背景
一般的应用场景对实时点播速度要求不高的情况下，可以设置探测码流的延时和探测数据的大小，代码如下：
pFormatContext->probesize = 500 *1024;
pFormatContext->max_analyze_duration = 5 * AV_TIME_BASE;//AV_TIME_BASE是定义的时间标准，代表1秒
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弊端
这样设置probesize和max_analyze_duration是可以减少探测时间，但是是以牺牲成功率为代价的，有时候探测不到流信息，就会播不出来，
出现在网络丢包的情况下（使用UDP进行视频数据的传输）或者网路特别复杂，跨越多个网段
探测码流有没有用于解码显示问题
AVFMT_FLAG_NOBUFFER宏定义剖析
默认情况下，读取的缓存数据将会用于解码，如果不想探测的码流用于显示，可以这样子设置：
pAVFormatContext->flags =pAVFormatContext->flags & AVFMT_FLAG_NOBUFFER;
代码剖析
函数调用int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
if (ic->flags & AVFMT_FLAG_NOBUFFER)
free_packet_buffer(&ic->packet_buffer, &ic->packet_buffer_end);
问题描述:因为没有设置该宏定义，传递进去I帧数据，应该会被保存下来，但是明显的得到PPS/SPS不存在的异常，究竟传输进去的码流哪里去了？？
场景
要求点播时间不超过1秒，允许指定摄像机的视频参数如下：
当前只考虑视频流，后续会添加音频流，已知输入的流格式video: H264 1920*1080 25fps
实施方案
当前采用FFmpeg 3.4版本，参考博客采用的是比较旧的版本2.2.0版本，有些函数已经被废弃，有些函数已经发生了改变，但是基本思路是不变的：
通过手动指定×××参数，来取代avformat_find_stream_info函数探测流格式
AVStream* CDecoder::CreateStream(AVFormatContext* pFormatContext, int nCodecType)
{
AVStream *st = avformat_new_stream(pFormatContext, NULL);
if (!st)
return NULL;
st->codecpar->codec_type = (AVMediaType)nCodecType;
return st;
}
int CDecoder::GetVideoExtraData(AVFormatContext* pFormatContext, int nVideoIndex)
{
int type, size, flags, pos, stream_type;
int ret = -1;
int64_t dts;
bool got_extradata = false;
if (!pFormatContext || nVideoIndex < 0 || nVideoIndex > 2)
return ret;
for (;; avio_skip(pFormatContext->pb, 4)) {
pos = avio_tell(pFormatContext->pb);
type = avio_r8(pFormatContext->pb);
size = avio_rb24(pFormatContext->pb);
dts = avio_rb24(pFormatContext->pb);
dts |= avio_r8(pFormatContext->pb) << 24;
avio_skip(pFormatContext->pb, 3);
if (0 == size)
break;
if (FLV_TAG_TYPE_AUDIO == type || FLV_TAG_TYPE_META == type) {
/*if audio or meta tags, skip them.*/
avio_seek(pFormatContext->pb, size, SEEK_CUR);
}
else if (type == FLV_TAG_TYPE_VIDEO) {
/*if the first video tag, read the sps/pps info from it. then break.*/
size -= 5;
pFormatContext->streams[nVideoIndex]->codecpar->extradata = (uint8_t*)av_malloc(size + FF_INPUT_BUFFER_PADDING_SIZE);
if (NULL == pFormatContext->streams[nVideoIndex]->codecpar->extradata)
break;
memset(pFormatContext->streams[nVideoIndex]->codecpar->extradata, 0, size + FF_INPUT_BUFFER_PADDING_SIZE);
memcpy(pFormatContext->streams[nVideoIndex]->codecpar->extradata, pFormatContext->pb->buf_ptr + 5, size);
pFormatContext->streams[nVideoIndex]->codecpar->extradata_size = size;
ret = 0;
got_extradata = true;
}
else {
/*The type unknown,something wrong.*/
break;
}
if (got_extradata)
break;
}
return ret;
}
int CDecoder::InitDecode(AVFormatContext *pFormatContext)
{
int video_index = -1;
int audio_index = -1;
int ret = -1;
if (!pFormatContext)
return ret;
/*
Get video stream index, if no video stream then create it.
And audio so on.
*/
if (0 == pFormatContext->nb_streams) {
CreateStream(pFormatContext, AVMEDIA_TYPE_VIDEO);
CreateStream(pFormatContext, AVMEDIA_TYPE_AUDIO);
video_index = 0;
audio_index = 1;
}
else if (1 == pFormatContext->nb_streams) {
if (AVMEDIA_TYPE_VIDEO == pFormatContext->streams[0]->codecpar->codec_type) {
CreateStream(pFormatContext, AVMEDIA_TYPE_AUDIO);
video_index = 0;
audio_index = 1;
}
else if (AVMEDIA_TYPE_AUDIO == pFormatContext->streams[0]->codecpar->codec_type) {
CreateStream(pFormatContext, AVMEDIA_TYPE_VIDEO);
video_index = 1;
audio_index = 0;
}
}
else if (2 == pFormatContext->nb_streams) {
if (AVMEDIA_TYPE_VIDEO == pFormatContext->streams[0]->codecpar->codec_type) {
video_index = 0;
audio_index = 1;
}
else if (AVMEDIA_TYPE_VIDEO == pFormatContext->streams[1]->codecpar->codec_type) {
video_index = 1;
audio_index = 0;
}
}
/*Error. I can't find video stream.*/
if (video_index != 0 && video_index != 1)
return ret;
//Init the audio codec(AAC).
pFormatContext->streams[audio_index]->codecpar->codec_id = AV_CODEC_ID_AAC;
pFormatContext->streams[audio_index]->codecpar->sample_rate = 44100;
pFormatContext->streams[audio_index]->codecpar->bits_per_coded_sample = 16;
pFormatContext->streams[audio_index]->codecpar->channels = 2;
pFormatContext->streams[audio_index]->codecpar->channel_layout = 3;
pFormatContext->streams[audio_index]->pts_wrap_bits = 32;
pFormatContext->streams[audio_index]->time_base.den = 1000;
pFormatContext->streams[audio_index]->time_base.num = 1;
//Init the video codec(H264).
pFormatContext->streams[video_index]->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
pFormatContext->streams[video_index]->codecpar->codec_id = AV_CODEC_ID_H264;
pFormatContext->streams[video_index]->codecpar->format = 12;
pFormatContext->streams[video_index]->codecpar->bits_per_raw_sample = 8;
pFormatContext->streams[video_index]->codecpar->profile = 66;
pFormatContext->streams[video_index]->codecpar->level = 42;
pFormatContext->streams[video_index]->codecpar->width = 1920;
pFormatContext->streams[video_index]->codecpar->height = 1080;
pFormatContext->streams[video_index]->codecpar->sample_aspect_ratio.num = 0;
pFormatContext->streams[video_index]->codecpar->sample_aspect_ratio.den = 1;
pFormatContext->streams[video_index]->pts_wrap_bits = 64;
pFormatContext->streams[video_index]->time_base.den = 1200000;
pFormatContext->streams[video_index]->time_base.num = 1;
pFormatContext->streams[video_index]->avg_frame_rate.den = 1;
pFormatContext->streams[video_index]->avg_frame_rate.num = 25;
/*Need to change, different condition has different frame_rate. 'r_frame_rate' is new in ffmepg2.3.3*/
pFormatContext->streams[video_index]->r_frame_rate.den = 25;
pFormatContext->streams[video_index]->r_frame_rate.num = 1;
/* H264 need sps/pps for decoding, so read it from the first video tag.*/
ret = GetVideoExtraData(pFormatContext, video_index);
/*Update the AVFormatContext Info*/
pFormatContext->nb_streams = 1;
/*empty the buffer.*/
pFormatContext->pb->buf_ptr = pFormatContext->pb->buf_end;
return ret;
}
已有的方案
AVDictionary* pOptions = NULL;
pFormatCtx->probesize = 200 *1024;
pFormatCtx->max_analyze_duration = 3 * AV_TIME_BASE;
//Retrieve stream information
if (avformat_find_stream_info(pFormatCtx, &pOptions) < 0)
{
return -1; // Couldn't find stream information
}
现有方案
InitDecode(pFormatCtx);
警告
在上述的解决方案中，采用了av_malloc分配内存，但是没有进行内存的释放，是否会随着pFormatCtx的释放而释放，不至于导致内存泄露，当然经过测试，并没有走到这一步
优化效果
通过测试，速度优化了1200毫秒
测试结果
探测ES流，avformat_open_input会非常快的返回，PS反而是一个例外。通过调用av_log_set_callback设置日志写文件的方式，
在调用avformat_open_input函数探测PS输入格式时候，
会打印如下的日志：
Probing mp3 score:1 size:2048
Probing mp3 score:1 size:4096
Probing mp3 score:1 size:8192
Probing mp3 score:1 size:16384
Probing h364 score:51 size:32768
Format h364 probed with size=32768 and score=51
Input #0, h364, from '':
Duration: N/A, bitrate: N/A
Stream #0:0, 0, 1/1200000: Video: h364 (Baseline), yuvj420p, 1920x1080, 25 fps, 0.04 tbr, 1200k tbn
deprecated pixel format used, make sure you did set range correctly
non-existing PPS 0 referenced
non-existing PPS 0 referenced
nal_unit_type: 1, nal_ref_idc: 3
non-existing PPS 0 referenced
non-existing PPS 0 referenced
decode_slice_header error
non-existing PPS 0 referenced
non-existing PPS 0 referenced
non-existing PPS 0 referenced
no frame!
通过跟踪源码Probing h364 score:51 size:32768日志打印在调用
av_probe_input_format3函数会打印
name h364
long_name raw H.264 video
raw_codec_id 28
说明如果指定了AVInputFormat结构体，就可以节省探测码流格式的时间
ffmpeg 针对指定的h364 es流延时优化
参考http://blog.csdn.net/rain_2011_kai/article/details/7746805文章，
是否只需要知道发送端发送的视屏的×××ID,视频帧的长和宽，就可以直接
直接省略掉ffmpeg库的视频流探测接口，avformat_open_input函数
和avformat_find_stream_info函数耗时超过500毫秒
手动指定解码格式 效果不明显
+buffer0x00000000002cbdc0 <字符串中的字符无效。>unsigned char *
+buf_end0x00000000002d3626 <字符串中的字符无效。>unsigned char *
30822
pos 262349
buffer 2932160
buf_end 2962982
buffsize 35840
buf_end-buffer 30822
pos的值从哪里来，值得考虑
pFormatContext->pb->pos = pFormatContext->pb->buf_end;
在已有的版本是编译不过的，因为pos是一个64位整型，buf_end是一个字符指针
但是从上面还是看不出它们之间的关系，尽管手动指定解码格式，但是效果并不理想
还有在这里读取一帧的数据
m_pVideoc->io_ctx = avio_alloc_context(m_pAvioBuf, BUF_SIZE, 0, this, ReadStreamData, NULL, NULL);
设置BUF_SIZE的大小为4*1024,实际上是否有效果，尚未可知
参考
http://blog.csdn.net/STN_LCD/article/details/74935760
https://jiya.io/archives/vlc_optimize_1.html
打印码流详情av_dump_format函数使用
函数说明
一般使用av_find_stream_info函数探测码流格式，它的作用是为pFormatContext->streams填充上正确的音视频格式信息。可以通过av_dump_format函数将音视频数据格式通过av_log输出到指定的文件或者控制台,方便开发者了解输入的视音频格式，对于程序的调用，删除该函数的调用没有任何的影响
/**
* Print detailed information about the input or output format, such as
* duration, bitrate, streams, container, programs, metadata, side data,
* codec and time base（打印输入或者输出格式的信息，例如视频总时长，波特率，流，容器，程序，元数据，边数据，编码器，时间戳）.
*
* @param ic the context to analyze（被分析的视音频上下文）
* @param index index of the stream to dump information about（需要解析的流索引号，例如0是视频流，1是音频流）
* @param url the URL to print, such as source or destination file（打印URL，例如源文件或者目的文件）
* @param is_output Select whether the specified context is an input(0) or output(1)（指定的视音频上下文是输入流（0）还是输出流（1））
*/
void av_dump_format(AVFormatContext *ic,
int index,
const char *url,
int is_output);
调用代码
av_dump_format(pFormatContext, 0, "", 0);
注意：最后一个参数如果是输入流取值0，如果是输出流取值1
通过av_log_set_callback设置日志文件输出，输出的结果如下：
Input #0, h364, from '':
Duration: N/A, bitrate: N/A
Stream #0:0, 0, 1/1200000: Video: h364 (Baseline), yuvj420p, 1920x1080, 25 fps, 0.04 tbr, 1200k tbn
当前没有打印视频总时长是因为这是h364裸留数据
函数代码剖析
void av_dump_format(AVFormatContext *ic, int index, const char *url, int is_output)
{
int i;
uint8_t *printed = ic->nb_streams ? av_mallocz(ic->nb_streams) : NULL;
if (ic->nb_streams && !printed)
return;
//如果is_output是0，说明是输入流
av_log(NULL, AV_LOG_INFO, "%s #%d, %s, %s '%s':\n",
is_output ? "Output" : "Input",
index,
is_output ? ic->oformat->name : ic->iformat->name,
is_output ? "to" : "from", url);
dump_metadata(NULL, ic->metadata, " ");
//如果is_output指定为0，开始计算视频总时长
if (!is_output) {
av_log(NULL, AV_LOG_INFO, " Duration: ");
//如果上下文中的总时长字段有效，说明不是h364裸留，h364裸留是没有dts/pts字段数据的，开始计算视频总时长
if (ic->duration != AV_NOPTS_VALUE) {
int hours, mins, secs, us;
int64_t duration = ic->duration + (ic->duration <= INT64_MAX - 5000 ? 5000 : 0);
secs = duration / AV_TIME_BASE;
us = duration % AV_TIME_BASE;
mins = secs / 60;
secs %= 60;
hours = mins / 60;
mins %= 60;
av_log(NULL, AV_LOG_INFO, "%02d:%02d:%02d.%02d", hours, mins, secs,
(100 * us) / AV_TIME_BASE);
} else {
av_log(NULL, AV_LOG_INFO, "N/A");
}
//判断开始是否开始时间有效
if (ic->start_time != AV_NOPTS_VALUE) {
int secs, us;
av_log(NULL, AV_LOG_INFO, ", start: ");
secs = llabs(ic->start_time / AV_TIME_BASE);
us = llabs(ic->start_time % AV_TIME_BASE);
av_log(NULL, AV_LOG_INFO, "%s%d.%06d",
ic->start_time >= 0 ? "" : "-",
secs,
(int) av_rescale(us, 1000000, AV_TIME_BASE));
}
av_log(NULL, AV_LOG_INFO, ", bitrate: ");
if (ic->bit_rate)
av_log(NULL, AV_LOG_INFO, "%"PRId64" kb/s", ic->bit_rate / 1000);
else
av_log(NULL, AV_LOG_INFO, "N/A");
av_log(NULL, AV_LOG_INFO, "\n");
}
for (i = 0; i < ic->nb_chapters; i++) {
AVChapter *ch = ic->chapters[i];
av_log(NULL, AV_LOG_INFO, " Chapter #%d:%d: ", index, i);
av_log(NULL, AV_LOG_INFO,
"start %f, ", ch->start * av_q2d(ch->time_base));
av_log(NULL, AV_LOG_INFO,
"end %f\n", ch->end * av_q2d(ch->time_base));
dump_metadata(NULL, ch->metadata, " ");
}
if (ic->nb_programs) {
int j, k, total = 0;
for (j = 0; j < ic->nb_programs; j++) {
AVDictionaryEntry *name = av_dict_get(ic->programs[j]->metadata,
"name", NULL, 0);
av_log(NULL, AV_LOG_INFO, " Program %d %s\n", ic->programs[j]->id,
name ? name->value : "");
dump_metadata(NULL, ic->programs[j]->metadata, " ");
for (k = 0; k < ic->programs[j]->nb_stream_indexes; k++) {
dump_stream_format(ic, ic->programs[j]->stream_index[k],
index, is_output);
printed[ic->programs[j]->stream_index[k]] = 1;
}
total += ic->programs[j]->nb_stream_indexes;
}
if (total < ic->nb_streams)
av_log(NULL, AV_LOG_INFO, " No Program\n");
}
for (i = 0; i < ic->nb_streams; i++)
if (!printed[i])
dump_stream_format(ic, i, index, is_output);
av_free(printed);
}
