本文主要是介绍FFmpeg源码分析：avformat_find_stream_info分析码流信息，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
FFmpeg在调用avformat_open_input()之后，可能码流信息不够完整，可以使用avformat_find_stream_info()获取更多的码流信息。比如获取视频帧率、视频宽高，重新计算最大分析时长，打开解码器解码获取codec数据。具体流程如下图所示：
avformat_find_stream_info方法位于libavformat/utils.c，具体代码如下(部分删减)：
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
{// 针对flv/mpeg/mpegts格式，max_analyze_duration另外赋值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;}for (i = 0; i < ic->nb_streams; i++) {// 查找解码器codec = find_probe_decoder(ic, st, st->codecpar->codec_id);// 确保字幕头信息已正确设置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__);}// 尝试打开解码器if (!has_codec_parameters(st, NULL) && st->internal->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__);}}......for (;;) {for (i = 0; i < ic->nb_streams; i++) {int fps_analyze_framecount = 20;int count;st = ic->streams[i];if (!has_codec_parameters(st, NULL))break;// 如果时间基是粗糙的，比如mkv采用ms作为单位// 需要分析更多帧数据来获取帧率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;count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?st->internal->info->codec_info_duration_fields/2 :st->internal->info->duration_count;if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {if (count < fps_analyze_framecount)break;}// 如果存在帧延时，查看前3帧if (st->internal->info->frame_delay_evidence && count < 2 && st->internal->avctx->has_b_frames == 0)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;}// 读取超过指定最大数据量，但还没获取到足够的解码器信息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]->internal->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;}// 读取一帧数据ret = read_frame_internal(ic, pkt1);......if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) {// 检测非单调递增的dtsif (st->internal->info->fps_last_dts != AV_NOPTS_VALUE &&st->internal->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->internal->info->fps_last_dts_idx,st->internal->info->fps_last_dts, st->codec_info_nb_frames,pkt->dts);st->internal->info->fps_first_dts =st->internal->info->fps_last_dts = AV_NOPTS_VALUE;}// 检测不连续的dts:序列的平均帧时长是否超过1000帧存在差异if (st->internal->info->fps_last_dts != AV_NOPTS_VALUE &&st->internal->info->fps_last_dts_idx > st->internal->info->fps_first_dts_idx &&(pkt->dts - (uint64_t)st->internal->info->fps_last_dts) / 1000 >(st->internal->info->fps_last_dts - (uint64_t)st->internal->info->fps_first_dts) /(st->internal->info->fps_last_dts_idx - st->internal->info->fps_first_dts_idx)) {st->internal->info->fps_first_dts =st->internal->info->fps_last_dts = AV_NOPTS_VALUE;}// 更新dtsif (st->internal->info->fps_first_dts == AV_NOPTS_VALUE) {st->internal->info->fps_first_dts = pkt->dts;st->internal->info->fps_first_dts_idx = st->codec_info_nb_frames;}st->internal->info->fps_last_dts = pkt->dts;st->internal->info->fps_last_dts_idx = st->codec_info_nb_frames;}// 提取额外数据数组if (!st->internal->avctx->extradata) {ret = extract_extradata(st, pkt);if (ret < 0)goto unref_then_goto_end;}// 如果仍然没有信息，则打开解码器尝试解码一帧// 针对MPEG-4, 需要为QuickTime进行解码try_decode_frame(ic, st, pkt,(options && i < orig_nb_streams) ? &options[i] : NULL);if (ic->flags & AVFMT_FLAG_NOBUFFER)av_packet_unref(pkt1);st->codec_info_nb_frames++;count++;}if (flush_codecs) {for (i = 0; i < ic->nb_streams; i++) {st = ic->streams[i];// 刷新解码器，尝试解码一帧数据if (st->internal->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));}}}......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;}// 如果demuxer没有设置帧率，则估算平均帧率if (st->internal->info->codec_info_duration_fields &&!st->avg_frame_rate.num &&st->internal->info->codec_info_duration) {int best_fps = 0;double best_error = 0.01;AVRational codec_frame_rate = avctx->framerate;if (st->internal->info->codec_info_duration >= INT64_MAX / st->time_base.num / 2||st->internal->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den ||st->internal->info->codec_info_duration < 0)continue;av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,st->internal->info->codec_info_duration_fields * (int64_t) st->time_base.den,st->internal->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000);// 推测标准帧率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);}// 计算帧率的分子与分母if (!st->r_frame_rate.num) {if ( avctx->time_base.den * (int64_t) st->time_base.num<= avctx->time_base.num * (uint64_t)avctx->ticks_per_frame * 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;}}// 计算宽高比if (st->internal->display_aspect_ratio.num && st->internal->display_aspect_ratio.den) {AVRational hw_ratio = { avctx->height, avctx->width };st->sample_aspect_ratio = av_mul_q(st->internal->display_aspect_ratio,hw_ratio);}}}......
}1、最大分析时长
max_analyze_duration用于设定探测格式阶段的分析时长，如果最大分析时长没有被赋值，默认值如下：
max_stream_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;2、查找解码器
使用find_probe_decoder()来查找解码器，具体代码如下：
static const AVCodec *find_probe_decoder(AVFormatContext *s, const AVStream *st, enum AVCodecID codec_id)
{const AVCodec *codec;#if CONFIG_H264_DECODERif (codec_id == AV_CODEC_ID_H264)return avcodec_find_decoder_by_name("h264");
#endifcodec = find_decoder(s, st, codec_id);if (!codec)return NULL;if (codec->capabilities & AV_CODEC_CAP_AVOID_PROBING) {const AVCodec *probe_codec = NULL;void *iter = NULL;while ((probe_codec = av_codec_iterate(&iter))) {if (probe_codec->id == codec->id &&av_codec_is_decoder(probe_codec) &&!(probe_codec->capabilities & (AV_CODEC_CAP_AVOID_PROBING | AV_CODEC_CAP_EXPERIMENTAL))) {return probe_codec;}}}return codec;
}3、打开解码器
使用avcodec_open2()打开解码器，传递三个参数：AVCodecContext、AVCodec、AVOption。分两种情况：字幕流和缺少codec参数。
3.1 字幕流
如果是字幕流，打开解码器，确保字幕头信息已经正确设置。
3.2 没有codec参数
如果缺少codec参数，也会打开解码器，来获取codec相关参数。判断是否有codec参数方法如下：
static int has_codec_parameters(AVStream *st, const char **errmsg_ptr)
{AVCodecContext *avctx = st->internal->avctx;#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->internal->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->internal->info->found_decoder >= 0 && !st->internal->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->internal->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;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;
}检测音频：sample_rate 、sample_format、channels、frame_size；
检测视频：width、pixel_format、sample_aspect_ratio;
检测字幕：如果是PGS图像字幕流，检测width；
4、读取一帧数据
使用read_frame_internal()来读取一帧数据，内部调用ff_read_packet()和parse_packet()，主要过程是读取packet进行组包，最终读取到一个完整帧。
5、检测dts
检测非单调递增的dts，如果dts不是单调递增，那么判定码流有问题。检测不连续的dts，如果序列的平均帧时长超过1000帧存在差异，那么判定dts不连续。最终更新dts。
6、解码一帧
调用try_decode_frame()进行解码，具体代码如下：
static int try_decode_frame(AVFormatContext *s, AVStream *st,const AVPacket *avpkt, AVDictionary **options)
{......if (!avcodec_is_open(avctx) &&st->internal->info->found_decoder <= 0 &&(st->codecpar->codec_id != -st->internal->info->found_decoder || !st->codecpar->codec_id)) {AVDictionary *thread_opt = NULL;codec = find_probe_decoder(s, st, st->codecpar->codec_id);if (!codec) {st->internal->info->found_decoder = -st->codecpar->codec_id;ret = -1;goto fail;}av_dict_set(options ? options : &thread_opt, "threads", "1", 0);av_dict_set(options ? options : &thread_opt, "lowres", "0", 0);if (s->codec_whitelist)av_dict_set(options ? options : &thread_opt, "codec_whitelist", s->codec_whitelist, 0);ret = avcodec_open2(avctx, codec, options ? options : &thread_opt);if (!options)av_dict_free(&thread_opt);if (ret < 0) {st->internal->info->found_decoder = -avctx->codec_id;goto fail;}st->internal->info->found_decoder = 1;} else if (!st->internal->info->found_decoder)st->internal->info->found_decoder = 1;......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 &&(avctx->codec->capabilities & AV_CODEC_CAP_CHANNEL_CONF)))) {got_picture = 0;if (avctx->codec_type == AVMEDIA_TYPE_VIDEO ||avctx->codec_type == AVMEDIA_TYPE_AUDIO) {ret = avcodec_send_packet(avctx, &pkt);if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)break;if (ret >= 0)pkt.size = 0;ret = avcodec_receive_frame(avctx, frame);if (ret >= 0)got_picture = 1;if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)ret = 0;} else if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) {ret = avcodec_decode_subtitle2(avctx, &subtitle,&got_picture, &pkt);if (got_picture)avsubtitle_free(&subtitle);if (ret >= 0)pkt.size = 0;}if (ret >= 0) {if (got_picture)st->internal->nb_decoded_frames++;ret = got_picture;}}......return ret;
}7、计算帧率
针对mkv格式，时间基单位是ms，需要读取更多帧来计算帧率。针对有帧延时情况，读取前3帧。然后计算帧率的分子与分母。
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