FFmpeg学习 av

FFmpeg学习 av"/>

FFmpeg学习 av

前言

本文分析av_read_frame函数，该函数在libavformat包下。

/*** 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_packet_unref 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);

调用流程如下：
av_read_frame
–> read_frame_internal
----> ff_read_packet
------> s->iformat->read_packet
----> av_parser_init
----> parse_packet
------> av_parser_parse2

av_read_frame

入口函数，从流中读取一个AVPacket，从相应格式中解封装出来的一个音频包或者视频包；
有packet_buffer的话从packet_buffer链表中获取，没有的话去读取一个包。

// utils.c
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{const int genpts = s->flags & AVFMT_FLAG_GENPTS;int eof = 0;int ret;AVStream *st;if (!genpts) {// 一般都会走到这里，在find_stream_info中会存包到packet_buffer，此时会从packet_buffer中获取ret = s->internal->packet_buffer? read_from_packet_buffer(&s->internal->packet_buffer, &s->internal->packet_buffer_end, pkt): read_frame_internal(s, pkt);if (ret < 0)return ret;goto return_packet;}// xxx
return_packet:return ret;
}

read_frame_internal

1. 调用ff_read_packet去读取一个包
2. 读到后初始化parser且进行parse_packet，后将读取到的packet返回

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{int ret = 0, i, got_packet = 0;AVDictionary *metadata = NULL;// 清空packetav_init_packet(pkt);while (!got_packet && !s->internal->parse_queue) {AVStream *st;AVPacket cur_pkt;// 通过s->iformat->read_packet(s, pkt);按格式读取一个包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];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];// 初始化流的parse，st->parser，对h264来说就是即ff_h264_parserif (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {// 根据codec id找到parserst->parser = av_parser_init(st->codecpar->codec_id);if (!st->parser) {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;got_packet = 1;} else if (st->discard < AVDISCARD_ALL) {// 去根据码流格式解析封装包if ((ret = parse_packet(s, &cur_pkt, cur_pkt.stream_index)) < 0)return ret;st->codecpar->sample_rate = st->internal->avctx->sample_rate;st->codecpar->bit_rate = st->internal->avctx->bit_rate;st->codecpar->channels = st->internal->avctx->channels;st->codecpar->channel_layout = st->internal->avctx->channel_layout;st->codecpar->codec_id = st->internal->avctx->codec_id;} else {// 直接释放了av_packet_unref(&cur_pkt);}// 是关键帧的话if (pkt->flags & AV_PKT_FLAG_KEY)st->skip_to_keyframe = 0;if (st->skip_to_keyframe) {av_packet_unref(&cur_pkt);if (got_packet) {*pkt = cur_pkt;}got_packet = 0;}}// 从parse_queue再去获取packetif (!got_packet && s->internal->parse_queue)ret = read_from_packet_buffer(&s->internal->parse_queue, &s->internal->parse_queue_end, pkt);return ret;
}

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ff_read_packet

代码贴的是无request_probe的，当request_probe不设置时，即始终是0，raw_packet_buffer始终是0，读到了就返回了；

是mp4格式时会设置request_probe，需要读取数据到raw_packet_buffer中，直到probe_codec成功为止；
是flv的话，request_probe一直是0，会通过flvdec.c#read_packet(s, pkt);读取包；当读取的是MetaData和第一个视频包(sps/pps)和第一个音频包(AAC sequence header)时会返回REDO，直到读到第二个视频包或音频包后返回。
在第一个视频包(sps/pps)和第一个音频包(AAC sequence header)时会根据协议设置相应的codec_id。

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{int ret, i, err;AVStream *st;for (;;) {AVPacketList *pktl = s->internal->raw_packet_buffer;if (pktl) {*pkt = pktl->pkt;st   = s->streams[pkt->stream_index];if (s->internal->raw_packet_buffer_remaining_size <= 0)if ((err = probe_codec(s, st, NULL)) < 0)return err;if (st->request_probe <= 0) {s->internal->raw_packet_buffer                 = pktl->next;s->internal->raw_packet_buffer_remaining_size += pkt->size;av_free(pktl);return 0;}}pkt->data = NULL;pkt->size = 0;av_init_packet(pkt);// 通过解封装器read_packetret = s->iformat->read_packet(s, pkt);if (ret < 0) {/* Some demuxers return FFERROR_REDO when they consumedata and discard it (ignored streams, junk, extradata).We must re-call the demuxer to get the real packet. */// 从flv日志看，是读到第二个音频包或第二个视频包为止，读metaData、第一个video包第一个audio包都返回REDO;if (ret == FFERROR_REDO)continue;if (!pktl || ret == AVERROR(EAGAIN))return ret;for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];if (st->probe_packets || st->request_probe > 0)if ((err = probe_codec(s, st, NULL)) < 0)return err;}continue;}if (!pktl && st->request_probe <= 0)return ret;err = add_to_pktbuf(&s->internal->raw_packet_buffer, pkt, &s->internal->raw_packet_buffer_end, 0);if (err)return err;s->internal->raw_packet_buffer_remaining_size -= pkt->size;if ((err = probe_codec(s, st, pkt)) < 0)return err;}}

s->iformat->read_packet
调用相应格式的read_packet，按相应协议从传输层读取一个包；

av_parser_init
遍历parser链表找到相应codec_id的parser。
如h264编码格式的h264_parser.c#ff_h264_parser。

AVCodecParser ff_h264_parser = {.codec_ids      = { AV_CODEC_ID_H264 },.priv_data_size = sizeof(H264ParseContext),.parser_init    = init,.parser_parse   = h264_parse,.parser_close   = h264_close,.split          = h264_split,
};

AVCodecParserContext *av_parser_init(int codec_id)
{AVCodecParserContext *s = NULL;AVCodecParser *parser;int ret;if (codec_id == AV_CODEC_ID_NONE)return NULL;// 遍历parser链表for (parser = av_first_parser; parser; parser = parser->next) {if (parser->codec_ids[0] == codec_id ||parser->codec_ids[1] == codec_id ||parser->codec_ids[2] == codec_id ||parser->codec_ids[3] == codec_id ||parser->codec_ids[4] == codec_id)goto found;}return NULL;found:// 初始化AVCodecParserContexts = av_mallocz(sizeof(AVCodecParserContext));s->parser = parser;s->priv_data = av_mallocz(parser->priv_data_size);s->fetch_timestamp=1;s->pict_type = AV_PICTURE_TYPE_I;if (parser->parser_init) {ret = parser->parser_init(s);}s->key_frame            = -1;s->dts_sync_point       = INT_MIN;s->dts_ref_dts_delta    = INT_MIN;s->pts_dts_delta        = INT_MIN;s->format               = -1;return s;
}

parse_packet

调用av_parser_parse2解析一个音频包或视频包，分割出来后都添加到parse_queue里；
核心会调用s->parser->parser_parse去解析一个包，会走到ff_h264_parser里。

/*** 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;}while (size > 0 || (pkt == &flush_pkt && got_output)) {int len;int64_t next_pts = pkt->pts;int64_t next_dts = pkt->dts;// 解析pktav_init_packet(&out_pkt);// 会调用s->parser->parser_parse，根据具体的流parser去解析一个包len = av_parser_parse2(st->parser, st->internal->avctx,&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;if (!out_pkt.size)continue;// ... 一系列赋值操作// 判断是否是关键帧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, next_dts, next_pts);// 添加parse_queue尾部ret = add_to_pktbuf(&s->internal->parse_queue, &out_pkt,&s->internal->parse_queue_end, 1);av_packet_unref(&out_pkt);if (ret < 0)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_packet_unref(pkt);return ret;
}

av_parser_parse2

会调研s->parser->parser_parse会走到h264_parser.c文件中，按照H264格式解析。
h264后续会专门写文章分析，这里就贴一下它的结构体吧。

// h264_parse.c
AVCodecParser ff_h264_parser = {.codec_ids      = { AV_CODEC_ID_H264 },.priv_data_size = sizeof(H264ParseContext),.parser_init    = init,.parser_parse   = h264_parse,.parser_close   = h264_close,.split          = h264_split,
};