virtualenv/FFmpeg
1
0
Fork 0
mirror of https://github.com/FFmpeg/FFmpeg.git synced 2024-11-21 10:55:51 +02:00
Code Issues Releases Activity

doc/avconv: add some details about the transcoding process.

Browse Source
This commit is contained in:
Anton Khirnov 2012-06-04 11:00:34 +02:00
parent a982e5a031
commit 2b1f105f1b
1 changed files with 120 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

120
doc/avconv.texi
View File

@ -79,6 +79,126 @@ The format option may be needed for raw input files.
@c man end DESCRIPTION
@chapter Detailed description
@c man begin DETAILED DESCRIPTION
The transcoding process in @command{avconv} for each output can be described by
the following diagram:
@example
_______ ______________ _________ ______________ ________
| | | | | | | | | |
| input | demuxer | encoded data | decoder | decoded | encoder | encoded data | muxer | output |
| file | ---------> | packets | ---------> | frames | ---------> | packets | -------> | file |
|_______| |______________| |_________| |______________| |________|
@end example
@command{avconv} calls the libavformat library (containing demuxers) to read
input files and get packets containing encoded data from them. When there are
multiple input files, @command{avconv} 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 again. Finally those are
passed to the muxer, which writes the encoded packets to the output file.
@section Filtering
Before encoding, @command{avconv} can process raw audio and video frames using
filters from the libavfilter library. Several chained filters form a filter
graph. @command{avconv} distinguishes between two types of filtergraphs -
simple and complex.
@subsection 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:
@example
_________ __________ ______________
| | | | | |
| decoded | simple filtergraph | filtered | encoder | encoded data |
| frames | -------------------> | frames | ---------> | packets |
|_________| |__________| |______________|
@end example
Simple filtergraphs are configured with the per-stream @option{-filter} option
(with @option{-vf} and @option{-af} aliases for video and audio respectively).
A simple filtergraph for video can look for example like this:
@example
_______ _____________ _______ _____ ________
| | | | | | | | | |
| input | ---> | deinterlace | ---> | scale | ---> | fps | ---> | output |
|_______| |_____________| |_______| |_____| |________|
@end example
Note that some filters change frame properties but not frame contents. E.g. the
@code{fps} filter in the example above changes number of frames, but does not
touch the frame contents. Another example is the @code{setpts} filter, which
only sets timestamps and otherwise passes the frames unchanged.
@subsection Complex filtergraphs
Complex filtergraphs are those which cannot be described as simply a linear
processing chain applied to one stream. This is the case e.g. 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:
@example
_________
| |
| input 0 |\ __________
|_________| \ | |
\ _________ /| output 0 |
\ | | / |__________|
_________ \| complex | /
| | | |/
| input 1 |---->| filter |\
|_________| | | \ __________
/| graph | \ | |
/ | | \| output 1 |
_________ / |_________| |__________|
| | /
| input 2 |/
|_________|
@end example
Complex filtergraphs are configured with the @option{-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.
A trivial example of a complex filtergraph is the @code{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 @code{amix} filter.
@section Stream copy
Stream copy is a mode selected by supplying the @code{copy} parameter to the
@option{-codec} option. It makes @command{avconv} 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:
@example
_______ ______________ ________
| | | | | |
| input | demuxer | encoded data | muxer | output |
| file | ---------> | packets | -------> | file |
|_______| |______________| |________|
@end example
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.
@c man end DETAILED DESCRIPTION
@chapter Stream selection
@c man begin STREAM SELECTION