doc/ffmpeg: rewrite Stream Selection chapter

Flesh out with details and examples to show quirks and limitations.

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.
+@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.
 
-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.
+@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