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How can I concatenate several .webm files (e.g. downloaded from youtube) into a single file? I've tried a simple cat followed by "mencoder -forceidx -oac copy -ovc copy ..." - didn't work. I'm using Linux.

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That's gonna be hard – most videos are not meant for concatenating. You can somewhat do this with MPEG-2 and h.264, but no idea for WebM, unless you want to re-encode them. –  slhck May 6 '12 at 9:39
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up vote 1 down vote accepted

As Lou mentioned, try mkvmerge like this (from command line):

mkvmerge -o output.webm -w file1.webm + file2.webm

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thanks, this works! –  eug May 22 '12 at 4:39
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Remember that the WebM container format is a strict subset of Matroska, so Matroska tools will work on WebM files. Try mkvmerge (which ships with mkvtoolnix) to re-mux files without re-encoding. The result may or may not be satisfactory for your purposes, but mkvtoolnix is mature and GPLv2-licensed, and well worth a try. There's even a mkvmerge GUI.

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Thanks for this info - I think this is definitely on the right track. I've tried the GUI but just feeding the files to it seems to be creating an output with multiple video tracks, not a single video whose length is the sum of all the inputs.. –  eug May 9 '12 at 6:38
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An mkvmerge GUI operation analogous to Laurent S's CLI solution above is to Add the first file and to Append the subsequent files in the series. Might not be obvious at first. –  Lou Quillio Jul 9 '12 at 19:46
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