问题
I'm creating a fragmented mp4 for html5 streaming, using the following command:
-i rtsp://172.20.28.52:554/h264 -vcodec copy -an -f mp4 -reset_timestamps 1 -movflags empty_moov+default_base_moof+frag_keyframe -loglevel quiet -
- "-i rtsp://172.20.28.52:554/h264" because the source is h264 in rtp packets stream from an ip camera. For the sake of testing, the camera is set with GOP of 1 (i.e. all frames are key frames)
- "-vcodec copy" because I don't need transcoding, only remuxing to mp4.
- "-movflags empty_moov+default_base_moof+frag_keyframe" to create a fragmented mp4 according to the media source extensions spec.
- "-" at the end in order to output the mp4 to stdout. I'm grabbing the ouput and sending it to the webclient through web sockets.
Everything is working well, expect for a latency issue which I'm trying to solve. If I'm logging every time a data is coming in from stdout, with the timestamp of arrival, I get this output:
16/06/2015 15:40:45.239 got data size = 24
16/06/2015 15:40:45.240 got data size = 7197
16/06/2015 15:40:45.241 got data size = 32768
16/06/2015 15:40:45.241 got data size = 4941
16/06/2015 15:40:45.241 got data size = 12606
16/06/2015 15:40:45.241 got data size = 6345
16/06/2015 15:40:45.241 got data size = 6339
16/06/2015 15:40:45.242 got data size = 6336
16/06/2015 15:40:45.242 got data size = 6361
16/06/2015 15:40:45.242 got data size = 6337
16/06/2015 15:40:45.242 got data size = 6331
16/06/2015 15:40:45.242 got data size = 6359
16/06/2015 15:40:45.243 got data size = 6346
16/06/2015 15:40:45.243 got data size = 6336
16/06/2015 15:40:45.243 got data size = 6338
16/06/2015 15:40:45.243 got data size = 6357
16/06/2015 15:40:45.243 got data size = 6357
16/06/2015 15:40:45.243 got data size = 6322
16/06/2015 15:40:45.243 got data size = 6359
16/06/2015 15:40:45.244 got data size = 6349
16/06/2015 15:40:45.244 got data size = 6353
16/06/2015 15:40:45.244 got data size = 6382
16/06/2015 15:40:45.244 got data size = 6403
16/06/2015 15:40:45.304 got data size = 6393
16/06/2015 15:40:45.371 got data size = 6372
16/06/2015 15:40:45.437 got data size = 6345
16/06/2015 15:40:45.504 got data size = 6352
16/06/2015 15:40:45.571 got data size = 6340
16/06/2015 15:40:45.637 got data size = 6331
16/06/2015 15:40:45.704 got data size = 6326
16/06/2015 15:40:45.771 got data size = 6360
16/06/2015 15:40:45.838 got data size = 6294
16/06/2015 15:40:45.904 got data size = 6328
16/06/2015 15:40:45.971 got data size = 6326
16/06/2015 15:40:46.038 got data size = 6326
16/06/2015 15:40:46.105 got data size = 6340
16/06/2015 15:40:46.171 got data size = 6341
16/06/2015 15:40:46.238 got data size = 6332
As you can see, the first 23 lines (which contain data of about 1.5 secs of video) are arriving almost instantly, and then the delay between each 2 consecutive lines is ~70ms which makes sense because the video is 15 frames per sec. This behavior introduces a latency of about 1.5 sec.
It looks like a flushing issue because I don't see any reason why would ffmpeg need to hold the first 23 frames in memory, especially since each frame is a fragment of it's own inside the mp4. I couldn't however, find any method that would cause ffmpeg to flush this data faster.
Has anyone got a suggestion?
I'd like to note that this is a follow up question to this one: Live streaming dash content using mp4box
回答1:
The key to removing the delay is to use the -probesize argument:
probesize integer (input)
Set probing size in bytes, i.e. the size of the data to analyze to get stream information. A higher value will enable detecting more information in case it is dispersed into the stream, but will increase latency. Must be an integer not lesser than 32. It is 5000000 by default.
By default the value is 5,000,000 bytes which was equivalent to ~1.5 sec of video. I was able to almost completely eliminate the delay by reducing the value to 200,000.
回答2:
I solved the latency issue by using the -g option to set the number of frames in the group. In my case I used -g 2. I suspect that if you don't make it explicit, the fragment either waits for the source to provide the keyframe or uses a really large default value to generate the keyframe before closing off the fragment and dumping it to stdout.
回答3:
Usually the buffering for stdout is disabled in case of console output. If you run ffmpeg from code, the buffering is enabled, so you will get your data only when the buffer is full or the command ends.
You have to eliminate the stdout buffering of your os. On windows its impossible imo, but on ubuntu for ex. There is http://manpages.ubuntu.com/manpages/maverick/man1/stdbuf.1.html
来源：https://stackoverflow.com/questions/30868854/flush-latency-issue-with-fragmented-mp4-creation-in-ffmpeg