On 2018-02-26 22:53, Pičugins Arsenijs wrote:
Hi! I'm writing an article detailing streaming to Twitch from a web camera,
using ffmpeg.
I tried to explain the ffmpeg command-line I'm using, however, as I don't know
ffmpeg
that well, my understanding of the command-line itself is limited. Could you
please
see if there's anything obviously (or non-obviously) wrong with the following
text?
First, let's cover a simple streaming case - no sound involved, just video. We
have a USB
camera (available as /dev/video0), and we have a Twitch RTMP URL, which we
should send our stream to. A basic ffmpeg command-line that does the job of is
as follows:
ffmpeg -f v4l2 -s "$INRES" -r "$FPS" -i /dev/video0 -vcodec h264_omx -g $GOP
-keyint_min $FPS -b:v $CBR -minrate "100k" -maxrate $CBR -pix_fmt yuv420p
-bufsize "500k" -preset "veryfast" -f flv
"rtmp://$SERVER.twitch.tv/app/$STREAM_KEY"
That's a lot of parameters for a single command! Let's go through it so that
you understand what's going on. First, here's the breakdown of all parts this
command-line consists of:
ffmpeg {global options} -f {input type} {input options} -i {input} {codec}
{codec options} -f {output type} {output}
We're only using one global option - "-hide-banner", which tells ffmpeg not to
print its version information on start. Our webcam is /dev/video0 - in your
case, it might end with another number (say, you're using a laptop with a
webcam). To capture video from the webcam, we're using the Video4Linux system
and the corresponding ffmpeg plugin called "v4l2", and we tell it the
resolution to use and FPS (frames per second) that we want. Twitch requires
that we compress our video as H.264 - this would usually be a CPU-intensive
task, but Raspberry Pi has hardware H264 encoding support. We can get that
support if we use the h264_omx ffmpeg plugin.
Even though compression means we don't send full frames all the time, we still
need to send a full frame once in a while - packets can get lost and glitches
can happen. A frame sent for the image synchronization purpose is called a
keyframe - the "-g" parameter tells how often keyframes will be made (say, $GOP
is double the $FPS, then a keyframe will be formed each 2 seconds), and the
"-keyint_min" parameter allows additional keyframes to appear if necessary
(say, the video changes rapidly) This explains the "-g $GOP -keyint_min $FPS"
part. Now, what about "-b:v $CBR -minrate "100k" -maxrate $CBR"? These are the
h264_omx codec parameters, and they restrict the bitrate of the resulting
stream - bitrate is, in our case, how much data we're sending per second.
... [a long not-ffmpeg-related explanation of what bitrates mean and why we need
a constant bitrate for Twitch]
After all the bitrate-related parameters, we have parameters defining the color
encoding scheme (Twitch requires the YUV420 scheme) and buffer size - the
buffer in question is the one ffmpeg uses to check whether bitrate is constant
enough; setting your "bufsize" to the same value as your bitrate is a good
starting point. The last parameter is the compression quality - as in, how much
time should be spent on compression. The "fast"-er the preset, the less time is
spent compressing and the more bandwidth will be taken by your video - you have
options like "veryslow", "slow", "medium", "fast" and "veryfast", and some more
in between.
I'm sorry if this request is taken as asking for too big of a favour (full
disclosure: I'm
getting paid to write this). I just really, really would hate to explain things
in a wrong
way and push it onto the readers, and I hope you can help make a ffmpeg
streaming
tutorial that explains things well (which, in my experience, is a rare
occurence).
Cheers!
Arsenijs
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I have never used ffmpeg for that purpose, so I can't comment much about
the technical correctness. However, I can give some comments about more
general topics.
1. Have you used this command, and does it in fact work? I assume that
you have, but you didn't say so in your introduction.
2. You use environment variables like $INRES, $FPS, $CBR, but I don't
see any text that explains they are variables, not literal parameters to
the command. Nor do I see an explanation of what values they should be
set to. Nor do I see an explanation of why these values, but not
others, are provided as environment variables instead of literally. I
imagine it's a good idea to make them variables, but it would help the
article to explain why.
I hope this helps. Good luck with the article!
--
--Jim DeLaHunt, [email protected] http://blog.jdlh.com/ (http://jdlh.com/)
multilingual websites consultant
355-1027 Davie St, Vancouver BC V6E 4L2, Canada
Canada mobile +1-604-376-8953
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