通过 ffmpeg 无损剪切/拼接视频

剪切/拼接视频文件是一种常见需求。在线视频网站现在往往将一个视频文件分割成 n 段，以减少流量消耗。使用 DownloadHelper/DownThemAll 这类工具下载下来的往往就是分割后的文件。能实现剪切/拼接视频文件的工具多种多样，但往往都需要进行视频重编码（transcoding），这就不可避免的带来了视频质量上的损耗，更不用提那长的令人发指的转换时间了…

其实借助 ffmpeg 我们就可以在不进行视频重编码的情况下完成此类任务：

剪切：

ffmpeg -i input.mp4 -ss **START_TIME** -t **STOP_TIME** -acodec copy -vcodec copy output.mp4

其中 START_TIME/STOP_TIME 的格式可以写成两种格式：

1. 以秒为单位计数： 80
2. 时:分:秒： 00:01:20

拼接 :

拼接的情况稍微复杂些，我们需要将需要拼接的视频文件按以下格式保存在一个列表 list.txt 中：

file '/path/to/file1'
file '/path/to/file2'
file '/path/to/file3'

相应的命令为：

ffmpeg -f concat -i **list.txt** -c copy output.mp4

由于不需要重编码，这两条命令几乎是即时完成的。

方便起见，我写了一个脚本来简化操作。放在 github 上，请自取：
https://gist.github.com/imcaspar/8771268

#!/bin/bash
#cut/join videos using ffmpeg without quality loss

if [ "$(uname)" == "Darwin" ]; then
    if ! [ -x "$(command -v brew)" ]; then
       echo 'homebrew is not installed.'
       /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
    fi
    if ! [ -x "$(command -v ffmpeg)" ]; then
       echo 'ffmpeg is not installed.'
       brew install ffmpeg
    fi

elif [ "$(expr substr $(uname -s) 1 5)" == "Linux" ]; then
    if ![ -x "$(command -v ffmpeg)" ]; then
       echo 'ffmpeg is not installed.'
    fi
fi

if [ -z $1 ] || [ -z $2 ]; then
   echo "Usage:$0 c[ut] seconds <File>"
   echo "   eg. $0 c 10 80 example.mp4"
   echo "   eg. $0 c 00:00:10.100 00:01:20.200 example.mp4"
   echo "Usage:$0 j[oin] <FileType>"
   echo "   eg. $0 j avi"
   exit
fi
 
case "$1" in
   c)
      echo "cuttig video..."
      echo $4
      fileName=$(echo $4 | rev | cut -f 2- -d '.' | rev)
      fileType=$(echo $4 | rev | cut -f 1 -d '.' | rev)
      echo $fileName
      echo $fileType
      startTime=$( echo "$2" | tr ':' '_')
      endTime=$( echo "$3" | tr ':' '_' )
      echo $escapedFileName
      ffmpeg -i "$4" -ss $2 -to $3 -acodec copy -vcodec copy "$fileName-$startTime-$endTime.mp4"
      ;;
   j)
      echo "joinning videos..."
      rm temp_list.txt      
      for f in `ls *.$2 | sort -k 1n -t '.'`; do echo "file '$f'" >> temp_list.txt; done
      #printf "file '%s'\n" ./*.$2 > temp_list.txt
      ffmpeg -f concat -i temp_list.txt -c copy output.$2
      rm temp_list.txt
      ;;
   *)
      echo "wrong arguments"
      ;;
esac
exit

以上拼接操作生效的前提是，所有视频文件的格式编码相同，如果需要拼接不同格式的视频文件可以借助以下脚本：

#!/bin/bash

################################################################################
#
# Script name: MultiMedia Concat Script (mmcat)
# Author: burek (burek021@gmail.com)
# License: GNU/GPL, see http://www.gnu.org/copyleft/gpl.html
# Date: 2012-07-14
#
# This script concatenates (joins, merges) several audio/video inputs into one
# final output (just like as if all the inputs were played in a playlist, one
# after another).
#
# All input files must have at least one audio and at least one video stream.
# If not, you can easily add audio silence, using FFmpeg. Just search the
# internet for "ffmpeg add silence".
#
# The script makes use of FFmpeg tool (www.ffmpeg.org) and is free for use under
# the GPL license. The inspiration for this script came from this FAQ item:
# http://ffmpeg.org/faq.html#How-can-I-join-video-files_003f
#
# If you find any bugs, please send me an e-mail so I can fix it.
#
################################################################################
#
# General syntax: mmcat <input1> <input2> <input3> ... <output>
#
# For example: mmcat file1.flv file2.flv output.flv
# would create "output.flv" out of "file1.flv" and "file2.flv".
#
################################################################################

# change this to what you need !!!
EXTRA_OPTIONS='-vcodec libx264 -crf 23 -preset medium -acodec aac -strict experimental -ac 2 -ar 44100 -ab 128k'

################################################################################
#
# NO NEED TO TOUCH ANYTHING AFTER THIS LINE!
#
################################################################################

# the version of the script
VERSION=1.3

# location of temp folder
TMP=/tmp

################################################################################

echo "MultiMedia Concat Script v$VERSION (mmcat) - A script to concatenate multiple multimedia files."
echo "Based on FFmpeg - www.ffmpeg.org"
echo "Don't forget to edit this script and change EXTRA_OPTIONS"
echo ""

################################################################################
# syntax check (has to have at least 3 params: infile1, infile2, outfile
################################################################################
if [ -z $3 ]; then
	echo "Syntax: $0 <input1> <input2> <input3> ... <output>"
	exit 1
fi

################################################################################
# get all the command line parameters, except for the last one, which is output
################################################################################
# $first  - first parameter
# $last   - last parameter (output file)
# $inputs - all the inputs, except the first input, because 1st input is
#           handled separately
################################################################################
first=${@:1:1}
last=${@:$#:1}
len=$(($#-2))
inputs=${@:2:$len}

# remove all previous tmp fifos (if exist)
rm -f $TMP/mcs_*

################################################################################
# decode first input differently, because the video header does not have to be
# kept for each video input, only the header from the first video is needed
################################################################################
mkfifo $TMP/mcs_a1 $TMP/mcs_v1

ffmpeg -y -i $first -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 $TMP/mcs_a1 2>/dev/null </dev/null &
ffmpeg -y -i $first -an -f yuv4mpegpipe -vcodec rawvideo $TMP/mcs_v1 2>/dev/null </dev/null &

# if you need to log the output of decoding processes (usually not necessary)
# then replace the "2>/dev/null" in 2 lines above with your log file names, like this:
#ffmpeg -y -i $first -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 $TMP/mcs_a1 2>$TMP/log.a.1 </dev/null &
#ffmpeg -y -i $first -an -f yuv4mpegpipe -vcodec rawvideo $TMP/mcs_v1 2>$TMP/log.v.1 </dev/null &

################################################################################
# decode all the other inputs, remove first line of video (header) with tail
# $all_a and $all_v are lists of all a/v fifos, to be used by "cat" later on
################################################################################
all_a=$TMP/mcs_a1
all_v=$TMP/mcs_v1
i=2
for f in $inputs
do
	mkfifo $TMP/mcs_a$i $TMP/mcs_v$i

	ffmpeg -y -i $f -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 $TMP/mcs_a$i 2>/dev/null </dev/null &
	{ ffmpeg -y -i $f -an -f yuv4mpegpipe -vcodec rawvideo - 2>/dev/null </dev/null | tail -n +2 > $TMP/mcs_v$i ; } &

	# if you need to log the output of decoding processes (usually not necessary)
	# then replace the "2>/dev/null" in 2 lines above with your log file names, like this:
	#ffmpeg -y -i $f -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 $TMP/mcs_a$i 2>$TMP/log.a.$i </dev/null &
	#{ ffmpeg -y -i $f -an -f yuv4mpegpipe -vcodec rawvideo - 2>$TMP/log.v.$i </dev/null | tail -n +2 > $TMP/mcs_v$i ; } &

	all_a="$all_a $TMP/mcs_a$i"
	all_v="$all_v $TMP/mcs_v$i"
	let i++
done

################################################################################
# concatenate all raw audio/video inputs into one audio/video
################################################################################
mkfifo $TMP/mcs_a_all
mkfifo $TMP/mcs_v_all
cat $all_a > $TMP/mcs_a_all &
cat $all_v > $TMP/mcs_v_all &

################################################################################
# finally, encode the raw concatenated audio/video into something useful
################################################################################
ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i $TMP/mcs_a_all \
       -f yuv4mpegpipe -vcodec rawvideo -i $TMP/mcs_v_all \
	$EXTRA_OPTIONS \
	$last

################################################################################
# remove all fifos
################################################################################
rm -f $TMP/mcs_*