个视频帧,需要先调用 av_read_frame()获得一帧视频的压缩数据,然后才能对该数据进行解码(例如H.264中一帧压缩数据通常对应一个NAL)。

对该函数源代码的分析是很久之前做的了,现在翻出来,用博客记录一下。


上代码之前,先参考了其他人对av_read_frame()的解释,在此做一个参考:

通过av_read_packet(***),读取一个包,需要说明的是此函数必须是包含整数帧的,不存在半帧的情况,以ts流为例,是读取一个完整的PES包(一个完整pes包包含若干视频或音频es包),读取完毕后,通过av_parser_parse2(***)分析出视频一帧(或音频若干帧),返回,下次进入循环的时候,如果上次的数据没有完全取完,则st = s->cur_st;不会是NULL,即再此进入av_parser_parse2(***)流程,而不是下面的av_read_packet(**)流程,这样就保证了,如果读取一次包含了N帧视频数据(以视频为例),则调用av_read_frame(***)N次都不会去读数据,而是返回第一次读取的数据,直到全部解析完毕。

av_read_frame()的声明位于libavformat\avformat.h,如下所示。

/**
 * Return the next frame of a stream.
 * This function returns what is stored in the file, and does not validate
 * that what is there are valid frames for the decoder. It will split what is
 * stored in the file into frames and return one for each call. It will not
 * omit invalid data between valid frames so as to give the decoder the maximum
 * information possible for decoding.
 *
 * If pkt->buf is NULL, then the packet is valid until the next
 * av_read_frame() or until avformat_close_input(). Otherwise the packet
 * is valid indefinitely. In both cases the packet must be freed with
 * av_free_packet when it is no longer needed. For video, the packet contains
 * exactly one frame. For audio, it contains an integer number of frames if each
 * frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
 * have a variable size (e.g. MPEG audio), then it contains one frame.
 *
 * pkt->pts, pkt->dts and pkt->duration are always set to correct
 * values in AVStream.time_base units (and guessed if the format cannot
 * provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
 * has B-frames, so it is better to rely on pkt->dts if you do not
 * decompress the payload.
 *
 * @return 0 if OK, < 0 on error or end of file
 */
int av_read_frame(AVFormatContext *s, AVPacket *pkt);

av_read_frame()使用方法在注释中写得很详细,用中文简单描述一下它的两个参数:

s:输入的AVFormatContext

pkt:输出的AVPacket

如果返回0则说明读取正常。

函数调用结构图

函数调用结构图如下所示。


av_read_frame()

av_read_frame()的定义位于libavformat\utils.c,如下所示:

//获取一个AVPacket
/*
 * av_read_frame - 新版本的ffmpeg用的是av_read_frame,而老版本的是av_read_packet
 * 。区别是av_read_packet读出的是包,它可能是半帧或多帧,不保证帧的完整性。av_read_frame对
 * av_read_packet进行了封装,使读出的数据总是完整的帧
 */
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int          eof = 0;

    if (!genpts)
    	/**
    	 * This buffer is only needed when packets were already buffered but
    	 * not decoded, for example to get the codec parameters in MPEG
    	 * streams.
    	 * 一般情况下会调用read_frame_internal(s, pkt)
    	 * 直接返回
    	 */
        return s->packet_buffer ? read_from_packet_buffer(s, pkt) :
                                  read_frame_internal(s, pkt);

    for (;;) {
        int ret;
        AVPacketList *pktl = s->packet_buffer;

        if (pktl) {
            AVPacket *next_pkt = &pktl->pkt;

            if (next_pkt->dts != AV_NOPTS_VALUE) {
                int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;
                while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {
                    if (pktl->pkt.stream_index == next_pkt->stream_index &&
                        (av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0) &&
                         av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) { //not b frame
                        next_pkt->pts = pktl->pkt.dts;
                    }
                    pktl = pktl->next;
                }
                pktl = s->packet_buffer;
            }

            /* read packet from packet buffer, if there is data */
            if (!(next_pkt->pts == AV_NOPTS_VALUE &&
                  next_pkt->dts != AV_NOPTS_VALUE && !eof))
                return read_from_packet_buffer(s, pkt);
        }

        ret = read_frame_internal(s, pkt);
        if (ret < 0) {
            if (pktl && ret != AVERROR(EAGAIN)) {
                eof = 1;
                continue;
            } else
                return ret;
        }

        if (av_dup_packet(add_to_pktbuf(&s->packet_buffer, pkt,
                          &s->packet_buffer_end)) < 0)
            return AVERROR(ENOMEM);
    }
}
可以从源代码中看出,av_read_frame()调用了read_frame_internal()。

read_frame_internal()

read_frame_internal()代码如下所示:

//av_read_frame对他进行了封装
static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
    int ret = 0, i, got_packet = 0;
    AVDictionary *metadata = NULL;
    //初始化
    av_init_packet(pkt);

    while (!got_packet && !s->parse_queue) {
        AVStream *st;
        AVPacket cur_pkt;

        /* read next packet */
        ret = ff_read_packet(s, &cur_pkt);
        if (ret < 0) {
            if (ret == AVERROR(EAGAIN))
                return ret;
            /* flush the parsers */
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
            	//需要解析
                if (st->parser && st->need_parsing)
                    parse_packet(s, NULL, st->index);
            }
            /* all remaining packets are now in parse_queue =>
             * really terminate parsing */
            break;
        }
        ret = 0;
        st  = s->streams[cur_pkt.stream_index];

        if (cur_pkt.pts != AV_NOPTS_VALUE &&
            cur_pkt.dts != AV_NOPTS_VALUE &&
            cur_pkt.pts < cur_pkt.dts) {
            av_log(s, AV_LOG_WARNING,
                   "Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",
                   cur_pkt.stream_index,
                   av_ts2str(cur_pkt.pts),
                   av_ts2str(cur_pkt.dts),
                   cur_pkt.size);
        }
        if (s->debug & FF_FDEBUG_TS)
            av_log(s, AV_LOG_DEBUG,
                   "ff_read_packet stream=%d, pts=%s, dts=%s, size=%d, duration=%d, flags=%d\n",
                   cur_pkt.stream_index,
                   av_ts2str(cur_pkt.pts),
                   av_ts2str(cur_pkt.dts),
                   cur_pkt.size, cur_pkt.duration, cur_pkt.flags);

        if (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {
            st->parser = av_parser_init(st->codec->codec_id);
            if (!st->parser) {
                av_log(s, AV_LOG_VERBOSE, "parser not found for codec "
                       "%s, packets or times may be invalid.\n",
                       avcodec_get_name(st->codec->codec_id));
                /* no parser available: just output the raw packets */
                st->need_parsing = AVSTREAM_PARSE_NONE;
            } else if (st->need_parsing == AVSTREAM_PARSE_HEADERS)
                st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
            else if (st->need_parsing == AVSTREAM_PARSE_FULL_ONCE)
                st->parser->flags |= PARSER_FLAG_ONCE;
            else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)
                st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
        }
        if (!st->need_parsing || !st->parser) {
            /* no parsing needed: we just output the packet as is */
            *pkt = cur_pkt;
            compute_pkt_fields(s, st, NULL, pkt);
            if ((s->iformat->flags & AVFMT_GENERIC_INDEX) &&
                (pkt->flags & AV_PKT_FLAG_KEY) && pkt->dts != AV_NOPTS_VALUE) {
                ff_reduce_index(s, st->index);
                av_add_index_entry(st, pkt->pos, pkt->dts,
                                   0, 0, AVINDEX_KEYFRAME);
            }
            got_packet = 1;
        } else if (st->discard < AVDISCARD_ALL) {
            if ((ret = parse_packet(s, &cur_pkt, cur_pkt.stream_index)) < 0)
                return ret;
        } else {
            /* free packet */
            av_free_packet(&cur_pkt);
        }
        if (pkt->flags & AV_PKT_FLAG_KEY)
            st->skip_to_keyframe = 0;
        if (st->skip_to_keyframe) {
            av_free_packet(&cur_pkt);
            if (got_packet) {
                *pkt = cur_pkt;
            }
            got_packet = 0;
        }
    }

    if (!got_packet && s->parse_queue)
        ret = read_from_packet_buffer(&s->parse_queue, &s->parse_queue_end, pkt);

    if (ret >= 0) {
        AVStream *st = s->streams[pkt->stream_index];
        int discard_padding = 0;
        if (st->first_discard_sample && pkt->pts != AV_NOPTS_VALUE) {
            int64_t pts = pkt->pts - (is_relative(pkt->pts) ? RELATIVE_TS_BASE : 0);
            int64_t sample = ts_to_samples(st, pts);
            int duration = ts_to_samples(st, pkt->duration);
            int64_t end_sample = sample + duration;
            if (duration > 0 && end_sample >= st->first_discard_sample &&
                sample < st->last_discard_sample)
                discard_padding = FFMIN(end_sample - st->first_discard_sample, duration);
        }
        if (st->skip_samples || discard_padding) {
            uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);
            if (p) {
                AV_WL32(p, st->skip_samples);
                AV_WL32(p + 4, discard_padding);
                av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %d\n", st->skip_samples);
            }
            st->skip_samples = 0;
        }

        if (st->inject_global_side_data) {
            for (i = 0; i < st->nb_side_data; i++) {
                AVPacketSideData *src_sd = &st->side_data[i];
                uint8_t *dst_data;

                if (av_packet_get_side_data(pkt, src_sd->type, NULL))
                    continue;

                dst_data = av_packet_new_side_data(pkt, src_sd->type, src_sd->size);
                if (!dst_data) {
                    av_log(s, AV_LOG_WARNING, "Could not inject global side data\n");
                    continue;
                }

                memcpy(dst_data, src_sd->data, src_sd->size);
            }
            st->inject_global_side_data = 0;
        }

        if (!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))
            av_packet_merge_side_data(pkt);
    }

    av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
    if (metadata) {
        s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
        av_dict_copy(&s->metadata, metadata, 0);
        av_dict_free(&metadata);
        av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
    }

    if (s->debug & FF_FDEBUG_TS)
        av_log(s, AV_LOG_DEBUG,
               "read_frame_internal stream=%d, pts=%s, dts=%s, "
               "size=%d, duration=%d, flags=%d\n",
               pkt->stream_index,
               av_ts2str(pkt->pts),
               av_ts2str(pkt->dts),
               pkt->size, pkt->duration, pkt->flags);

    return ret;
}

read_frame_internal()代码比较长,这里只简单看一下它前面的部分。它前面部分有2步是十分关键的:

(1)调用了ff_read_packet()从相应的AVInputFormat读取数据。

(2)如果媒体频流需要使用AVCodecParser,则调用parse_packet()解析相应的AVPacket。

下面我们分成分别看一下ff_read_packet()和parse_packet()的源代码。


ff_read_packet()

ff_read_packet()的代码比较长,如下所示。

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, err;
    AVStream *st;

    for (;;) {
        AVPacketList *pktl = s->raw_packet_buffer;

        if (pktl) {
            *pkt = pktl->pkt;
            st   = s->streams[pkt->stream_index];
            if (s->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            if (st->request_probe <= 0) {
                s->raw_packet_buffer                 = pktl->next;
                s->raw_packet_buffer_remaining_size += pkt->size;
                av_free(pktl);
                return 0;
            }
        }

        pkt->data = NULL;
        pkt->size = 0;
        av_init_packet(pkt);
        //关键:读取Packet
        ret = s->iformat->read_packet(s, pkt);
        if (ret < 0) {
            if (!pktl || ret == AVERROR(EAGAIN))
                return ret;
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
                if (st->probe_packets)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(st->request_probe <= 0);
            }
            continue;
        }

        if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&
            (pkt->flags & AV_PKT_FLAG_CORRUPT)) {
            av_log(s, AV_LOG_WARNING,
                   "Dropped corrupted packet (stream = %d)\n",
                   pkt->stream_index);
            av_free_packet(pkt);
            continue;
        }

        if (pkt->stream_index >= (unsigned)s->nb_streams) {
            av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);
            continue;
        }

        st = s->streams[pkt->stream_index];

        if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
            // correct first time stamps to negative values
            if (!is_relative(st->first_dts))
                st->first_dts = wrap_timestamp(st, st->first_dts);
            if (!is_relative(st->start_time))
                st->start_time = wrap_timestamp(st, st->start_time);
            if (!is_relative(st->cur_dts))
                st->cur_dts = wrap_timestamp(st, st->cur_dts);
        }

        pkt->dts = wrap_timestamp(st, pkt->dts);
        pkt->pts = wrap_timestamp(st, pkt->pts);

        force_codec_ids(s, st);

        /* TODO: audio: time filter; video: frame reordering (pts != dts) */
        if (s->use_wallclock_as_timestamps)
            pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);

        if (!pktl && st->request_probe <= 0)
            return ret;

        add_to_pktbuf(&s->raw_packet_buffer, pkt, &s->raw_packet_buffer_end);
        s->raw_packet_buffer_remaining_size -= pkt->size;

        if ((err = probe_codec(s, st, pkt)) < 0)
            return err;
    }
}

ff_read_packet()中最关键的地方就是调用了AVInputFormat的read_packet()方法。AVInputFormat的read_packet()是一个函数指针,指向当前的AVInputFormat的读取数据的函数。在这里我们以FLV封装格式对应的AVInputFormat为例,看看read_packet()的实现函数是什么样子的。

FLV封装格式对应的AVInputFormat的定义位于libavformat\flvdec.c,如下所示。

AVInputFormat ff_flv_demuxer = {
    .name           = "flv",
    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
    .priv_data_size = sizeof(FLVContext),
    .read_probe     = flv_probe,
    .read_header    = flv_read_header,
    .read_packet    = flv_read_packet,
    .read_seek      = flv_read_seek,
    .read_close     = flv_read_close,
    .extensions     = "flv",
    .priv_class     = &flv_class,
};

从ff_flv_demuxer的定义可以看出,read_packet()对应的是flv_read_packet()函数。在看flv_read_packet()函数之前,我们先回顾一下FLV封装格式的结构,如下图所示。


从图中可以看出,FLV文件体部分是由一个一个的Tag连接起来的(中间间隔着Previous Tag Size)。每个Tag包含了Tag Header和Tag Data两个部分。Tag Data根据Tag的Type不同而不同:可以分为音频Tag Data,视频Tag Data以及Script Tag Data。下面简述一下音频Tag Data和视频Tag Data。

Audio Tag Data

Audio Tag在官方标准中定义如下。
 
Audio Tag开始的第1个字节包含了音频数据的参数信息,从第2个字节开始为音频流数据。
第1个字节的前4位的数值表示了音频数据格式:
0 = Linear PCM, platform endian
1 = ADPCM
2 = MP3
3 = Linear PCM, little endian
4 = Nellymoser 16-kHz mono
5 = Nellymoser 8-kHz mono
6 = Nellymoser
7 = G.711 A-law logarithmic PCM
8 = G.711 mu-law logarithmic PCM
9 = reserved
10 = AAC
14 = MP3 8-Khz
15 = Device-specific sound
第1个字节的第5-6位的数值表示采样率:0 = 5.5kHz,1 = 11KHz,2 = 22 kHz,3 = 44 kHz。
第1个字节的第7位表示采样精度:0 = 8bits,1 = 16bits。
第1个字节的第8位表示音频类型:0 = sndMono,1 = sndStereo。
其中,当音频编码为AAC的时候,第一个字节后面存储的是AACAUDIODATA,格式如下所示。
 

Video Tag Data

Video Tag在官方标准中的定义如下。

Video Tag也用开始的第1个字节包含视频数据的参数信息,从第2个字节为视频流数据。
第1个字节的前4位的数值表示帧类型(FrameType):
1: keyframe (for AVC, a seekableframe)(关键帧)
2: inter frame (for AVC, a nonseekableframe)
3: disposable inter frame (H.263only)
4: generated keyframe (reservedfor server use only)
5: video info/command frame
第1个字节的后4位的数值表示视频编码ID(CodecID):
1: JPEG (currently unused)
2: Sorenson H.263
3: Screen video
4: On2 VP6
5: On2 VP6 with alpha channel
6: Screen video version 2
7: AVC
其中,当音频编码为AVC(H.264)的时候,第一个字节后面存储的是AVCVIDEOPACKET,格式如下所示。
 

了解了FLV的基本格式之后,就可以看一下FLV解析Tag的函数flv_read_packet()了。

flv_read_packet()

flv_read_packet()的定义位于libavformat\flvdec.c,如下所示。

static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    FLVContext *flv = s->priv_data;
    int ret, i, type, size, flags;
    int stream_type=-1;
    int64_t next, pos, meta_pos;
    int64_t dts, pts = AV_NOPTS_VALUE;
    int av_uninit(channels);
    int av_uninit(sample_rate);
    AVStream *st    = NULL;

    /* pkt size is repeated at end. skip it */
    for (;; avio_skip(s->pb, 4)) {
        pos  = avio_tell(s->pb);
        //解析Tag Header==========
        //Tag类型
        type = (avio_r8(s->pb) & 0x1F);
        //Datasize数据大小
        size = avio_rb24(s->pb);
        //Timstamp时间戳
        dts  = avio_rb24(s->pb);
        dts |= avio_r8(s->pb) << 24;
        av_dlog(s, "type:%d, size:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, dts, avio_tell(s->pb));
        if (avio_feof(s->pb))
            return AVERROR_EOF;
        //StreamID
        avio_skip(s->pb, 3); /* stream id, always 0 */
        flags = 0;
        //========================
        if (flv->validate_next < flv->validate_count) {
            int64_t validate_pos = flv->validate_index[flv->validate_next].pos;
            if (pos == validate_pos) {
                if (FFABS(dts - flv->validate_index[flv->validate_next].dts) <=
                    VALIDATE_INDEX_TS_THRESH) {
                    flv->validate_next++;
                } else {
                    clear_index_entries(s, validate_pos);
                    flv->validate_count = 0;
                }
            } else if (pos > validate_pos) {
                clear_index_entries(s, validate_pos);
                flv->validate_count = 0;
            }
        }

        if (size == 0)
            continue;

        next = size + avio_tell(s->pb);

        if (type == FLV_TAG_TYPE_AUDIO) {
        	//Type是音频
            stream_type = FLV_STREAM_TYPE_AUDIO;
            //Tag Data的第一个字节
            flags    = avio_r8(s->pb);
            size--;
        } else if (type == FLV_TAG_TYPE_VIDEO) {
        	//Type是音频
            stream_type = FLV_STREAM_TYPE_VIDEO;
            //Tag Data的第一个字节
            flags    = avio_r8(s->pb);
            size--;
            if ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_VIDEO_INFO_CMD)
                goto skip;
        } else if (type == FLV_TAG_TYPE_META) {
            stream_type=FLV_STREAM_TYPE_DATA;
            if (size > 13 + 1 + 4 && dts == 0) { // Header-type metadata stuff
                meta_pos = avio_tell(s->pb);
                if (flv_read_metabody(s, next) <= 0) {
                    goto skip;
                }
                avio_seek(s->pb, meta_pos, SEEK_SET);
            }
        } else {
            av_log(s, AV_LOG_DEBUG,
                   "Skipping flv packet: type %d, size %d, flags %d.\n",
                   type, size, flags);
skip:
            avio_seek(s->pb, next, SEEK_SET);
            continue;
        }

        /* skip empty data packets */
        if (!size)
            continue;

        /* now find stream */
        for (i = 0; i < s->nb_streams; i++) {
            st = s->streams[i];
            if (stream_type == FLV_STREAM_TYPE_AUDIO) {
                if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO &&
                    (s->audio_codec_id || flv_same_audio_codec(st->codec, flags)))
                    break;
            } else if (stream_type == FLV_STREAM_TYPE_VIDEO) {
                if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO &&
                    (s->video_codec_id || flv_same_video_codec(st->codec, flags)))
                    break;
            } else if (stream_type == FLV_STREAM_TYPE_DATA) {
                if (st->codec->codec_type == AVMEDIA_TYPE_DATA)
                    break;
            }
        }
        if (i == s->nb_streams) {
            static const enum AVMediaType stream_types[] = {AVMEDIA_TYPE_VIDEO, AVMEDIA_TYPE_AUDIO, AVMEDIA_TYPE_DATA};
            av_log(s, AV_LOG_WARNING, "Stream discovered after head already parsed\n");
            st = create_stream(s, stream_types[stream_type]);
            if (!st)
                return AVERROR(ENOMEM);

        }
        av_dlog(s, "%d %X %d \n", stream_type, flags, st->discard);

        if ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY ||
            stream_type == FLV_STREAM_TYPE_AUDIO)
            av_add_index_entry(st, pos, dts, size, 0, AVINDEX_KEYFRAME);

        if (  (st->discard >= AVDISCARD_NONKEY && !((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY || (stream_type == FLV_STREAM_TYPE_AUDIO)))
            ||(st->discard >= AVDISCARD_BIDIR  &&  ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_DISP_INTER && (stream_type == FLV_STREAM_TYPE_VIDEO)))
            || st->discard >= AVDISCARD_ALL
        ) {
            avio_seek(s->pb, next, SEEK_SET);
            continue;
        }
        break;
    }

    // if not streamed and no duration from metadata then seek to end to find
    // the duration from the timestamps
    if (s->pb->seekable && (!s->duration || s->duration == AV_NOPTS_VALUE) && !flv->searched_for_end) {
        int size;
        const int64_t pos   = avio_tell(s->pb);
        // Read the last 4 bytes of the file, this should be the size of the
        // previous FLV tag. Use the timestamp of its payload as duration.
        int64_t fsize       = avio_size(s->pb);
retry_duration:
        avio_seek(s->pb, fsize - 4, SEEK_SET);
        size = avio_rb32(s->pb);
        // Seek to the start of the last FLV tag at position (fsize - 4 - size)
        // but skip the byte indicating the type.
        avio_seek(s->pb, fsize - 3 - size, SEEK_SET);
        if (size == avio_rb24(s->pb) + 11) {
            uint32_t ts = avio_rb24(s->pb);
            ts         |= avio_r8(s->pb) << 24;
            if (ts)
                s->duration = ts * (int64_t)AV_TIME_BASE / 1000;
            else if (fsize >= 8 && fsize - 8 >= size) {
                fsize -= size+4;
                goto retry_duration;
            }
        }

        avio_seek(s->pb, pos, SEEK_SET);
        flv->searched_for_end = 1;
    }

    if (stream_type == FLV_STREAM_TYPE_AUDIO) {
        int bits_per_coded_sample;
        channels = (flags & FLV_AUDIO_CHANNEL_MASK) == FLV_STEREO ? 2 : 1;
        sample_rate = 44100 << ((flags & FLV_AUDIO_SAMPLERATE_MASK) >>
                                FLV_AUDIO_SAMPLERATE_OFFSET) >> 3;
        bits_per_coded_sample = (flags & FLV_AUDIO_SAMPLESIZE_MASK) ? 16 : 8;
        if (!st->codec->channels || !st->codec->sample_rate ||
            !st->codec->bits_per_coded_sample) {
            st->codec->channels              = channels;
            st->codec->channel_layout        = channels == 1
                                               ? AV_CH_LAYOUT_MONO
                                               : AV_CH_LAYOUT_STEREO;
            st->codec->sample_rate           = sample_rate;
            st->codec->bits_per_coded_sample = bits_per_coded_sample;
        }
        if (!st->codec->codec_id) {
            flv_set_audio_codec(s, st, st->codec,
                                flags & FLV_AUDIO_CODECID_MASK);
            flv->last_sample_rate =
            sample_rate           = st->codec->sample_rate;
            flv->last_channels    =
            channels              = st->codec->channels;
        } else {
            AVCodecContext ctx = {0};
            ctx.sample_rate = sample_rate;
            ctx.bits_per_coded_sample = bits_per_coded_sample;
            flv_set_audio_codec(s, st, &ctx, flags & FLV_AUDIO_CODECID_MASK);
            sample_rate = ctx.sample_rate;
        }
    } else if (stream_type == FLV_STREAM_TYPE_VIDEO) {
        size -= flv_set_video_codec(s, st, flags & FLV_VIDEO_CODECID_MASK, 1);
    }
    //几种特殊的格式
    if (st->codec->codec_id == AV_CODEC_ID_AAC ||
        st->codec->codec_id == AV_CODEC_ID_H264 ||
        st->codec->codec_id == AV_CODEC_ID_MPEG4) {
    	//对应AACPacketType或者AVCPacketType
        int type = avio_r8(s->pb);
        size--;
        //H.264
        if (st->codec->codec_id == AV_CODEC_ID_H264 || st->codec->codec_id == AV_CODEC_ID_MPEG4) {
            // sign extension
        	//对应CompositionTime
            int32_t cts = (avio_rb24(s->pb) + 0xff800000) ^ 0xff800000;
            //计算PTS
            pts = dts + cts;
            if (cts < 0) { // dts might be wrong
                if (!flv->wrong_dts)
                    av_log(s, AV_LOG_WARNING,
                        "Negative cts, previous timestamps might be wrong.\n");
                flv->wrong_dts = 1;
            } else if (FFABS(dts - pts) > 1000*60*15) {
                av_log(s, AV_LOG_WARNING,
                       "invalid timestamps %"PRId64" %"PRId64"\n", dts, pts);
                dts = pts = AV_NOPTS_VALUE;
            }
        }
        //如果编码器是AAC或者H.264
        if (type == 0 && (!st->codec->extradata || st->codec->codec_id == AV_CODEC_ID_AAC ||
            st->codec->codec_id == AV_CODEC_ID_H264)) {
            AVDictionaryEntry *t;

            if (st->codec->extradata) {
                if ((ret = flv_queue_extradata(flv, s->pb, stream_type, size)) < 0)
                    return ret;
                ret = AVERROR(EAGAIN);
                goto leave;
            }
            if ((ret = flv_get_extradata(s, st, size)) < 0)
                return ret;

            /* Workaround for buggy Omnia A/XE encoder */
            t = av_dict_get(s->metadata, "Encoder", NULL, 0);
            if (st->codec->codec_id == AV_CODEC_ID_AAC && t && !strcmp(t->value, "Omnia A/XE"))
                st->codec->extradata_size = 2;
            //AAC
            if (st->codec->codec_id == AV_CODEC_ID_AAC && 0) {
                MPEG4AudioConfig cfg;

                if (avpriv_mpeg4audio_get_config(&cfg, st->codec->extradata,
                                             st->codec->extradata_size * 8, 1) >= 0) {
                st->codec->channels       = cfg.channels;
                st->codec->channel_layout = 0;
                if (cfg.ext_sample_rate)
                    st->codec->sample_rate = cfg.ext_sample_rate;
                else
                    st->codec->sample_rate = cfg.sample_rate;
                av_dlog(s, "mp4a config channels %d sample rate %d\n",
                        st->codec->channels, st->codec->sample_rate);
                }
            }

            ret = AVERROR(EAGAIN);
            goto leave;
        }
    }

    /* skip empty data packets */
    if (!size) {
        ret = AVERROR(EAGAIN);
        goto leave;
    }

    ret = av_get_packet(s->pb, pkt, size);
    if (ret < 0)
        return ret;
    //设置PTS、DTS等等
    pkt->dts          = dts;
    pkt->pts          = pts == AV_NOPTS_VALUE ? dts : pts;
    pkt->stream_index = st->index;
    if (flv->new_extradata[stream_type]) {
        uint8_t *side = av_packet_new_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA,
                                                flv->new_extradata_size[stream_type]);
        if (side) {
            memcpy(side, flv->new_extradata[stream_type],
                   flv->new_extradata_size[stream_type]);
            av_freep(&flv->new_extradata[stream_type]);
            flv->new_extradata_size[stream_type] = 0;
        }
    }
    if (stream_type == FLV_STREAM_TYPE_AUDIO &&
                    (sample_rate != flv->last_sample_rate ||
                     channels    != flv->last_channels)) {
        flv->last_sample_rate = sample_rate;
        flv->last_channels    = channels;
        ff_add_param_change(pkt, channels, 0, sample_rate, 0, 0);
    }
    //标记上Keyframe
    if (    stream_type == FLV_STREAM_TYPE_AUDIO ||
            ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY) ||
            stream_type == FLV_STREAM_TYPE_DATA)
        pkt->flags |= AV_PKT_FLAG_KEY;

leave:
    avio_skip(s->pb, 4);
    return ret;
}

flv_read_packet()的代码比较长,但是逻辑比较简单。它的主要功能就是根据FLV文件格式的规范,逐层解析Tag以及TagData,获取Tag以及TagData中的信息。比较关键的地方已经写上了注释,不再详细叙述。


parse_packet()

parse_packet()给需要AVCodecParser的媒体流提供解析AVPacket的功能。它的代码如下所示:

/**
 * Parse a packet, add all split parts to parse_queue.
 *
 * @param pkt Packet to parse, NULL when flushing the parser at end of stream.
 */
static int parse_packet(AVFormatContext *s, AVPacket *pkt, int stream_index)
{
    AVPacket out_pkt = { 0 }, flush_pkt = { 0 };
    AVStream *st = s->streams[stream_index];
    uint8_t *data = pkt ? pkt->data : NULL;
    int size      = pkt ? pkt->size : 0;
    int ret = 0, got_output = 0;

    if (!pkt) {
        av_init_packet(&flush_pkt);
        pkt        = &flush_pkt;
        got_output = 1;
    } else if (!size && st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {
        // preserve 0-size sync packets
        compute_pkt_fields(s, st, st->parser, pkt);
    }

    while (size > 0 || (pkt == &flush_pkt && got_output)) {
        int len;

        av_init_packet(&out_pkt);
        //解析
        len = av_parser_parse2(st->parser, st->codec,
                               &out_pkt.data, &out_pkt.size, data, size,
                               pkt->pts, pkt->dts, pkt->pos);

        pkt->pts = pkt->dts = AV_NOPTS_VALUE;
        pkt->pos = -1;
        /* increment read pointer */
        data += len;
        size -= len;

        got_output = !!out_pkt.size;
        //继续
        if (!out_pkt.size)
            continue;

        if (pkt->side_data) {
            out_pkt.side_data       = pkt->side_data;
            out_pkt.side_data_elems = pkt->side_data_elems;
            pkt->side_data          = NULL;
            pkt->side_data_elems    = 0;
        }

        /* set the duration */
        out_pkt.duration = 0;
        if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (st->codec->sample_rate > 0) {
                out_pkt.duration =
                    av_rescale_q_rnd(st->parser->duration,
                                     (AVRational) { 1, st->codec->sample_rate },
                                     st->time_base,
                                     AV_ROUND_DOWN);
            }
        }
        //设置属性值
        out_pkt.stream_index = st->index;
        out_pkt.pts          = st->parser->pts;
        out_pkt.dts          = st->parser->dts;
        out_pkt.pos          = st->parser->pos;

        if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)
            out_pkt.pos = st->parser->frame_offset;

        if (st->parser->key_frame == 1 ||
            (st->parser->key_frame == -1 &&
             st->parser->pict_type == AV_PICTURE_TYPE_I))
            out_pkt.flags |= AV_PKT_FLAG_KEY;

        if (st->parser->key_frame == -1 && st->parser->pict_type ==AV_PICTURE_TYPE_NONE && (pkt->flags&AV_PKT_FLAG_KEY))
            out_pkt.flags |= AV_PKT_FLAG_KEY;

        compute_pkt_fields(s, st, st->parser, &out_pkt);

        if (out_pkt.data == pkt->data && out_pkt.size == pkt->size) {
            out_pkt.buf = pkt->buf;
            pkt->buf    = NULL;
#if FF_API_DESTRUCT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS
            out_pkt.destruct = pkt->destruct;
            pkt->destruct = NULL;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
        }
        if ((ret = av_dup_packet(&out_pkt)) < 0)
            goto fail;

        if (!add_to_pktbuf(&s->parse_queue, &out_pkt, &s->parse_queue_end)) {
            av_free_packet(&out_pkt);
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    /* end of the stream => close and free the parser */
    if (pkt == &flush_pkt) {
        av_parser_close(st->parser);
        st->parser = NULL;
    }

fail:
    av_free_packet(pkt);
    return ret;
}

从代码中可以看出,最终调用了相应AVCodecParser的av_parser_parse2()函数,解析出来AVPacket。此后根据解析的信息还进行了一系列的赋值工作,不再详细叙述。



本文转载自:http://blog.csdn.net/leixiaohua1020/article/details/12678577

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