选项详情（部分）
更多：https://www.cnblogs.com/skiy/p/ffmpeg-usage.html
man ffmpeg
FFMPEG(1) FFMPEG(1)
NAME
ffmpeg - ffmpeg video converter
SYNOPSIS
ffmpeg [global_options] {[input_file_options] -i input_url} ... {[output_file_options] output_url} ...
DESCRIPTION
ffmpeg is a very fast video and audio converter that can also grab from a live audio/video source. It can also convert between
arbitrary sample rates and resize video on the fly with a high quality polyphase filter.
ffmpeg reads from an arbitrary number of input "files" (which can be regular files, pipes, network streams, grabbing devices,
etc.), specified by the "-i" option, and writes to an arbitrary number of output "files", which are specified by a plain output
url. Anything found on the command line which cannot be interpreted as an option is considered to be an output url.
Each input or output url can, in principle, contain any number of streams of different types
(video/audio/subtitle/attachment/data). The allowed number and/or types of streams may be limited by the container format.
Selecting which streams from which inputs will go into which output is either done automatically or with the "-map" option (see
the Stream selection chapter).
To refer to input files in options, you must use their indices (0-based). E.g. the first input file is 0, the second is 1, etc.
Similarly, streams within a file are referred to by their indices. E.g. "2:3" refers to the fourth stream in the third input
file. Also see the Stream specifiers chapter.
As a general rule, options are applied to the next specified file. Therefore, order is important, and you can have the same
option on the command line multiple times. Each occurrence is then applied to the next input or output file. Exceptions from
this rule are the global options (e.g. verbosity level), which should be specified first.
Do not mix input and output files -- first specify all input files, then all output files. Also do not mix options which belong
to different files. All options apply ONLY to the next input or output file and are reset between files.
• To set the video bitrate of the output file to 64 kbit/s:
ffmpeg -i input.avi -b:v 64k -bufsize 64k output.avi
• To force the frame rate of the output file to 24 fps:
ffmpeg -i input.avi -r 24 output.avi
• To force the frame rate of the input file (valid for raw formats only) to 1 fps and the frame rate of the output file to 24
fps:
ffmpeg -r 1 -i input.m2v -r 24 output.avi
The format option may be needed for raw input files.
DETAILED DESCRIPTION
The transcoding process in ffmpeg for each output can be described by the following diagram:
_______ ______________
| | | |
| input | demuxer | encoded data | decoder
| file | ---------> | packets | -----+
|_______| |______________| |
v
_________
| |
| decoded |
| frames |
|_________|
________ ______________ |
| | | | |
| output | <-------- | encoded data | <----+
| file | muxer | packets | encoder
|________| |______________|
ffmpeg calls the libavformat library (containing demuxers) to read input files and get packets containing encoded data from them.
When there are multiple input files, ffmpeg tries to keep them synchronized by tracking lowest timestamp on any active input
stream.
Encoded packets are then passed to the decoder (unless streamcopy is selected for the stream, see further for a description). The
decoder produces uncompressed frames (raw video/PCM audio/...) which can be processed further by filtering (see next section).
After filtering, the frames are passed to the encoder, which encodes them and outputs encoded packets. Finally those are passed
to the muxer, which writes the encoded packets to the output file.
Filtering
Before encoding, ffmpeg can process raw audio and video frames using filters from the libavfilter library. Several chained
filters form a filter graph. ffmpeg distinguishes between two types of filtergraphs: simple and complex.
Simple filtergraphs
Simple filtergraphs are those that have exactly one input and output, both of the same type. In the above diagram they can be
represented by simply inserting an additional step between decoding and encoding:
_________ ______________
| | | |
| decoded | | encoded data |
| frames |\ _ | packets |
|_________| \ /||______________|
\ __________ /
simple _\|| | / encoder
filtergraph | filtered |/
| frames |
|__________|
Simple filtergraphs are configured with the per-stream -filter option (with -vf and -af aliases for video and audio
respectively). A simple filtergraph for video can look for example like this:
_______ _____________ _______ ________
| | | | | | | |
| input | ---> | deinterlace | ---> | scale | ---> | output |
|_______| |_____________| |_______| |________|
Note that some filters change frame properties but not frame contents. E.g. the "fps" filter in the example above changes number
of frames, but does not touch the frame contents. Another example is the "setpts" filter, which only sets timestamps and
otherwise passes the frames unchanged.
Complex filtergraphs
Complex filtergraphs are those which cannot be described as simply a linear processing chain applied to one stream. This is the
case, for example, when the graph has more than one input and/or output, or when output stream type is different from input. They
can be represented with the following diagram:
_________
| |
| input 0 |\ __________
|_________| \ | |
\ _________ /| output 0 |
\ | | / |__________|
_________ \| complex | /
| | | |/
| input 1 |---->| filter |\
|_________| | | \ __________
/| graph | \ | |
/ | | \| output 1 |
_________ / |_________| |__________|
| | /
| input 2 |/
|_________|
Complex filtergraphs are configured with the -filter_complex option. Note that this option is global, since a complex
filtergraph, by its nature, cannot be unambiguously associated with a single stream or file.
The -lavfi option is equivalent to -filter_complex.
A trivial example of a complex filtergraph is the "overlay" filter, which has two video inputs and one video output, containing
one video overlaid on top of the other. Its audio counterpart is the "amix" filter.
Stream copy
Stream copy is a mode selected by supplying the "copy" parameter to the -codec option. It makes ffmpeg omit the decoding and
encoding step for the specified stream, so it does only demuxing and muxing. It is useful for changing the container format or
modifying container-level metadata. The diagram above will, in this case, simplify to this:
_______ ______________ ________
| | | | | |
| input | demuxer | encoded data | muxer | output |
| file | ---------> | packets | -------> | file |
|_______| |______________| |________|
Since there is no decoding or encoding, it is very fast and there is no quality loss. However, it might not work in some cases
because of many factors. Applying filters is obviously also impossible, since filters work on uncompressed data.