Changes between Version 37 and Version 38 of Concatenate

Timestamp:

Nov 14, 2017, 10:19:27 PM (7 months ago)

Author:

slhck

Comment:

formatting, grammar, more explanations and caveats, add example image to concat filter

Concatenate

@@ -8 +8 @@
 == Concatenation of files with same codecs ==#samecodec
 
-There are two methods within ffmpeg that can be used to concatenate files of the same type: [#demuxer the concat ''demuxer''] and [#protocol the concat ''protocol'']. The demuxer is more flexible - it requires the same codecs, but different container formats can be used; and it can be used with any container formats, while the protocol only works with a select few containers. However, the concat protocol is available in older versions of ffmpeg, where the demuxer isn't.
+There are two methods within ffmpeg that can be used to concatenate files of the same type:
+
+1. [#demuxer the concat ''demuxer'']
+2. [#protocol the concat ''protocol'']
+
+The demuxer is more flexible – it requires the same codecs, but different container formats can be used; and it can be used with any container formats, while the protocol only works with a select few containers.
 
 === Concat demuxer ===#demuxer
 
-The concat demuxer was added to FFmpeg 1.1. You can read about it in the [https://ffmpeg.org/ffmpeg-formats.html#concat documentation].
+You can read about the concat demuxer in the [https://ffmpeg.org/ffmpeg-formats.html#concat documentation]. This demuxer reads a list of files and other directives from a text file and demuxes them one after the other, as if all their packets had been muxed together. All files must have the same streams (same codecs, same time base, etc.) but can be wrapped in different container formats.
 
 ==== Instructions ====
@@ -31 +36 @@
 The `-safe 0` above is not required if the paths are relative.
 
+==== Automatically generating the input file ====
+
 It is possible to generate this list file with a bash for loop, or using `printf`. '''Either''' of the following would generate a list file containing every *.wav in the working directory:
 
@@ -67 +74 @@
 }}}
 
-Concatenation becomes troublesome, if next clip for concatenation does not exist at the moment, because decoding won't start until the whole list is read. However, it is possible to refer another list at the end of the current list:
+==== Automatically appending to the list file ====
+
+Concatenation does not work if the next clip for does not exist at the moment, because decoding won't start until the whole list is read. However, it is possible to refer to another list at the end of the current list. The following script provides an example for this mechanism:
 
 {{{
@@ -112 +121 @@
 ffplaypid=$!
 
-
 echo "generating some test data..."
 i=0; for c in red yellow green blue; do
@@ -131 +139 @@
 === Concat protocol ===#protocol
 
-While the demuxer works at the stream level, the concat protocol works at the file level. Certain files (mpg and mpeg transport streams, possibly others) can be concatenated. This is analogous to using cat on UNIX-like systems or copy on Windows.
+While the demuxer works at the stream level, the concat protocol works at the file level. Certain files (MPEG-2 transport streams, possibly others) can be concatenated. This is analogous to using `cat` on UNIX-like systems or `copy` on Windows.
 
 ==== Instructions ====
 
-{{{
-ffmpeg -i "concat:input1.mpg|input2.mpg|input3.mpg" -c copy output.mpg
-}}}
-
-If you have MP4 files, these could be losslessly concatenated by first transcoding them to mpeg transport streams. With h.264 video and AAC audio, the following can be used:
+The following command concatenates three MPEG-2 TS files and concatenates them without re-encoding:
+
+{{{
+ffmpeg -i "concat:input1.ts|input2.ts|input3.ts" -c copy output.ts
+}}}
+
+==== Using intermediate files ====
+
+If you have MP4 files, these could be losslessly concatenated by first transcoding them to MPEG-2 transport streams. With H.264 video and AAC audio, the following can be used:
 
 {{{
@@ -147 +159 @@
 }}}
 
-If you're using a system that supports named pipes, you can use those to avoid creating intermediate files - this sends stderr (which ffmpeg sends all the written data to) to /dev/null, to avoid cluttering up the command-line:
+==== Using named pipes to avoid intermediate files ====
+
+If you're using a system that supports named pipes, you can use those to avoid creating intermediate files. This sends stderr (to which ffmpeg sends all the written data) to `/dev/null`, to avoid cluttering up the command-line:
 
 {{{
@@ -156 +170 @@
 }}}
 
-All MPEG codecs (H.264, MPEG4/divx/xvid, MPEG2; MP2, MP3, AAC) are supported in the mpegts container format, though the commands above would require some alteration (the `-bsf` bitstream filters will have to be changed).
+All MPEG codecs (MPEG-4 Part 10 / AVC, MPEG-4 Part 2, MPEG-2 Video, MPEG-1 Audio Layer II, MPEG-2 Audio Layer III (MP3), MPEG-4 Part III (AAC)) are supported in the MPEG-TS container format, although the commands above would require some alteration (e.g., the `-bsf` bitstream filters will have to be changed).
 
 == Concatenation of files with different codecs ==#differentcodec
 
+In many cases, input files will have different codecs or different codec properties, which makes it impossible to use any of the above methods.
+
 === Concat filter ===#filter
 
-The concat filter is available in recent versions of ffmpeg. See the [https://ffmpeg.org/ffmpeg-filters.html#concat concat filter documentation] for more info.
+See the [https://ffmpeg.org/ffmpeg-filters.html#concat concat filter documentation] for more info. The filter works on segments of synchronized video and audio streams. All segments must have the same number of streams of each type, and that will also be the number of streams at output.
+
+{{{
+#!div style="border: 1px solid #e5e5c7; margin: 1em; background-color: #ffd;"
+'''Note:''' Filters are incompatible with [https://ffmpeg.org/ffmpeg.html#Stream-copy stream copying]; you can't use `-c copy` with this method. Since you have to re-encode the video and audio stream(s), and since re-encoding may introduce compression artifacts, make sure to add proper target bitrate or quality settings. See the [[Encode|encoding guides]] for more info.
+}}}
+
+For the concat filter to work, the inputs have to be of the same frame dimensions (e.g., 1920⨉1080 pixels) and should have the same framerate. Therefore, you may at least have to add a [http://ffmpeg.org/ffmpeg-filters.html#scale-1 scale] or [http://ffmpeg.org/ffmpeg-filters.html#scale2ref scale2ref] filter before concatenating videos. A handful of other attributes have to match as well, like the stream aspect ratio. Refer to the documentation of the filter for more info.
 
 ==== Instructions ====
 
-This is easiest to explain using an example:
-
-{{{
-ffmpeg -i input1.mp4 -i input2.webm \
--filter_complex "[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]" \
--map "[v]" -map "[a]" <encoding options> output.mkv
-}}}
-
-On the `-filter_complex` line, the following:
-
-{{{
-[0:v:0] [0:a:0] [1:v:0] [1:a:0]
-}}}
-
-tells ffmpeg what streams to send to the concat filter; in this case, video stream 0 [0:v:0] and audio stream 0 [0:a:0] from input 0 (`input1.mp4` in this example), and video stream 0 [1:v:0] and audio stream 0 [1:v:0] from input 1 (`input2.webm`).
-
-{{{
-concat=n=2:v=1:a=1 [v] [a]'
-}}}
-
-This is the concat filter itself. `n=2` is telling the filter that there are two input files; `v=1` is telling it that there will be one video stream; `a=1` is telling it that there will be one audio stream. `[v]` and `[a]` are names for the output streams to allow the rest of the ffmpeg line to use the output of the concat filter.
-
-Note that the single quotes around the whole filter section are required.
-
-{{{
--map '[v]' -map '[a]'
+Let's say we have three files that we want to concatenate – each of them with one video and audio stream. The concat filter command would look like this:
+
+{{{
+ffmpeg -i input1.mp4 -i input2.webm -i input3.mov \
+-filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0][2:v:0][2:a:0]concat=n=3:v=1:a=1[outv][outa]" \
+-map "[outv]" -map "[outa]" output.mkv
+}}}
+
+Now, let's dissect that command. We first specify all the input files, then instantiate a `-filter_complex` filtergraph – this is needed instead of `-filter:v` because it has multiple inputs and outputs.
+
+The following line:
+
+{{{
+[0:v:0][0:a:0][1:v:0][1:a:0][2:v:0][2:a:0]
+}}}
+
+tells ffmpeg which streams to take from the input files and send as input to the concat filter. In this case, video stream 0 [0:v:0] and audio stream 0 [0:a:0] from input 0 (`input1.mp4` in this example), and video stream 0 [1:v:0] and audio stream 0 [1:v:0] from input 1 (`input2.webm`), etc.
+
+{{{
+concat=n=3:v=1:a=1[outv][outa]'
+}}}
+
+This is the concat filter itself. `n=3` is telling the filter that there are three input segments; `v=1` is telling it that there will be one video stream per segment; `a=1` is telling it that there will be one audio stream per segment. The filter then concatenates these segments and produces two output streams. `[outv]` and `[outa]` are names for these output streams. Note that the quotes around the filter section are required.
+
+The following image shows the stream mapping to and from the filter in the above example:
+
+[[Image(concat_filter.png)]]
+
+You can then either re-use these streams in other filters, or [[Map|map them to the output file]]:
+
+{{{
+-map "[outv]" -map "[outa]" output.mkv
 }}}
 
 This tells ffmpeg to use the results of the concat filter rather than the streams directly from the input files.
 
-Note that filters are incompatible with [https://ffmpeg.org/ffmpeg.html#Stream-copy stream copying]; you can't use `-c copy` with this method. Also, I'm not sure whether softsubs are supported.
-
-As you can infer from this example, multiple types of input are supported, and anything readable by ffmpeg should work. The inputs have to be of the same frame size, and a handful of other attributes have to match.
-
 === Using an external script ===#extscript