FFmpeg中，流的探測是一個比較耗時的過程，那他的具體流程是怎樣呢，本文進行簡單梳理。並對流探測的兩個重要函數：avformat_open_input和avformat_find_stream_info進行分析。
avformat_open_input
avformat_open_input作用是打開文件，並且對流格式進行初步探測。
第一部分，init_input，其中主要分爲三種情況：
1、FFmpeg中，用戶自己定了avio，這種情況一般是用戶希望從內存中讀取文件，需要創建avio，並賦給pb，那麼從pb中調用av_probe_input_buffer2探測。
2、如果用戶已經指定了AVInputFormat，就直接返回；否則調用av_probe_input_format2函數探測，要注意av_probe_input_format2的第二個參數is_opened爲0，那麼即是在不打開文件的情況下進行探測，僅會對AVFMT_NOFILE標誌的AVInputFormat進行探測。例如RTP/RTSP類型。
帶AVFMT_NOFILE標誌的AVInputFormat，將不會攜帶AVIOContext，而是由AVInputFormat的read_packet自己來控制如何讀取數據。
3、一般的情況，都是打開文件，從av_probe_input_buffer2探測。
init_input代碼如下：
static int init_input(AVFormatContext *s, const char *filename,
AVDictionary **options)
{
int ret;
AVProbeData pd = { filename, NULL, 0 };
int score = AVPROBE_SCORE_RETRY;
if (s->pb) {
s->flags |= AVFMT_FLAG_CUSTOM_IO;
if (!s->iformat)
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
else if (s->iformat->flags & AVFMT_NOFILE)
av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
"will be ignored with AVFMT_NOFILE format.\n");
return 0;
}
if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
return score;
if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
return ret;
if (s->iformat)
return 0;
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
}
av_probe_input_format2()會調用av_probe_input_format3()，該函數最主要的部分是一個循環。該循環調用av_demuxer_iterate()遍歷FFmpeg中所有的AVInputFormat，因爲is_opened爲0，僅探測帶AVFMT_NOFILE標誌的AVInputFormat，並根據以下規則確定AVInputFormat和輸入媒體數據的匹配分數。
最主要的就是read_probe()，定義在各個AVInputFormat之中。
1、如果AVInputFormat中包含read_probe()，就調用read_probe()函數獲取匹配分數（這一方法如果結果匹配的話，一般會獲得AVPROBE_SCORE_MAX的分值，即100分）。如果不包含該函數，就使
2、用av_match_ext()函數比較輸入媒體的擴展名和AVInputFormat的擴展名是否匹配，如果匹配的話，設定匹配分數爲AVPROBE_SCORE_EXTENSION（AVPROBE_SCORE_EXTENSION取值爲50，即50分）。
3、使用av_match_name()比較輸入媒體的mime_type和AVInputFormat的mime_type，如果匹配的話，設定匹配分數爲AVPROBE_SCORE_MIME（AVPROBE_SCORE_MIME取值爲75，即75分）。
如果該AVInputFormat的匹配分數大於此前的最大匹配分數，則記錄當前的匹配分數爲最大匹配分數，並且記錄當前的AVInputFormat爲最佳匹配的AVInputFormat。
av_probe_input_buffer2()根據輸入的媒體數據推測該媒體數據的AVInputFormat，重要的參數如下：
- pb：AVIOContext，用於讀取流數據
- fmt：輸出參數，爲探測到的AVInputFormat
- filename：文件路徑
- offset：讀數據偏移量
- max_probe_size：探測時可用的最大數據量
函數主要是在一個循環中，調用avio_read讀取數據，再調用av_probe_input_format2(&pd, 1, &score);探測（注意這裏is_opened爲1）。
函數首先從PROBE_BUF_MIN（取值爲2048）個字節開始讀取，如果通過這些數據已經可以推測出AVInputFormat，那麼就可以直接退出循環了（參考for循環的判斷條件“!*fmt”）；如果沒有推測出來，就增加probe_size的量爲過去的2倍（參考for循環的表達式“probe_size << 1”），繼續推測AVInputFormat；如果一直讀取到max_probe_size字節的數據依然沒能確定AVInputFormat，則會退出循環並且返回錯誤信息。
第二部分，調用s->iformat->read_header探測流信息
該部分會調用具體的demux的read_header函數。一般來說，可以得到流數量steam_nb，音視頻類型和codec_id
mpegts，會探測解析PAT,PMT獲取音視頻流的PID，並添加到音視頻流。
rtsp，會進行rtsp的建連，包括describe->setup->play，同時通過SDP信息解析出音視頻類型
HLS，會下載解析m3u8，並自己內部再去調用mpegts，探測ts音視頻信息。
第三部分，update_stream_avctx更新解碼器信息
填充已經探測到的音視頻信息。
如下：
case AVMEDIA_TYPE_VIDEO:
codec->pix_fmt = par->format;
codec->width = par->width;
codec->height = par->height;
codec->field_order = par->field_order;
codec->color_range = par->color_range;
codec->color_primaries = par->color_primaries;
codec->color_trc = par->color_trc;
codec->colorspace = par->color_space;
codec->chroma_sample_location = par->chroma_location;
codec->sample_aspect_ratio = par->sample_aspect_ratio;
codec->has_b_frames = par->video_delay;
break;
case AVMEDIA_TYPE_AUDIO:
codec->sample_fmt = par->format;
codec->channel_layout = par->channel_layout;
codec->channels = par->channels;
codec->sample_rate = par->sample_rate;
codec->block_align = par->block_align;
codec->frame_size = par->frame_size;
codec->delay =
codec->initial_padding = par->initial_padding;
codec->trailing_padding = par->trailing_padding;
codec->seek_preroll = par->seek_preroll;
對於解碼播放，像ts，hls，rtsp，一般來說avformat_open_input就足夠了，但對於FFmpeg，完整的音視頻信息是不夠的，FFmpeg中通過has_codec_parameters來看是否有完整的解碼器信息。
主要如下：
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->codecpar->codec_id == AV_CODEC_ID_RV30 || st->codecpar->codec_id == AV_CODEC_ID_RV40)
if (!st->sample_aspect_ratio.num && !st->codecpar->sample_aspect_ratio.num && !st->codec_info_nb_frames)
FAIL("no frame in rv30/40 and no sar");
break;
對於大部分的demux，都是無法完全獲取這些信息的，另外像duration信息，很多demux在此階段也無法獲取。所以如果需要完整的獲取，需要通過avformat_find_stream_info函數。
avformat_find_stream_info
從has_codec_parameters函數中可以看出，最起碼得獲取數據類型及code_id。
對於視頻，需要width，pix_fmt。對音頻，需要frame_size，sample_fmt，sample_rate，channels
第一部分，設置探測參數
主要是max_stream_analyze_duration和probesize，這些應用層均可以設置。
int64_t max_analyze_duration = ic->max_analyze_duration;
int64_t max_stream_analyze_duration;
int64_t max_subtitle_analyze_duration;
int64_t probesize = ic->probesize;
int eof_reached = 0;
int *missing_streams = av_opt_ptr(ic->iformat->priv_class, ic->priv_data, "missing_streams");
flush_codecs = probesize > 0;
av_opt_set(ic, "skip_clear", "1", AV_OPT_SEARCH_CHILDREN);
max_stream_analyze_duration = max_analyze_duration;
max_subtitle_analyze_duration = max_analyze_duration;
if (!max_analyze_duration) {
max_stream_analyze_duration =
max_analyze_duration = 5*AV_TIME_BASE;
max_subtitle_analyze_duration = 30*AV_TIME_BASE;
if (!strcmp(ic->iformat->name, "flv"))
max_stream_analyze_duration = 90*AV_TIME_BASE;
if (!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts"))
max_stream_analyze_duration = 7*AV_TIME_BASE;
}
第二部分，初始化codec及parser
遍歷音視頻流，主要完成：
1、獲得編碼器上下文環境avctx，設置avctx的time_base,code_id,codec_type,orig_codec_id。
2、如果解析器paser爲空，那麼會初始化解析器。
3、找到對應的解碼器，打開解碼器
for (i = 0; i < ic->nb_streams; i++) {
const AVCodec *codec;
AVDictionary *thread_opt = NULL;
st = ic->streams[i];
avctx = st->internal->avctx;
if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
/* if (!st->time_base.num)
st->time_base = */
if (!avctx->time_base.num)
avctx->time_base = st->time_base;
}
/* check if the caller has overridden the codec id */
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
if (st->codec->codec_id != st->internal->orig_codec_id) {
st->codecpar->codec_id = st->codec->codec_id;
st->codecpar->codec_type = st->codec->codec_type;
st->internal->orig_codec_id = st->codec->codec_id;
}
FF_ENABLE_DEPRECATION_WARNINGS
#endif
// only for the split stuff
if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE) && st->request_probe <= 0) {
st->parser = av_parser_init(st->codecpar->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.\n",
avcodec_get_name(st->codecpar->codec_id));
}
}
if (st->codecpar->codec_id != st->internal->orig_codec_id)
st->internal->orig_codec_id = st->codecpar->codec_id;
ret = avcodec_parameters_to_context(avctx, st->codecpar);
if (ret < 0)
goto find_stream_info_err;
if (st->request_probe <= 0)
st->internal->avctx_inited = 1;
codec = find_probe_decoder(ic, st, st->codecpar->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->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE
&& codec && !avctx->codec) {
if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
av_log(ic, AV_LOG_WARNING,
"Failed to open codec in %s\n",__FUNCTION__);
}
// 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 && !avctx->codec)
if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
av_log(ic, AV_LOG_WARNING,
"Failed to open codec in %s\n",__FUNCTION__);
}
if (!options)
av_dict_free(&thread_opt);
}
第三部分，循環探測
1、再一次遍歷流，檢查是不是還有編解碼器需要進一步處理。這裏會用has_codec_parameters檢查編解碼器的各個參數，如果還有些編解碼器的信息不全，那麼繼續探測。
2、如果探測的數據大於probesize，則退出。
4、讀一幀數據進來。使用read_frame_internal函數。read_frame_internal函數中會調用解析器parser的parser_parse函數，像H264的話，在此階段會解析SPS/PPS信息。然後將解析到的信息更新到解碼器上下文中。
5、把讀出來的數據添加到緩衝區。使用的是ff_packet_list_put函數。然後更新讀到的總的數據的大小：read_size += pkt->size;
6、avcodec_parameters_to_context更新解碼器信息。
7、更新一些dts信息。
8、如果還沒解析全，調用try_decode_frame嘗試解碼獲取。需要注意的是，try_decode_frame進行解碼會耗時更多，但是像H264中，pix_fmt是需要到解碼階段纔行的。
儘量是不要到這一步，在實際使用中，我們對pix_fmt進行裁剪，因爲對於上層應用，並不需要。
雖然有的H264 SPS中帶有bit_depth_luma和chroma_format_idc時，可以獲取到pix_fmt，但是FFmpeg中也不會使用，而是在解碼纔給其賦值。
9、如果已經try_decode_frame，耗時已超過max_stream_analyze_duration，退出循環。
for (;;) {
int analyzed_all_streams;
if (ff_check_interrupt(&ic->interrupt_callback)) {
ret = AVERROR_EXIT;
av_log(ic, AV_LOG_DEBUG, "interrupted\n");
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 (ic->metadata) {
AVDictionaryEntry *t = av_dict_get(ic->metadata, "skip-calc-frame-rate", NULL, AV_DICT_MATCH_CASE);
if (t) {
int fps_flag = (int) strtol(t->value, NULL, 10);
if (!st->r_frame_rate.num && st->avg_frame_rate.num > 0 && st->avg_frame_rate.den > 0 && fps_flag > 0) {
int avg_fps = st->avg_frame_rate.num / st->avg_frame_rate.den;
if (avg_fps > 0 && avg_fps <= 120) {
st->r_frame_rate.num = st->avg_frame_rate.num;
st->r_frame_rate.den = st->avg_frame_rate.den;
}
}
}
}
/* 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->internal->avctx))
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->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
int count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?
st->info->codec_info_duration_fields/2 :
st->info->duration_count;
if (count < fps_analyze_framecount)
break;
}
if (!st->internal->avctx->extradata &&
(!st->internal->extract_extradata.inited ||
st->internal->extract_extradata.bsf) &&
extract_extradata_check(st))
break;
if (st->first_dts == AV_NOPTS_VALUE &&
!(ic->iformat->flags & AVFMT_NOTIMESTAMPS) &&
st->codec_info_nb_frames < ((st->disposition & AV_DISPOSITION_ATTACHED_PIC) ? 1 : ic->max_ts_probe) &&
(st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO))
break;
}
analyzed_all_streams = 0;
if (!missing_streams || !*missing_streams)
if (i == ic->nb_streams) {
analyzed_all_streams = 1;
/* 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\n");
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\n", 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]->codecpar->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\n", 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*/
eof_reached = 1;
break;
}
pkt = &pkt1;
if (!(ic->flags & AVFMT_FLAG_NOBUFFER)) {
ret = add_to_pktbuf(&ic->internal->packet_buffer, pkt,
&ic->internal->packet_buffer_end, 0);
if (ret < 0)
goto find_stream_info_err;
}
st = ic->streams[pkt->stream_index];
if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC))
read_size += pkt->size;
avctx = st->internal->avctx;
if (!st->internal->avctx_inited) {
ret = avcodec_parameters_to_context(avctx, st->codecpar);
if (ret < 0)
goto find_stream_info_err;
st->internal->avctx_inited = 1;
}
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"\n",
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"\n",
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;
int64_t limit;
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 (analyzed_all_streams) limit = max_analyze_duration;
else if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) limit = max_subtitle_analyze_duration;
else limit = max_stream_analyze_duration;
if (t >= limit) {
av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds st:%d\n",
limit,
t, pkt->stream_index);
if (ic->flags & AVFMT_FLAG_NOBUFFER)
av_packet_unref(pkt);
break;
}
if (pkt->duration) {
if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
st->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->info->codec_info_duration + pkt->duration);
} else
st->info->codec_info_duration += pkt->duration;
st->info->codec_info_duration_fields += st->parser && st->need_parsing && avctx->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2;
}
}
#if FF_API_R_FRAME_RATE
if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
ff_rfps_add_frame(ic, st, pkt->dts);
#endif
if (!st->internal->avctx->extradata) {
ret = extract_extradata(st, pkt);
if (ret < 0)
goto find_stream_info_err;
}
/* 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 MPEG-4, we need to
* decompress for QuickTime.
*
* If AV_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++;
}
第四部分，繼續處理一些音視頻信息
循環用來處理音頻流和視頻流。
對視頻流而言，首先會獲得原始數據的圖像格式，然後據此找到對應的編解碼器的tag。之後會計算平均幀率。另外就是設置寬高比display_aspect_ratio。
對音頻流而言，基於音頻流的服務類型初始化disposition。disposition可能是設置音效相關的處理，不確認。
ff_rfps_calculate(ic);
for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
avctx = st->internal->avctx;
if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
if (avctx->codec_id == AV_CODEC_ID_RAWVIDEO && !avctx->codec_tag && !avctx->bits_per_coded_sample) {
uint32_t tag= avcodec_pix_fmt_to_codec_tag(avctx->pix_fmt);
if (avpriv_find_pix_fmt(avpriv_get_raw_pix_fmt_tags(), tag) == avctx->pix_fmt)
avctx->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;
AVRational codec_frame_rate = avctx->framerate;
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 (ic->internal->prefer_codec_framerate && codec_frame_rate.num > 0 && codec_frame_rate.den > 0) {
error = fabs(av_q2d(codec_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 ( avctx->time_base.den * (int64_t) st->time_base.num
<= avctx->time_base.num * avctx->ticks_per_frame * (int64_t) st->time_base.den) {
av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den,
avctx->time_base.den, (int64_t)avctx->time_base.num * avctx->ticks_per_frame, INT_MAX);
} else {
st->r_frame_rate.num = st->time_base.den;
st->r_frame_rate.den = st->time_base.num;
}
}
if (st->display_aspect_ratio.num && st->display_aspect_ratio.den) {
AVRational hw_ratio = { avctx->height, avctx->width };
st->sample_aspect_ratio = av_mul_q(st->display_aspect_ratio,
hw_ratio);
}
} else if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {
if (!avctx->bits_per_coded_sample)
avctx->bits_per_coded_sample =
av_get_bits_per_sample(avctx->codec_id);
// set stream disposition based on audio service type
switch (avctx->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;
}
}
}
第五部分，計算duration
這個在之前的文章中有分析過，不贅述，主要是通過三種方式來計算時長。
1、通過pts，estimate_timings_from_pts()。它的基本思想就是讀取視音頻流中的結束位置AVPacket的PTS和起始位置AVPacket的PTS，兩者相減得到時長信息。
2、通過已知流的時長。部分AVInputFormat本身協議信息中就是有duration的，如HLS/RTSP
3、通過bitrate，estimate_timings_from_bit_rate()。它的基本思想就是獲得整個文件大小，以及整個文件的bitrate，兩者相除之後得到時長信息。
if (probesize)
estimate_timings(ic, old_offset);
對於mpegts類型，duration的獲取是比較耗時的，如果服務端可以URL裏攜帶duration，那麼也能節省一些起播時間