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FFmpeg源代码简单分析:avformat_find_stream_info()

栏目:php教程时间:2015-03-24 08:22:44
本文简单分析FFmpeg中1个经常使用的函数:avformat_find_stream_info()。该函数可以读取1部份视音频数据并且取得1些相干的信息。avformat_find_stream_info()的声明位于libavformatavformat.h,以下所示。 
/** * Read packets of a media file to get stream information. This * is useful for file formats with no headers such as MPEG. This * function also computes the real framerate in case of MPEG⑵ repeat * frame mode. * The logical file position is not changed by this function; * examined packets may be buffered for later processing. * * @param ic media file handle * @param options If non-NULL, an ic.nb_streams long array of pointers to * dictionaries, where i-th member contains options for * codec corresponding to i-th stream. * On return each dictionary will be filled with options that were not found. * @return >=0 if OK, AVERROR_xxx on error * * @note this function isn't guaranteed to open all the codecs, so * options being non-empty at return is a perfectly normal behavior. * * @todo Let the user decide somehow what information is needed so that * we do not waste time getting stuff the user does not need. */ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

简单解释1下它的参数的含义:
ic:输入的AVFormatContext。
options:额外的选项,目前没有深入研究过。
函数正常履行后返回值大于等于0。
该函数最典型的例子可以参考:最简单的基于FFMPEG+SDL的视频播放器 ver2 (采取SDL2.0)

PS:由于该函数比较复杂,所以只看了1部份代码,以后有时间再进1步分析。

函数调用关系图

函数的调用关系以下图所示。


avformat_find_stream_info()

avformat_find_stream_info()的定义位于libavformatutils.c。它的代码比较长,以下所示。
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options) { int i, count, ret = 0, j; int64_t read_size; AVStream *st; AVPacket pkt1, *pkt; int64_t old_offset = avio_tell(ic->pb); // new streams might appear, no options for those int orig_nb_streams = ic->nb_streams; int flush_codecs; int64_t max_analyze_duration = ic->max_analyze_duration2; int64_t probesize = ic->probesize2; if (!max_analyze_duration) max_analyze_duration = ic->max_analyze_duration; if (ic->probesize) probesize = ic->probesize; flush_codecs = probesize > 0; av_opt_set(ic, "skip_clear", "1", AV_OPT_SEARCH_CHILDREN); if (!max_analyze_duration) { if (!strcmp(ic->iformat->name, "flv") && !(ic->ctx_flags & AVFMTCTX_NOHEADER)) { max_analyze_duration = 10*AV_TIME_BASE; } else max_analyze_duration = 5*AV_TIME_BASE; } if (ic->pb) av_log(ic, AV_LOG_DEBUG, "Before avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d ", avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count); for (i = 0; i < ic->nb_streams; i++) { const AVCodec *codec; AVDictionary *thread_opt = NULL; st = ic->streams[i]; if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO || st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE) { /* if (!st->time_base.num) st->time_base = */ if (!st->codec->time_base.num) st->codec->time_base = st->time_base; } // only for the split stuff if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE)) { st->parser = av_parser_init(st->codec->codec_id); if (st->parser) { if (st->need_parsing == AVSTREAM_PARSE_HEADERS) { st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES; } else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW) { st->parser->flags |= PARSER_FLAG_USE_CODEC_TS; } } else if (st->need_parsing) { av_log(ic, AV_LOG_VERBOSE, "parser not found for codec " "%s, packets or times may be invalid. ", avcodec_get_name(st->codec->codec_id)); } } codec = find_decoder(ic, st, st->codec->codec_id); /* Force thread count to 1 since the H.264 decoder will not extract * SPS and PPS to extradata during multi-threaded decoding. */ av_dict_set(options ? &options[i] : &thread_opt, "threads", "1", 0); if (ic->codec_whitelist) av_dict_set(options ? &options[i] : &thread_opt, "codec_whitelist", ic->codec_whitelist, 0); /* Ensure that subtitle_header is properly set. */ if (st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE && codec && !st->codec->codec) { if (avcodec_open2(st->codec, codec, options ? &options[i] : &thread_opt) < 0) av_log(ic, AV_LOG_WARNING, "Failed to open codec in av_find_stream_info "); } // Try to just open decoders, in case this is enough to get parameters. if (!has_codec_parameters(st, NULL) && st->request_probe <= 0) { if (codec && !st->codec->codec) if (avcodec_open2(st->codec, codec, options ? &options[i] : &thread_opt) < 0) av_log(ic, AV_LOG_WARNING, "Failed to open codec in av_find_stream_info "); } if (!options) av_dict_free(&thread_opt); } for (i = 0; i < ic->nb_streams; i++) { #if FF_API_R_FRAME_RATE ic->streams[i]->info->last_dts = AV_NOPTS_VALUE; #endif ic->streams[i]->info->fps_first_dts = AV_NOPTS_VALUE; ic->streams[i]->info->fps_last_dts = AV_NOPTS_VALUE; } count = 0; read_size = 0; for (;;) { if (ff_check_interrupt(&ic->interrupt_callback)) { ret = AVERROR_EXIT; av_log(ic, AV_LOG_DEBUG, "interrupted "); break; } /* check if one codec still needs to be handled */ for (i = 0; i < ic->nb_streams; i++) { int fps_analyze_framecount = 20; st = ic->streams[i]; if (!has_codec_parameters(st, NULL)) break; /* If the timebase is coarse (like the usual millisecond precision * of mkv), we need to analyze more frames to reliably arrive at * the correct fps. */ if (av_q2d(st->time_base) > 0.0005) fps_analyze_framecount *= 2; if (!tb_unreliable(st->codec)) fps_analyze_framecount = 0; if (ic->fps_probe_size >= 0) fps_analyze_framecount = ic->fps_probe_size; if (st->disposition & AV_DISPOSITION_ATTACHED_PIC) fps_analyze_framecount = 0; /* variable fps and no guess at the real fps */ if (!(st->r_frame_rate.num && st->avg_frame_rate.num) && st->info->duration_count < fps_analyze_framecount && st->codec->codec_type == AVMEDIA_TYPE_VIDEO) break; if (st->parser && st->parser->parser->split && !st->codec->extradata) break; if (st->first_dts == AV_NOPTS_VALUE && !(ic->iformat->flags & AVFMT_NOTIMESTAMPS) && st->codec_info_nb_frames < ic->max_ts_probe && (st->codec->codec_type == AVMEDIA_TYPE_VIDEO || st->codec->codec_type == AVMEDIA_TYPE_AUDIO)) break; } if (i == ic->nb_streams) { /* NOTE: If the format has no header, then we need to read some * packets to get most of the streams, so we cannot stop here. */ if (!(ic->ctx_flags & AVFMTCTX_NOHEADER)) { /* If we found the info for all the codecs, we can stop. */ ret = count; av_log(ic, AV_LOG_DEBUG, "All info found "); flush_codecs = 0; break; } } /* We did not get all the codec info, but we read too much data. */ if (read_size >= probesize) { ret = count; av_log(ic, AV_LOG_DEBUG, "Probe buffer size limit of %"PRId64" bytes reached ", probesize); for (i = 0; i < ic->nb_streams; i++) if (!ic->streams[i]->r_frame_rate.num && ic->streams[i]->info->duration_count <= 1 && ic->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO && strcmp(ic->iformat->name, "image2")) av_log(ic, AV_LOG_WARNING, "Stream #%d: not enough frames to estimate rate; " "consider increasing probesize ", i); break; } /* NOTE: A new stream can be added there if no header in file * (AVFMTCTX_NOHEADER). */ ret = read_frame_internal(ic, &pkt1); if (ret == AVERROR(EAGAIN)) continue; if (ret < 0) { /* EOF or error*/ break; } if (ic->flags & AVFMT_FLAG_NOBUFFER) free_packet_buffer(&ic->packet_buffer, &ic->packet_buffer_end); { pkt = add_to_pktbuf(&ic->packet_buffer, &pkt1, &ic->packet_buffer_end); if (!pkt) { ret = AVERROR(ENOMEM); goto find_stream_info_err; } if ((ret = av_dup_packet(pkt)) < 0) goto find_stream_info_err; } st = ic->streams[pkt->stream_index]; if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC)) read_size += pkt->size; if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) { /* check for non-increasing dts */ if (st->info->fps_last_dts != AV_NOPTS_VALUE && st->info->fps_last_dts >= pkt->dts) { av_log(ic, AV_LOG_DEBUG, "Non-increasing DTS in stream %d: packet %d with DTS " "%"PRId64", packet %d with DTS %"PRId64" ", st->index, st->info->fps_last_dts_idx, st->info->fps_last_dts, st->codec_info_nb_frames, pkt->dts); st->info->fps_first_dts = st->info->fps_last_dts = AV_NOPTS_VALUE; } /* Check for a discontinuity in dts. If the difference in dts * is more than 1000 times the average packet duration in the * sequence, we treat it as a discontinuity. */ if (st->info->fps_last_dts != AV_NOPTS_VALUE && st->info->fps_last_dts_idx > st->info->fps_first_dts_idx && (pkt->dts - st->info->fps_last_dts) / 1000 > (st->info->fps_last_dts - st->info->fps_first_dts) / (st->info->fps_last_dts_idx - st->info->fps_first_dts_idx)) { av_log(ic, AV_LOG_WARNING, "DTS discontinuity in stream %d: packet %d with DTS " "%"PRId64", packet %d with DTS %"PRId64" ", st->index, st->info->fps_last_dts_idx, st->info->fps_last_dts, st->codec_info_nb_frames, pkt->dts); st->info->fps_first_dts = st->info->fps_last_dts = AV_NOPTS_VALUE; } /* update stored dts values */ if (st->info->fps_first_dts == AV_NOPTS_VALUE) { st->info->fps_first_dts = pkt->dts; st->info->fps_first_dts_idx = st->codec_info_nb_frames; } st->info->fps_last_dts = pkt->dts; st->info->fps_last_dts_idx = st->codec_info_nb_frames; } if (st->codec_info_nb_frames>1) { int64_t t = 0; if (st->time_base.den > 0) t = av_rescale_q(st->info->codec_info_duration, st->time_base, AV_TIME_BASE_Q); if (st->avg_frame_rate.num > 0) t = FFMAX(t, av_rescale_q(st->codec_info_nb_frames, av_inv_q(st->avg_frame_rate), AV_TIME_BASE_Q)); if ( t == 0 && st->codec_info_nb_frames>30 && st->info->fps_first_dts != AV_NOPTS_VALUE && st->info->fps_last_dts != AV_NOPTS_VALUE) t = FFMAX(t, av_rescale_q(st->info->fps_last_dts - st->info->fps_first_dts, st->time_base, AV_TIME_BASE_Q)); if (t >= max_analyze_duration) { av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds ", max_analyze_duration, t); if (ic->flags & AVFMT_FLAG_NOBUFFER) av_packet_unref(pkt); break; } if (pkt->duration) { st->info->codec_info_duration += pkt->duration; st->info->codec_info_duration_fields += st->parser && st->need_parsing && st->codec->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2; } } #if FF_API_R_FRAME_RATE if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO) ff_rfps_add_frame(ic, st, pkt->dts); #endif if (st->parser && st->parser->parser->split && !st->codec->extradata) { int i = st->parser->parser->split(st->codec, pkt->data, pkt->size); if (i > 0 && i < FF_MAX_EXTRADATA_SIZE) { if (ff_alloc_extradata(st->codec, i)) return AVERROR(ENOMEM); memcpy(st->codec->extradata, pkt->data, st->codec->extradata_size); } } /* If still no information, we try to open the codec and to * decompress the frame. We try to avoid that in most cases as * it takes longer and uses more memory. For MPEG⑷, we need to * decompress for QuickTime. * * If CODEC_CAP_CHANNEL_CONF is set this will force decoding of at * least one frame of codec data, this makes sure the codec initializes * the channel configuration and does not only trust the values from * the container. */ try_decode_frame(ic, st, pkt, (options && i < orig_nb_streams) ? &options[i] : NULL); if (ic->flags & AVFMT_FLAG_NOBUFFER) av_packet_unref(pkt); st->codec_info_nb_frames++; count++; } if (flush_codecs) { AVPacket empty_pkt = { 0 }; int err = 0; av_init_packet(&empty_pkt); for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; /* flush the decoders */ if (st->info->found_decoder == 1) { do { err = try_decode_frame(ic, st, &empty_pkt, (options && i < orig_nb_streams) ? &options[i] : NULL); } while (err > 0 && !has_codec_parameters(st, NULL)); if (err < 0) { av_log(ic, AV_LOG_INFO, "decoding for stream %d failed ", st->index); } } } } // close codecs which were opened in try_decode_frame() for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; avcodec_close(st->codec); } ff_rfps_calculate(ic); for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO) { if (st->codec->codec_id == AV_CODEC_ID_RAWVIDEO && !st->codec->codec_tag && !st->codec->bits_per_coded_sample) { uint32_t tag= avcodec_pix_fmt_to_codec_tag(st->codec->pix_fmt); if (avpriv_find_pix_fmt(avpriv_get_raw_pix_fmt_tags(), tag) == st->codec->pix_fmt) st->codec->codec_tag= tag; } /* estimate average framerate if not set by demuxer */ if (st->info->codec_info_duration_fields && !st->avg_frame_rate.num && st->info->codec_info_duration) { int best_fps = 0; double best_error = 0.01; if (st->info->codec_info_duration >= INT64_MAX / st->time_base.num / 2|| st->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den || st->info->codec_info_duration < 0) continue; av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den, st->info->codec_info_duration_fields * (int64_t) st->time_base.den, st->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000); /* Round guessed framerate to a "standard" framerate if it's * within 1% of the original estimate. */ for (j = 0; j < MAX_STD_TIMEBASES; j++) { AVRational std_fps = { get_std_framerate(j), 12 * 1001 }; double error = fabs(av_q2d(st->avg_frame_rate) / av_q2d(std_fps) - 1); if (error < best_error) { best_error = error; best_fps = std_fps.num; } } if (best_fps) av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den, best_fps, 12 * 1001, INT_MAX); } if (!st->r_frame_rate.num) { if ( st->codec->time_base.den * (int64_t) st->time_base.num <= st->codec->time_base.num * st->codec->ticks_per_frame * (int64_t) st->time_base.den) { st->r_frame_rate.num = st->codec->time_base.den; st->r_frame_rate.den = st->codec->time_base.num * st->codec->ticks_per_frame; } else { st->r_frame_rate.num = st->time_base.den; st->r_frame_rate.den = st->time_base.num; } } } else if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) { if (!st->codec->bits_per_coded_sample) st->codec->bits_per_coded_sample = av_get_bits_per_sample(st->codec->codec_id); // set stream disposition based on audio service type switch (st->codec->audio_service_type) { case AV_AUDIO_SERVICE_TYPE_EFFECTS: st->disposition = AV_DISPOSITION_CLEAN_EFFECTS; break; case AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED: st->disposition = AV_DISPOSITION_VISUAL_IMPAIRED; break; case AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED: st->disposition = AV_DISPOSITION_HEARING_IMPAIRED; break; case AV_AUDIO_SERVICE_TYPE_COMMENTARY: st->disposition = AV_DISPOSITION_COMMENT; break; case AV_AUDIO_SERVICE_TYPE_KARAOKE: st->disposition = AV_DISPOSITION_KARAOKE; break; } } } if (probesize) estimate_timings(ic, old_offset); av_opt_set(ic, "skip_clear", "0", AV_OPT_SEARCH_CHILDREN); if (ret >= 0 && ic->nb_streams) /* We could not have all the codec parameters before EOF. */ ret = ⑴; for (i = 0; i < ic->nb_streams; i++) { const char *errmsg; st = ic->streams[i]; if (!has_codec_parameters(st, &errmsg)) { char buf[256]; avcodec_string(buf, sizeof(buf), st->codec, 0); av_log(ic, AV_LOG_WARNING, "Could not find codec parameters for stream %d (%s): %s " "Consider increasing the value for the 'analyzeduration' and 'probesize' options ", i, buf, errmsg); } else { ret = 0; } } compute_chapters_end(ic); find_stream_info_err: for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; if (ic->streams[i]->codec->codec_type != AVMEDIA_TYPE_AUDIO) ic->streams[i]->codec->thread_count = 0; if (st->info) av_freep(&st->info->duration_error); av_freep(&ic->streams[i]->info); } if (ic->pb) av_log(ic, AV_LOG_DEBUG, "After avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d frames:%d ", avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, count); return ret; }

由于avformat_find_stream_info()代码比较长,难以全部份析,在这里只能简单记录1下它的要点。该函数主要用于给每一个媒体流(音频/视频)的AVStream结构体赋值。我们大致阅读1下这个函数的代码,会发现它其实已实现了解码器的查找,解码器的打开,视音频帧的读取,视音频帧的解码等工作。换句话说,该函数实际上已“走通”的解码的全部流程。下面看1下除成员变量赋值以外,该函数的几个关键流程。
1.查找解码器:find_decoder()
2.打开解码器:avcodec_open2()
3.读取完全的1帧紧缩编码的数据:read_frame_internal()
注:av_read_frame()内部实际上就是调用的read_frame_internal()。
4.解码1些紧缩编码数据:try_decode_frame()
下面选择上述流程中几个关键函数的代码简单看1下。

find_decoder()

find_decoder()用于找到适合的解码器,它的定义以下所示。
static const AVCodec *find_decoder(AVFormatContext *s, AVStream *st, enum AVCodecID codec_id) { if (st->codec->codec) return st->codec->codec; switch (st->codec->codec_type) { case AVMEDIA_TYPE_VIDEO: if (s->video_codec) return s->video_codec; break; case AVMEDIA_TYPE_AUDIO: if (s->audio_codec) return s->audio_codec; break; case AVMEDIA_TYPE_SUBTITLE: if (s->subtitle_codec) return s->subtitle_codec; break; } return avcodec_find_decoder(codec_id); }

从代码中可以看出,如果指定的AVStream已包括了解码器,则函数甚么也不做直接返回。否则调用avcodec_find_decoder()获得解码器。avcodec_find_decoder()是1个FFmpeg的API函数,在这里不做详细分析。

read_frame_internal()

read_frame_internal()的功能是读取1帧紧缩码流数据。FFmpeg的API函数av_read_frame()内部调用的就是read_frame_internal()。有关这方面的知识可以参考文章:

ffmpeg 源代码简单分析 : av_read_frame()
因此,可以认为read_frame_internal()和av_read_frame()的功能基本上是同等的。

try_decode_frame()

try_decode_frame()的功能可以从字面上的意思进行理解:“尝试解码1些帧”,它的定义以下所示。
/* returns 1 or 0 if or if not decoded data was returned, or a negative error */ static int try_decode_frame(AVFormatContext *s, AVStream *st, AVPacket *avpkt, AVDictionary **options) { const AVCodec *codec; int got_picture = 1, ret = 0; AVFrame *frame = av_frame_alloc(); AVSubtitle subtitle; AVPacket pkt = *avpkt; if (!frame) return AVERROR(ENOMEM); if (!avcodec_is_open(st->codec) && st->info->found_decoder <= 0 && (st->codec->codec_id != -st->info->found_decoder || !st->codec->codec_id)) { AVDictionary *thread_opt = NULL; codec = find_decoder(s, st, st->codec->codec_id); if (!codec) { st->info->found_decoder = -st->codec->codec_id; ret = ⑴; goto fail; } /* Force thread count to 1 since the H.264 decoder will not extract * SPS and PPS to extradata during multi-threaded decoding. */ av_dict_set(options ? options : &thread_opt, "threads", "1", 0); if (s->codec_whitelist) av_dict_set(options ? options : &thread_opt, "codec_whitelist", s->codec_whitelist, 0); ret = avcodec_open2(st->codec, codec, options ? options : &thread_opt); if (!options) av_dict_free(&thread_opt); if (ret < 0) { st->info->found_decoder = -st->codec->codec_id; goto fail; } st->info->found_decoder = 1; } else if (!st->info->found_decoder) st->info->found_decoder = 1; if (st->info->found_decoder < 0) { ret = ⑴; goto fail; } while ((pkt.size > 0 || (!pkt.data && got_picture)) && ret >= 0 && (!has_codec_parameters(st, NULL) || !has_decode_delay_been_guessed(st) || (!st->codec_info_nb_frames && st->codec->codec->capabilities & CODEC_CAP_CHANNEL_CONF))) { got_picture = 0; switch (st->codec->codec_type) { case AVMEDIA_TYPE_VIDEO: ret = avcodec_decode_video2(st->codec, frame, &got_picture, &pkt); break; case AVMEDIA_TYPE_AUDIO: ret = avcodec_decode_audio4(st->codec, frame, &got_picture, &pkt); break; case AVMEDIA_TYPE_SUBTITLE: ret = avcodec_decode_subtitle2(st->codec, &subtitle, &got_picture, &pkt); ret = pkt.size; break; default: break; } if (ret >= 0) { if (got_picture) st->nb_decoded_frames++; pkt.data += ret; pkt.size -= ret; ret = got_picture; } } if (!pkt.data && !got_picture) ret = ⑴; fail: av_frame_free(&frame); return ret; }

从try_decode_frame()的定义可以看出,该函数首先判断视音频流的解码器是不是已打开,如果没有打开的话,先打开相应的解码器。接下来根据视音频流类型的不同,调用不同的解码函数进行解码:视频流调用avcodec_decode_video2(),音频流调用avcodec_decode_audio4(),字幕流调用avcodec_decode_subtitle2()。解码的循环会1直延续下去直到满足了while()的所有条件。

while()语句的条件中有1个has_codec_parameters()函数,用于判断AVStream中的成员变量是不是都已设置终了。该函数在avformat_find_stream_info()中的多个地方被使用过。下面简单看1下该函数。


has_codec_parameters()

has_codec_parameters()用于检查AVStream中的成员变量是不是都已设置终了。函数的定义以下。
static int has_codec_parameters(AVStream *st, const char **errmsg_ptr) { AVCodecContext *avctx = st->codec; #define FAIL(errmsg) do { if (errmsg_ptr) *errmsg_ptr = errmsg; return 0; } while (0) if ( avctx->codec_id == AV_CODEC_ID_NONE && avctx->codec_type != AVMEDIA_TYPE_DATA) FAIL("unknown codec"); switch (avctx->codec_type) { case AVMEDIA_TYPE_AUDIO: if (!avctx->frame_size && determinable_frame_size(avctx)) FAIL("unspecified frame size"); if (st->info->found_decoder >= 0 && avctx->sample_fmt == AV_SAMPLE_FMT_NONE) FAIL("unspecified sample format"); if (!avctx->sample_rate) FAIL("unspecified sample rate"); if (!avctx->channels) FAIL("unspecified number of channels"); if (st->info->found_decoder >= 0 && !st->nb_decoded_frames && avctx->codec_id == AV_CODEC_ID_DTS) FAIL("no decodable DTS frames"); break; case AVMEDIA_TYPE_VIDEO: if (!avctx->width) FAIL("unspecified size"); if (st->info->found_decoder >= 0 && avctx->pix_fmt == AV_PIX_FMT_NONE) FAIL("unspecified pixel format"); if (st->codec->codec_id == AV_CODEC_ID_RV30 || st->codec->codec_id == AV_CODEC_ID_RV40) if (!st->sample_aspect_ratio.num && !st->codec->sample_aspect_ratio.num && !st->codec_info_nb_frames) FAIL("no frame in rv30/40 and no sar"); break; case AVMEDIA_TYPE_SUBTITLE: if (avctx->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE && !avctx->width) FAIL("unspecified size"); break; case AVMEDIA_TYPE_DATA: if (avctx->codec_id == AV_CODEC_ID_NONE) return 1; } return 1; }


estimate_timings()

estimate_timings()位于avformat_find_stream_info()最后面,用于估算AVFormatContext和AVStream的时长duration。它的代码以下所示。
static void estimate_timings(AVFormatContext *ic, int64_t old_offset) { int64_t file_size; /* get the file size, if possible */ if (ic->iformat->flags & AVFMT_NOFILE) { file_size = 0; } else { file_size = avio_size(ic->pb); file_size = FFMAX(0, file_size); } if ((!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts")) && file_size && ic->pb->seekable) { /* get accurate estimate from the PTSes */ estimate_timings_from_pts(ic, old_offset); ic->duration_estimation_method = AVFMT_DURATION_FROM_PTS; } else if (has_duration(ic)) { /* at least one component has timings - we use them for all * the components */ fill_all_stream_timings(ic); ic->duration_estimation_method = AVFMT_DURATION_FROM_STREAM; } else { /* less precise: use bitrate info */ estimate_timings_from_bit_rate(ic); ic->duration_estimation_method = AVFMT_DURATION_FROM_BITRATE; } update_stream_timings(ic); { int i; AVStream av_unused *st; for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; av_dlog(ic, "%d: start_time: %0.3f duration: %0.3f ", i, (double) st->start_time / AV_TIME_BASE, (double) st->duration / AV_TIME_BASE); } av_dlog(ic, "stream: start_time: %0.3f duration: %0.3f bitrate=%d kb/s ", (double) ic->start_time / AV_TIME_BASE, (double) ic->duration / AV_TIME_BASE, ic->bit_rate / 1000); } }

从estimate_timings()的代码中可以看出,有3种估算方法:
(1)通过pts(显示时间戳)。该方法调用estimate_timings_from_pts()。它的基本思想就是读取视音频流中的结束位置AVPacket的PTS和起始位置AVPacket的PTS,二者相减得到时长信息。
(2)通过已知流的时长。该方法调用fill_all_stream_timings()。它的代码没有细看,但从函数的注释的意思来讲,应当是当有些视音频流有时长信息的时候,直接赋值给其他视音频流。
(3)通过bitrate(码率)。该方法调用estimate_timings_from_bit_rate()。它的基本思想就是取得全部文件大小,和全部文件的bitrate,二者相除以后得到时长信息。

estimate_timings_from_bit_rate()

在这里附上上述几种方法中最简单的函数estimate_timings_from_bit_rate()的代码。
static void estimate_timings_from_bit_rate(AVFormatContext *ic) { int64_t filesize, duration; int i, show_warning = 0; AVStream *st; /* if bit_rate is already set, we believe it */ if (ic->bit_rate <= 0) { int bit_rate = 0; for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; if (st->codec->bit_rate > 0) { if (INT_MAX - st->codec->bit_rate < bit_rate) { bit_rate = 0; break; } bit_rate += st->codec->bit_rate; } } ic->bit_rate = bit_rate; } /* if duration is already set, we believe it */ if (ic->duration == AV_NOPTS_VALUE && ic->bit_rate != 0) { filesize = ic->pb ? avio_size(ic->pb) : 0; if (filesize > ic->data_offset) { filesize -= ic->data_offset; for (i = 0; i < ic->nb_streams; i++) { st = ic->streams[i]; if ( st->time_base.num <= INT64_MAX / ic->bit_rate && st->duration == AV_NOPTS_VALUE) { duration = av_rescale(8 * filesize, st->time_base.den, ic->bit_rate * (int64_t) st->time_base.num); st->duration = duration; show_warning = 1; } } } } if (show_warning) av_log(ic, AV_LOG_WARNING, "Estimating duration from bitrate, this may be inaccurate "); }

从代码中可以看出,该函数做了两步工作:
(1)如果AVFormatContext中没有bit_rate信息,就把所有AVStream的bit_rate加起来作为AVFormatContext的bit_rate信息。
(2)使用文件大小filesize除以bitrate得到时长信息。具体的方法是:
AVStream->duration=(filesize*8/bit_rate)/time_base
PS:
1)filesize乘以8是由于需要把Byte转换为Bit
2)具体的实现函数是那个av_rescale()函数。x=av_rescale(a,b,c)的含义是x=a*b/c。
3)之所以要除以time_base,是由于AVStream中的duration的单位是time_base,注意这和AVFormatContext中的duration的单位(单位是AV_TIME_BASE,固定取值为1000000)是不1样的。


至此,avformat_find_stream_info()主要的函数就分析完了。


雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020

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