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3.5 Audio Options

`-aframes number'
    Set the number of audio frames to record. 
`-ar freq'
    Set the audio sampling frequency (default = 44100 Hz). 
`-ab bitrate'
    Set the audio bitrate in bit/s (default = 64k). 
`-ac channels'
    Set the number of audio channels (default = 1). 
`-an'
    Disable audio recording. 
`-acodec codec'
    Force audio codec to codec. Use the copy special 
    value to specify that the raw codec data must be 
    copied as is. 
`-newaudio'
    Add a new audio track to the output file. If you 
    want to specify parameters, do so before -newaudio 
    (-acodec, -ab, etc..). Mapping will be done 
    automatically, if the number of output streams 
    is equal to the number of input streams, else it 
    will pick the first one that matches. You can 
    override the mapping using -map as usual. 
    Example:

ffmpeg -i file.mpg -vcodec copy -acodec ac3 -ab 384k test.mpg -acodec mp2 -ab 192k -newaudio

`-alang code'
    Set the ISO 639 language code (3 letters) of the 
    current audio stream. 

3.6 Advanced Audio options:

`-atag fourcc/tag'
    Force audio tag/fourcc. 
`-absf bitstream filter'
    Bitstream filters available are "dump_extra", 
    "remove_extra", "noise", "mp3comp", "mp3decomp". 

3.7 Subtitle options:

`-scodec codec'
    Force subtitle codec ('copy' to copy stream). 
`-newsubtitle'
    Add a new subtitle stream to the current output stream. 
`-slang code'
    Set the ISO 639 language code (3 letters) of the 
    current subtitle stream. 

3.8 Audio/Video grab options

`-vc channel'
    Set video grab channel (DV1394 only). 
`-tvstd standard'
    Set television standard (NTSC, PAL (SECAM)). 
`-isync'
    Synchronize read on input. 

3.9 Advanced options

`-map input stream id[:input stream id]'
    Set stream mapping from input streams to output 
    streams. Just enumerate the input streams in the 
    order you want them in the output. [input stream id] 
    sets the (input) stream to sync against. 
`-map_meta_data outfile:infile'
    Set meta data information of outfile from infile. 
`-debug'
    Print specific debug info. 
`-benchmark'
    Add timings for benchmarking. 
`-dump'
    Dump each input packet. 
`-hex'
    When dumping packets, also dump the payload. 
`-bitexact'
    Only use bit exact algorithms (for codec testing). 
`-ps size'
    Set packet size in bits. 
`-re'
    Read input at native frame rate. Mainly used to 
    simulate a grab device. 
`-loop_input'
    Loop over the input stream. Currently it works 
    only for image streams. This option is used for 
    automatic FFserver testing. 
`-loop_output number_of_times'
    Repeatedly loop output for formats that support 
    looping such as animated GIF (0 will loop the 
    output infinitely). 
`-threads count'
    Thread count. 
`-vsync parameter'
    Video sync method. Video will be stretched/squeezed 
    to match the timestamps, it is done by duplicating 
    and dropping frames. With -map you can select from 
    which stream the timestamps should be taken. You 
    can leave either video or audio unchanged and sync 
    the remaining stream(s) to the unchanged one. 
`-async samples_per_second'
    Audio sync method. "Stretches/squeezes" the audio 
    stream to match the timestamps, the parameter is 
    the maximum samples per second by which the audio 
    is changed. -async 1 is a special case where only 
    the start of the audio stream is corrected without 
    any later correction. 
`-copyts'
    Copy timestamps from input to output. 
`-shortest'
    Finish encoding when the shortest input stream ends. 
`-dts_delta_threshold'
    Timestamp discontinuity delta threshold. 
`-muxdelay seconds'
    Set the maximum demux-decode delay. 
`-muxpreload seconds'
    Set the initial demux-decode delay. 

3.10 FFmpeg formula evaluator

When evaluating a rate control string, FFmpeg uses an 
internal formula evaluator.

The following binary operators are available: +, -, *, /, ^.

The following unary operators are available: +, -, (...).

The following functions are available:

sinh(x)
cosh(x)
tanh(x)
sin(x)
cos(x)
tan(x)
exp(x)
log(x)
squish(x)
gauss(x)
abs(x)
max(x, y)
min(x, y)
gt(x, y)
lt(x, y)
eq(x, y)
bits2qp(bits)
qp2bits(qp)

The following constants are available:

PI
E
iTex
pTex
tex
mv
fCode
iCount
mcVar
var
isI
isP
isB
avgQP
qComp
avgIITex
avgPITex
avgPPTex
avgBPTex
avgTex

3.11 Protocols

The filename can be `-' to read from standard input 
or to write to standard output.

FFmpeg also handles many protocols specified with 
an URL syntax.

Use 'ffmpeg -formats' to see a list of the supported 
protocols.

The protocol http: is currently used only to 
communicate with FFserver (see the FFserver documentation). 
When FFmpeg will be a video player it will also be 
used for streaming :-)
4. Tips

    * For streaming at very low bitrate application, use 
    a low frame rate and a small GOP size. This is 
    especially true for RealVideo where the Linux player 
    does not seem to be very fast, so it can miss frames. 
    An example is:

ffmpeg -g 3 -r 3 -t 10 -b 50k -s qcif -f rv10 /tmp/b.rm

    * The parameter 'q' which is displayed while encoding 
    is the current quantizer. The value 1 indicates that 
    a very good quality could be achieved. The value 31 
    indicates the worst quality. If q=31 appears too often, 
    it means that the encoder cannot compress enough to 
    meet your bitrate. You must either increase the bitrate, 
    decrease the frame rate or decrease the frame size.
    * If your computer is not fast enough, you can speed 
    up the compression at the expense of the compression 
    ratio. You can use '-me zero' to speed up motion 
    estimation, and '-intra' to disable motion estimation 
    completely (you have only I-frames, which means it is 
    about as good as JPEG compression).
    * To have very low audio bitrates, reduce the sampling 
    frequency (down to 22050 kHz for MPEG audio, 22050 or 11025 for AC3).
    * To have a constant quality (but a variable bitrate), 
    use the option '-qscale n' when 'n' is between 1 
    (excellent quality) and 31 (worst quality).
    * When converting video files, you can use the 
    '-sameq' option which uses the same quality factor 
    in the encoder as in the decoder. It allows almost 
    lossless encoding. 

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