On 27-05-2018 02:37 AM, Lou Logan wrote:
On Sat, May 26, 2018, at 12:33 AM, Gyan Doshi wrote:
Part of the ugliness is due to how the code fragments are rendered: with
vertical margins, creating uneven line spacing. Do you mind if I reduce
or eliminate those margins?
Which file sets those margins? The ugly justified text in the HTML doc that I was
referring to before appears to be due to "body {text-align: justify;}" in
style.min.css. I don't see any value to that at the moment, but maybe I'll take a closer
look some other time and we can save this discussion for later since it should be dealt
with separately from this patch.
v2 attached.
I was talking about the vertical margins appled to the 'code' element.
Regards,
Gyan
From 60ed76348e70f1b0a25eadde8d886d47be3fca69 Mon Sep 17 00:00:00 2001
From: Gyan Doshi <ffm...@gyani.pro>
Date: Thu, 24 May 2018 19:11:00 +0530
Subject: [PATCH v2] doc/ffmpeg - rewrite Stream Selection chapter
Flesh out with details and examples to show quirks and limitations.
---
doc/ffmpeg.texi | 187 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 177 insertions(+), 10 deletions(-)
diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
index 88dbdeb95a..803490ce7b 100644
--- a/doc/ffmpeg.texi
+++ b/doc/ffmpeg.texi
@@ -216,16 +216,183 @@ 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.
+
+@section Description
+@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:
+@*
+@*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 found but there's a caveat.
+The output format's default subtitle encoder may be text-based or image-based,
and only a
+subtitle stream of the same type can be chosen.
+
+Data or attachment streams are not automatically selected and can only be
included
+using @code{-map}.
+
+In the case where several streams of the same type rate equally, the stream
with the lowest
+index is chosen.
+
+The @code{-vn}, @code{-an}, @code{-sn} options can be used to skip automatic
stream selection
+for video, audio, and subtitle streams respectively.
+
+@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. An exception
exists for subtitles.
+If a subtitle encoder is specified for an output file, the first subtitle
stream found, of 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 automatic
+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} 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
+
+This command will fail, as the output pad labelled @code{[outv]} has been
mapped twice.
+No 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 will also fail as the hue filter output is labelled
(@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,
+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 video output from the hue filter and the first
audio stream from
+@file{B.mp4}.
+@*
@c man end STREAM SELECTION
--
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