此前写了好几篇ffmpeg源代码分析文章,列表如下:
图解FFMPEG打开媒体的函数avformat_open_input
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 : av_read_frame()
ffmpeg 源代码简单分析 : avcodec_decode_video2()
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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()的源代码如下:
//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); } }
一般情况下,av_read_frame()会调用read_frame_internal(),其代码如下所示:
//av_read_frame对他进行了封装 static int read_frame_internal(AVFormatContext *s, AVPacket *pkt) { AVStream *st; int len, ret, i; //初始化 av_init_packet(pkt); for(;;) { /* 选择当前的 input stream */ st = s->cur_st; if (st) { //不需要解析。不清楚哪些数据属于这类 if (!st->need_parsing || !st->parser) { /* no parsing needed: we just output the packet as is */ /* raw data support */ *pkt = st->cur_pkt; st->cur_pkt.data= NULL; st->cur_pkt.side_data_elems = 0; st->cur_pkt.side_data = NULL; compute_pkt_fields(s, st, NULL, pkt); s->cur_st = NULL; 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); } break; } //需要解析 else if (st->cur_len > 0 && st->discard < AVDISCARD_ALL) { //解析 len = av_parser_parse2(st->parser, st->codec, &pkt->data, &pkt->size, st->cur_ptr, st->cur_len, st->cur_pkt.pts, st->cur_pkt.dts, st->cur_pkt.pos); st->cur_pkt.pts = AV_NOPTS_VALUE; st->cur_pkt.dts = AV_NOPTS_VALUE; /* increment read pointer */ st->cur_ptr += len; st->cur_len -= len; /* return packet if any */ if (pkt->size) { got_packet: pkt->duration = 0; pkt->stream_index = st->index; pkt->pts = st->parser->pts; pkt->dts = st->parser->dts; pkt->pos = st->parser->pos; if(pkt->data == st->cur_pkt.data && pkt->size == st->cur_pkt.size){ s->cur_st = NULL; pkt->destruct= st->cur_pkt.destruct; st->cur_pkt.destruct= NULL; st->cur_pkt.data = NULL; assert(st->cur_len == 0); }else{ pkt->destruct = NULL; } compute_pkt_fields(s, st, st->parser, pkt); if((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY){ int64_t pos= (st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->pos : st->parser->frame_offset; ff_reduce_index(s, st->index); av_add_index_entry(st, pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME); } break; } } else { /* free packet */ av_free_packet(&st->cur_pkt); s->cur_st = NULL; } } else { AVPacket cur_pkt; /* read next packet */ //读取AVPacket,老版本里只有av_read_packet,现在被封装了 ret = av_read_packet(s, &cur_pkt); if (ret < 0) { if (ret == AVERROR(EAGAIN)) return ret; /* return the last frames, if any */ for(i = 0; i < s->nb_streams; i++) { st = s->streams[i]; if (st->parser && st->need_parsing) { av_parser_parse2(st->parser, st->codec, &pkt->data, &pkt->size, NULL, 0, AV_NOPTS_VALUE, AV_NOPTS_VALUE, AV_NOPTS_VALUE); if (pkt->size) goto got_packet; } } /* no more packets: really terminate parsing */ return ret; } st = s->streams[cur_pkt.stream_index]; st->cur_pkt= cur_pkt; if(st->cur_pkt.pts != AV_NOPTS_VALUE && st->cur_pkt.dts != AV_NOPTS_VALUE && st->cur_pkt.pts < st->cur_pkt.dts){ av_log(s, AV_LOG_WARNING, "Invalid timestamps stream=%d, pts=%"PRId64", dts=%"PRId64", size=%d\n", st->cur_pkt.stream_index, st->cur_pkt.pts, st->cur_pkt.dts, st->cur_pkt.size); // av_free_packet(&st->cur_pkt); // return -1; } if(s->debug & FF_FDEBUG_TS) av_log(s, AV_LOG_DEBUG, "av_read_packet stream=%d, pts=%"PRId64", dts=%"PRId64", size=%d, duration=%d, flags=%d\n", st->cur_pkt.stream_index, st->cur_pkt.pts, st->cur_pkt.dts, st->cur_pkt.size, st->cur_pkt.duration, st->cur_pkt.flags); s->cur_st = st; st->cur_ptr = st->cur_pkt.data; st->cur_len = st->cur_pkt.size; 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; } } } } if(s->debug & FF_FDEBUG_TS) av_log(s, AV_LOG_DEBUG, "read_frame_internal stream=%d, pts=%"PRId64", dts=%"PRId64", size=%d, duration=%d, flags=%d\n", pkt->stream_index, pkt->pts, pkt->dts, pkt->size, pkt->duration, pkt->flags); return 0; }
一般的码流都需要解析,这是需要调用av_paser_parse2(),它的代码如下所示:
//解析。例如解析264里的NAL等等 int av_parser_parse2(AVCodecParserContext *s, AVCodecContext *avctx, uint8_t **poutbuf, int *poutbuf_size, const uint8_t *buf, int buf_size, int64_t pts, int64_t dts, int64_t pos) { int index, i; uint8_t dummy_buf[FF_INPUT_BUFFER_PADDING_SIZE]; if(!(s->flags & PARSER_FLAG_FETCHED_OFFSET)) { s->next_frame_offset = s->cur_offset = pos; s->flags |= PARSER_FLAG_FETCHED_OFFSET; } if (buf_size == 0) { /* padding is always necessary even if EOF, so we add it here */ memset(dummy_buf, 0, sizeof(dummy_buf)); buf = dummy_buf; } else if (s->cur_offset + buf_size != s->cur_frame_end[s->cur_frame_start_index]) { /* skip remainder packets */ /* add a new packet descriptor */ i = (s->cur_frame_start_index + 1) & (AV_PARSER_PTS_NB - 1); s->cur_frame_start_index = i; s->cur_frame_offset[i] = s->cur_offset; s->cur_frame_end[i] = s->cur_offset + buf_size; s->cur_frame_pts[i] = pts; s->cur_frame_dts[i] = dts; s->cur_frame_pos[i] = pos; } if (s->fetch_timestamp){ s->fetch_timestamp=0; s->last_pts = s->pts; s->last_dts = s->dts; s->last_pos = s->pos; ff_fetch_timestamp(s, 0, 0); } /* WARNING: the returned index can be negative */ //H264里对应的就是parser_parse=h264_parse, index = s->parser->parser_parse(s, avctx, (const uint8_t **)poutbuf, poutbuf_size, buf, buf_size); //av_log(NULL, AV_LOG_DEBUG, "parser: in:%"PRId64", %"PRId64", out:%"PRId64", %"PRId64", in:%d out:%d id:%d\n", pts, dts, s->last_pts, s->last_dts, buf_size, *poutbuf_size, avctx->codec_id); /* update the file pointer */ if (*poutbuf_size) { /* fill the data for the current frame */ s->frame_offset = s->next_frame_offset; /* offset of the next frame */ s->next_frame_offset = s->cur_offset + index; s->fetch_timestamp=1; } if (index < 0) index = 0; s->cur_offset += index; return index; }
从index = s->parser->parser_parse(s, avctx, (const uint8_t **)poutbuf, poutbuf_size, buf, buf_size);这句代码可以看出,最终调用了相应解码器的parser_parse()函数。
有点累了,先不做详细分析,以后有机会再补上。