[Ffmpeg-devel] ffserver Documentation
Piero Bugoni
crboca32
Thu Mar 15 18:25:45 CET 2007
>
> However, In order to document correctly, I have had
> to
> turn to the ffmpeg program documentation, which is
> also somewhat terse.
>
> So, now I have some questions regarding ffmepg.
>
Regarding video bit rate, -sameq, -qmin, -qmax, and so
forth, I have some questions. I understand these are
typically ffmpeg-user type questions, but the answers
I intend to include in documentation.
So, here is one example:
This command: ./ffmpeg -t 10 -y -r 29.97 -s 320x240
-f video4linux2 -i /dev/video1 out.yuv
produced the following output:
Input #0, video4linux2, from '/dev/video1':
Duration: N/A, bitrate: 27620 kb/s
Stream #0.0: Video: rawvideo, yuv420p, 320x240,
27620 kb/s, 29.97 fps(r)
Output #0, rawvideo, to 'out.yuv':
Stream #0.0: Video: rawvideo, yuv420p, 320x240,
q=2-31, 200 kb/s, 29.97 fps(c)
Stream mapping:
Stream #0.0 -> #0.0
Press [q] to stop encoding
frame= 300 q=0.0 Lsize= 33750kB time=10.0
bitrate=27620.3kbits/s
video:33750kB audio:0kB global headers:0kB muxing
overhead 0.000000%
The "Output #0" line indicates that for Stream #0.0
that the bit rate is 200 kb/s (the default). But the
status line indicates a bit rate of 27620.3kbits/s,
and the output file was, not surprisingly, around 33M.
Also, is there a relationship between "q" and bitrate?
Does specifying one override the other?
The doc says that -sameq implies VBR. What does this
mean?
I know that -sameq produces a bigger file, and better
output than high "q" values. For instance, setting
-qmin 32 and -qmax 32 produces a very small file,
howerver, that is almost unviewable.
When grabbing raw video from a capture card, and
outputting to a raw file, are "q" and bitrate
meaningless? i.e. spew data to disk as fast as the
card captures it, no encoding, just a simple recording
operation?
The end goal of all this is to help people tune the
"q", and bitrate values to their bandwidth for best
results with ffserver/ffmpeg. It is probably less
important if you have tons of disk space, (-sameq is
very convenient), but I think it is definitely
critical for network usage.
Thanks.
Piero.
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