FFmpeg源代码：av_read_frame

FFmpeg源代码：av_read_frame

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）。

/** * 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. * * On success, the returned packet is reference-counted (pkt->buf is set) and * valid indefinitely. 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. On error, pkt will be blank * (as if it came from av_packet_alloc()). * * @note pkt will be initialized, so it may be uninitialized, but it must not * contain data that needs to be freed. */int av_read_frame(AVFormatContext *s, AVPacket *pkt);

该函数参数：

s: 输入流的AVFormatContextpkt: 输出的AVPacket

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

//获取一个AVPacketint av_read_frame(AVFormatContext *s, AVPacket *pkt){ const int genpts = s->flags & AVFMT_FLAG_GENPTS; int eof = 0; int ret; AVStream *st; if (!genpts) { //一般情况下会调用read_frame_internal ret = s->internal->packet_buffer ? ff_packet_list_get(&s->internal->packet_buffer, &s->internal->packet_buffer_end, pkt) : read_frame_internal(s, pkt); if (ret < 0) return ret; goto return_packet; } for (;;) { AVPacketList *pktl = s->internal->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; // last dts seen for this stream. if any of packets following // current one had no dts, we will set this to AV_NOPTS_VALUE. int64_t last_dts = next_pkt->dts; av_assert2(wrap_bits <= 64); 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, 2ULL << (wrap_bits - 1)) < 0) { if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) { // not B-frame next_pkt->pts = pktl->pkt.dts; } if (last_dts != AV_NOPTS_VALUE) { // Once last dts was set to AV_NOPTS_VALUE, we don't change it. last_dts = pktl->pkt.dts; } } pktl = pktl->next; } if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) { // Fixing the last reference frame had none pts issue (For MXF etc). // We only do this when // 1. eof. // 2. we are not able to resolve a pts value for current packet. // 3. the packets for this stream at the end of the files had valid dts. next_pkt->pts = last_dts + next_pkt->duration; } pktl = s->internal->packet_buffer; } /* read packet from packet buffer, if there is data */ st = s->streams[next_pkt->stream_index]; if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL && next_pkt->dts != AV_NOPTS_VALUE && !eof)) { ret = ff_packet_list_get(&s->internal->packet_buffer, &s->internal->packet_buffer_end, pkt); goto return_packet; } } ret = read_frame_internal(s, pkt); if (ret < 0) { if (pktl && ret != AVERROR(EAGAIN)) { eof = 1; continue; } else return ret; } ret = ff_packet_list_put(&s->internal->packet_buffer, &s->internal->packet_buffer_end, pkt, 0); if (ret < 0) { av_packet_unref(pkt); return ret; } }return_packet: st = s->streams[pkt->stream_index]; if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) { ff_reduce_index(s, st->index); av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME); } if (is_relative(pkt->dts)) pkt->dts -= RELATIVE_TS_BASE; if (is_relative(pkt->pts)) pkt->pts -= RELATIVE_TS_BASE; return ret;}

从接口定义中可以看出，av_read_frame()调用了read_frame_internal()

read_frame_internal()是一个内部函数

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt){ int ret, i, got_packet = 0; AVDictionary *metadata = NULL; while (!got_packet && !s->internal->parse_queue) { AVStream *st; /* read next packet */ //Read a transport packet from a media file. // 很重要的一个函数 ret = ff_read_packet(s, 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]; //解析pkt if (st->parser && st->need_parsing) parse_packet(s, pkt, st->index, 1); } /* all remaining packets are now in parse_queue => * really terminate parsing */ break; } ret = 0; st = s->streams[pkt->stream_index]; /* update context if required */ if (st->internal->need_context_update) { if (avcodec_is_open(st->internal->avctx)) { av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n"); avcodec_close(st->internal->avctx); st->info->found_decoder = 0; } /* close parser, because it depends on the codec */ if (st->parser && st->internal->avctx->codec_id != st->codecpar->codec_id) { av_parser_close(st->parser); st->parser = NULL; } ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar); if (ret < 0) { av_packet_unref(pkt); return ret; }#if FF_API_LAVF_AVCTXFF_DISABLE_DEPRECATION_WARNINGS /* update deprecated public codec context */ ret = avcodec_parameters_to_context(st->codec, st->codecpar); if (ret < 0) { av_packet_unref(pkt); return ret; }FF_ENABLE_DEPRECATION_WARNINGS#endif st->internal->need_context_update = 0; } if (pkt->pts != AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE && pkt->pts < pkt->dts) { av_log(s, AV_LOG_WARNING, "Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n", pkt->stream_index, av_ts2str(pkt->pts), av_ts2str(pkt->dts), 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=%"PRId64", flags=%d\n", pkt->stream_index, av_ts2str(pkt->pts), av_ts2str(pkt->dts), pkt->size, pkt->duration, pkt->flags); if (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) { st->parser = av_parser_init(st->codecpar->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->codecpar->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 */ compute_pkt_fields(s, st, NULL, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE); 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, pkt, pkt->stream_index, 0)) < 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 { /* free packet */ av_packet_unref(pkt); } if (pkt->flags & AV_PKT_FLAG_KEY) st->skip_to_keyframe = 0; if (st->skip_to_keyframe) { av_packet_unref(pkt); got_packet = 0; } } if (!got_packet && s->internal->parse_queue) ret = ff_packet_list_get(&s->internal->parse_queue, &s->internal->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->start_skip_samples && (pkt->pts == 0 || pkt->pts == RELATIVE_TS_BASE)) st->skip_samples = st->start_skip_samples; 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 / discard %d\n", st->skip_samples, discard_padding); } 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; } } 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 FF_API_LAVF_AVCTX update_stream_avctx(s);#endif if (s->debug & FF_FDEBUG_TS) av_log(s, AV_LOG_DEBUG, "read_frame_internal stream=%d, pts=%s, dts=%s, " "size=%d, duration=%"PRId64", flags=%d\n", pkt->stream_index, av_ts2str(pkt->pts), av_ts2str(pkt->dts), pkt->size, pkt->duration, pkt->flags); /* A demuxer might have returned EOF because of an IO error, let's * propagate this back to the user. */ if (ret == AVERROR_EOF && s->pb && s->pb->error < 0 && s->pb->error != AVERROR(EAGAIN)) ret = s->pb->error; return ret;}

read_frame_internal() 的功能主要由以下两个接口实现：

1. ff_read_packet()从相应的AVInputFormat读取AVPacket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

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