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• Why does ffmpeg have bigger latency on dark images ?

  19 novembre 2017, par doodoroma

  I have a c# application to stream real camera images using ffmpeg. The input images are in raw, 8-bit gray-scale format. I created an ffmpeg stream using the standard input to feed the images and send the output packages to websocket clients.

  I start an external ffmpeg process using this config :

  -f rawvideo -pixel_format gray -video_size " + camera.Width.ToString() + "x" + camera.Height.ToString() + " -framerate 25 -i - -f mpeg1video -b:v 512k -s 320x240 -

  Typical image size is 1040*1392 pixels

  I display the stream on the browser, using jsmpeg library

  This works with a reasonable latency ( 500ms on localhost), but when the camera image is really dark (black image), the latency is extremely big ( 2-3 seconds on localhost). When there is something bright on the image again after a black period, it takes 2-3 seconds to "synchronize".

  I was thinking that black images are really easy to compress and will generate really small packages, jsmpeg has almost no information to display and wait until a complete data package arrives, but I couldn’t prove my theory.

  I played with ffmpeg parameters like bitrate and fps but nothing has changed.

  Is there any settings which I could try ?

• FFmpeg drawtext and live coordinates with sendcmd/zmq

  9 mai 2019, par DavidK

  I’d like to use a marker on a live video source which has external live data to set x,y coordinates. sendcmd can read the text file but it won’t update it, so later updates are not executed. Is it possible only with zmq ? And if so, can I use zmq as a single filter (with drawtext), not with filter complex ?

  I have a python that exports live coordinates in the appropriate format to a cmd.txt file. I use unix time for the coordinates and I also copy the input timestamps from the live loopback device so they have almost the same time. There’s a small delay so I have compensated the exported timestamps with +1.5s. This means the marker is moving for this extra period (while timestamps in cmd.txt are a bit ahead of the live source), but it won’t update any more. I assume that FFmpeg reads this cmd.txt and won’t update it any more but my python is writing it continuously.

  Example line of the cmd.txt :

  1557402120.3119707 drawtext reinit 'x=752:y=480';

  This is the actual ffmpeg pipe :

  ffmpeg -fflags nobuffer -vaapi_device /dev/dri/renderD128 -f v4l2 -i /dev/video0 -vf "sendcmd=f=cmd.txt,drawtext=fontfile=font.ttf:fontsize=30:fontcolor=white:r=25:text='o',format=nv12,hwupload" -copyts -c:v h264_vaapi -qp 24 -y 0.mp4

  Source is a loopback device with unix time as input timestamp.

• Swapping FFMPEG input source

  21 novembre 2017, par stevendesu

  I’m using FFMPEG to stream RMTP to a server. I wish to change what I’m streaming without breaking the connection to this server.

  My current FFMPEG command looks like so :

  ffmpeg -f v4l2 -s 1280x720 -r 10 -i /dev/video0 -c:v libx264 -f flv -r 30 -pix_fmt yuv420p "rtmp://server live=true pubUser=user pubPasswd=pass playpath=stream"

  If I wanted to change from /dev/video0 to /dev/video1 then I need to stop this program and re-run the command swapping out the -i bit.

  Since FFMPEG can read from stdin as well as files, I believe it should be possible to switch the input source on the fly by either piping the output of a different program, or utilizing UNIX sockets. There may also be a solution built into FFMPEG which I’m not aware of.

  My question now is : what’s the simplest / least code / most recommended way of switching these inputs ? Is there a third-party tool that’s recommended ? Does FFMPEG have an alternate command-line parameter I’ve never heard of ? If I do use a UNIX socket of stdin, is there a recommended way to change what’s being written to them ?

  One of my concerns is that if I have FFMPEG read in from a UNIX socket and in a separate shell I have a different instance of FFMPEG writing to this UNIX socket, during the brief period when I’m switching sources the first instance of FFMPEG (which is doing the broadcasting) would die, as it couldn’t find any video data to stream.