wehub-resource-sync
115 lines
5.2 KiB
Plaintext
115 lines
5.2 KiB
Plaintext
DETAILED DESCRIPTION
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The transcoding process in ffmpeg for each output can be described by the following diagram:
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_______ ______________
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| | | |
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| input | demuxer | encoded data | decoder
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| file | ---------> | packets | -----+
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|_______| |______________| |
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v
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_________
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| decoded |
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| frames |
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|_________|
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________ ______________ |
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| | | | |
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| output | <-------- | encoded data | <----+
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| file | muxer | packets | encoder
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|________| |______________|
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ffmpeg calls the libavformat library (containing demuxers) to read input files and get packets
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containing encoded data from them. When there are multiple input files, ffmpeg tries to keep them
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synchronized by tracking lowest timestamp on any active input stream.
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Encoded packets are then passed to the decoder (unless streamcopy is selected for the stream, see
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further for a description). The decoder produces uncompressed frames (raw video/PCM audio/...)
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which can be processed further by filtering (see next section). After filtering, the frames are
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passed to the encoder, which encodes them and outputs encoded packets. Finally those are passed to
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the muxer, which writes the encoded packets to the output file.
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Filtering
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Before encoding, ffmpeg can process raw audio and video frames using filters from the libavfilter
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library. Several chained filters form a filter graph. ffmpeg distinguishes between two types of
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filtergraphs: simple and complex.
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Simple filtergraphs
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Simple filtergraphs are those that have exactly one input and output, both of the same type. In
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the above diagram they can be represented by simply inserting an additional step between decoding
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and encoding:
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_________ ______________
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| decoded | | encoded data |
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| frames |\ _ | packets |
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|_________| \ /||______________|
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\ __________ /
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simple _\|| | / encoder
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filtergraph | filtered |/
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| frames |
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|__________|
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Simple filtergraphs are configured with the per-stream -filter option (with -vf and -af aliases
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for video and audio respectively). A simple filtergraph for video can look for example like this:
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_______ _____________ _______ ________
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| input | ---> | deinterlace | ---> | scale | ---> | output |
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|_______| |_____________| |_______| |________|
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Note that some filters change frame properties but not frame contents. E.g. the "fps" filter in
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the example above changes number of frames, but does not touch the frame contents. Another example
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is the "setpts" filter, which only sets timestamps and otherwise passes the frames unchanged.
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Complex filtergraphs
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Complex filtergraphs are those which cannot be described as simply a linear processing chain
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applied to one stream. This is the case, for example, when the graph has more than one input
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and/or output, or when output stream type is different from input. They can be represented with
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the following diagram:
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_________
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| input 0 |\ __________
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|_________| \ | |
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\ _________ /| output 0 |
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\ | | / |__________|
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_________ \| complex | /
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| | | |/
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| input 1 |---->| filter |\
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|_________| | | \ __________
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/| graph | \ | |
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/ | | \| output 1 |
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_________ / |_________| |__________|
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| | /
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| input 2 |/
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|_________|
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Complex filtergraphs are configured with the -filter_complex option. Note that this option is
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global, since a complex filtergraph, by its nature, cannot be unambiguously associated with a
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single stream or file.
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The -lavfi option is equivalent to -filter_complex.
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A trivial example of a complex filtergraph is the "overlay" filter, which has two video inputs and
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one video output, containing one video overlaid on top of the other. Its audio counterpart is the
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"amix" filter.
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Stream copy
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Stream copy is a mode selected by supplying the "copy" parameter to the -codec option. It makes
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ffmpeg omit the decoding and encoding step for the specified stream, so it does only demuxing and
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muxing. It is useful for changing the container format or modifying container-level metadata. The
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diagram above will, in this case, simplify to this:
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_______ ______________ ________
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| input | demuxer | encoded data | muxer | output |
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| file | ---------> | packets | -------> | file |
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|_______| |______________| |________|
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Since there is no decoding or encoding, it is very fast and there is no quality loss. However, it
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might not work in some cases because of many factors. Applying filters is obviously also
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impossible, since filters work on uncompressed data.
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