From e50a5c9c4eb76ff2ff7147ae98e7fbe1c51011b5 Mon Sep 17 00:00:00 2001 From: Gyan Doshi Date: Thu, 24 May 2018 19:11:00 +0530 Subject: doc/ffmpeg: rewrite Stream Selection chapter Flesh out with details and examples to show quirks and limitations. --- doc/ffmpeg.texi | 212 +++++++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 202 insertions(+), 10 deletions(-) (limited to 'doc/ffmpeg.texi') diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi index d9dd256ad7..79da1ae14d 100644 --- a/doc/ffmpeg.texi +++ b/doc/ffmpeg.texi @@ -216,16 +216,208 @@ filters is obviously also impossible, since filters work on uncompressed data. @chapter Stream selection @c man begin STREAM SELECTION -By default, @command{ffmpeg} includes only one stream of each type (video, audio, subtitle) -present in the input files and adds them to each output file. It picks the -"best" of each based upon the following criteria: for video, it is the stream -with the highest resolution, for audio, it is the stream with the most channels, for -subtitles, it is the first subtitle stream. In the case where several streams of -the same type rate equally, the stream with the lowest index is chosen. - -You can disable some of those defaults by using the @code{-vn/-an/-sn/-dn} options. For -full manual control, use the @code{-map} option, which disables the defaults just -described. +@command{ffmpeg} provides the @code{-map} option for manual control of stream selection in each +output file. Users can skip @code{-map} and let ffmpeg perform automatic stream selection as +described below. The @code{-vn / -an / -sn / -dn} options can be used to skip inclusion of +video, audio, subtitle and data streams respectively, whether manually mapped or automatically +selected, except for those streams which are outputs of complex filtergraphs. + +@section Description +The sub-sections that follow describe the various rules that are involved in stream selection. +The examples that follow next show how these rules are applied in practice. + +While every effort is made to accurately reflect the behavior of the program, FFmpeg is under +continuous development and the code may have changed since the time of this writing. + +@subsection Automatic stream selection + +In the absence of any map options for a particular output file, ffmpeg inspects the output +format to check which type of streams can be included in it, viz. video, audio and/or +subtitles. For each acceptable stream type, ffmpeg will pick one stream, when available, +from among all the inputs. + +It will select that stream based upon the following criteria: +@itemize +@item +for video, it is the stream with the highest resolution, +@item +for audio, it is the stream with the most channels, +@item +for subtitles, it is the first subtitle stream found but there's a caveat. +The output format's default subtitle encoder can be either text-based or image-based, +and only a subtitle stream of the same type will be chosen. +@end itemize + +In the case where several streams of the same type rate equally, the stream with the lowest +index is chosen. + +Data or attachment streams are not automatically selected and can only be included +using @code{-map}. +@subsection Manual stream selection + +When @code{-map} is used, only user-mapped streams are included in that output file, +with one possible exception for filtergraph outputs described below. + +@subsection Complex filtergraphs + +If there are any complex filtergraph output streams with unlabeled pads, they will be added +to the first output file. This will lead to a fatal error if the stream type is not supported +by the output format. In the absence of the map option, the inclusion of these streams leads +to the automatic stream selection of their types being skipped. If map options are present, +these filtergraph streams are included in addition to the mapped streams. + +Complex filtergraph output streams with labeled pads must be mapped once and exactly once. + +@subsection Stream handling + +Stream handling is independent of stream selection, with an exception for subtitles described +below. Stream handling is set via the @code{-codec} option addressed to streams within a +specific @emph{output} file. In particular, codec options are applied by ffmpeg after the +stream selection process and thus do not influence the latter. If no @code{-codec} option is +specified for a stream type, ffmpeg will select the default encoder registered by the output +file muxer. + +An exception exists for subtitles. If a subtitle encoder is specified for an output file, the +first subtitle stream found of any type, text or image, will be included. ffmpeg does not validate +if the specified encoder can convert the selected stream or if the converted stream is acceptable +within the output format. This applies generally as well: when the user sets an encoder manually, +the stream selection process cannot check if the encoded stream can be muxed into the output file. +If it cannot, ffmpeg will abort and @emph{all} output files will fail to be processed. + +@section Examples + +The following examples illustrate the behavior, quirks and limitations of ffmpeg's stream +selection methods. + +They assume the following three input files. + +@verbatim + +input file 'A.avi' + stream 0: video 640x360 + stream 1: audio 2 channels + +input file 'B.mp4' + stream 0: video 1920x1080 + stream 1: audio 2 channels + stream 2: subtitles (text) + stream 3: audio 5.1 channels + stream 4: subtitles (text) + +input file 'C.mkv' + stream 0: video 1280x720 + stream 1: audio 2 channels + stream 2: subtitles (image) +@end verbatim + +@subsubheading Example: automatic stream selection +@example +ffmpeg -i A.avi -i B.mp4 out1.mkv out2.wav -map 1:a -c:a copy out3.mov +@end example +There are three output files specified, and for the first two, no @code{-map} options +are set, so ffmpeg will select streams for these two files automatically. + +@file{out1.mkv} is a Matroska container file and accepts video, audio and subtitle streams, +so ffmpeg will try to select one of each type.@* +For video, it will select @code{stream 0} from @file{B.mp4}, which has the highest +resolution among all the input video streams.@* +For audio, it will select @code{stream 3} from @file{B.mp4}, since it has the greatest +number of channels.@* +For subtitles, it will select @code{stream 2} from @file{B.mp4}, which is the first subtitle +stream from among @file{A.avi} and @file{B.mp4}. + +@file{out2.wav} accepts only audio streams, so only @code{stream 3} from @file{B.mp4} is +selected. + +For @file{out3.mov}, since a @code{-map} option is set, no automatic stream selection will +occur. The @code{-map 1:a} option will select all audio streams from the second input +@file{B.mp4}. No other streams will be included in this output file. + +For the first two outputs, all included streams will be transcoded. The encoders chosen will +be the default ones registered by each output format, which may not match the codec of the +selected input streams. + +For the third output, codec option for audio streams has been set +to @code{copy}, so no decoding-filtering-encoding operations will occur, or @emph{can} occur. +Packets of selected streams shall be conveyed from the input file and muxed within the output +file. + +@subsubheading Example: automatic subtitles selection +@example +ffmpeg -i C.mkv out1.mkv -c:s dvdsub -an out2.mkv +@end example +Although @file{out1.mkv} is a Matroska container file which accepts subtitle streams, only a +video and audio stream shall be selected. The subtitle stream of @file{C.mkv} is image-based +and the default subtitle encoder of the Matroska muxer is text-based, so a transcode operation +for the subtitles is expected to fail and hence the stream isn't selected. However, in +@file{out2.mkv}, a subtitle encoder is specified in the command and so, the subtitle stream is +selected, in addition to the video stream. The presence of @code{-an} disables audio stream +selection for @file{out2.mkv}. + +@subsubheading Example: unlabeled filtergraph outputs +@example +ffmpeg -i A.avi -i C.mkv -i B.mp4 -filter_complex "overlay" out1.mp4 out2.srt +@end example +A filtergraph is setup here using the @code{-filter_complex} option and consists of a single +video filter. The @code{overlay} filter requires exactly two video inputs, but none are +specified, so the first two available video streams are used, those of @file{A.avi} and +@file{C.mkv}. The output pad of the filter has no label and so is sent to the first output file +@file{out1.mp4}. Due to this, automatic selection of the video stream is skipped, which would +have selected the stream in @file{B.mp4}. The audio stream with most channels viz. @code{stream 3} +in @file{B.mp4}, is chosen automatically. No subtitle stream is chosen however, since the MP4 +format has no default subtitle encoder registered, and the user hasn't specified a subtitle encoder. + +The 2nd output file, @file{out2.srt}, only accepts text-based subtitle streams. So, even though +the first subtitle stream available belongs to @file{C.mkv}, it is image-based and hence skipped. +The selected stream, @code{stream 2} in @file{B.mp4}, is the first text-based subtitle stream. + +@subsubheading Example: labeled filtergraph outputs +@example +ffmpeg -i A.avi -i B.mp4 -i C.mkv -filter_complex "[1:v]hue=s=0[outv];overlay;aresample" \ + -map '[outv]' -an out1.mp4 \ + out2.mkv \ + -map '[outv]' -map 1:a:0 out3.mkv +@end example + +The above command will fail, as the output pad labelled @code{[outv]} has been mapped twice. +None of the output files shall be processed. + +@example +ffmpeg -i A.avi -i B.mp4 -i C.mkv -filter_complex "[1:v]hue=s=0[outv];overlay;aresample" \ + -an out1.mp4 \ + out2.mkv \ + -map 1:a:0 out3.mkv +@end example + +This command above will also fail as the hue filter output has a label, @code{[outv]}, +and hasn't been mapped anywhere. + +The command should be modified as follows, +@example +ffmpeg -i A.avi -i B.mp4 -i C.mkv -filter_complex "[1:v]hue=s=0,split=2[outv1][outv2];overlay;aresample" \ + -map '[outv1]' -an out1.mp4 \ + out2.mkv \ + -map '[outv2]' -map 1:a:0 out3.mkv +@end example +The video stream from @file{B.mp4} is sent to the hue filter, whose output is cloned once using +the split filter, and both outputs labelled. Then a copy each is mapped to the first and third +output files. + +The overlay filter, requiring two video inputs, uses the first two unused video streams. Those +are the streams from @file{A.avi} and @file{C.mkv}. The overlay output isn't labelled, so it is +sent to the first output file @file{out1.mp4}, regardless of the presence of the @code{-map} option. + +The aresample filter is sent the first unused audio stream, that of @file{A.avi}. Since this filter +output is also unlabelled, it too is mapped to the first output file. The presence of @code{-an} +only suppresses automatic or manual stream selection of audio streams, not outputs sent from +filtergraphs. Both these mapped streams shall be ordered before the mapped stream in @file{out1.mp4}. + +The video, audio and subtitle streams mapped to @code{out2.mkv} are entirely determined by +automatic stream selection. + +@file{out3.mkv} consists of the cloned video output from the hue filter and the first audio +stream from @file{B.mp4}. +@* @c man end STREAM SELECTION -- cgit v1.2.3