ffmpeg 使用心得
為了要將影片轉成 ipod touch 3代，可用的格式，特別學習 ffmpeg，試了好幾天，終於找出來滿意的方法，特此記錄下來，免得忘了。
轉檔時的指令
ffmpeg -i $1 -s 720x406 -vpre ipod -ac 2 -sn -y -vf subtitles=$1.srt $1.m4v
其中
-i $1 是要轉檔的檔名
-s 720x406 是轉檔後的解析度，406 = 720 * 720 / 1280，（原始檔的解析度為 1280x720）。這個一定要去計算，不然，在電視上看，畫面比例會不對，字幕可能出不來。
-vpre ipod 是為了 ipod 特別找出來的 preset，設定如後。
--ac 2 是轉成立體聲，不然 ipod touch 沒辦法播放。
--sn 不顯示原始檔中的字幕。因為，我要用另外的字幕檔。
--vf subtitles 為字幕檔。
preset設定
preset 可以放在 ~/.ffmpeg，檔名為 arg.ffpreset，例如，我設定了一個專門轉到 ipod touch 的 preset 檔，命名為 ipod.ffpreset。
strict=-2
vcodec=libx264
vprofile=baseline
level=30
maxrate=10000000
bufsize=10000000
#preset=fast
preset=slower
crf=19
bf=0
refs=1
ab=192k
調整字幕時間
這個很少用到，在 linux 中，就找到 aegisub 是可用的，那就拿來用了，沒有什麼特別的考慮。
提取 mkv 內嵌字幕
為了要把 mkv 中的字幕，轉成繁體。使用以下三道指令即可：
ffmpeg -i $1.mkv -vn -an -scodec copy $1.ssa
opencc -i $1.ssa -o o.ssa
ffmpeg -i $1 -s 720x406 -vpre ipod -ac 2 -sn -y -vf ass=$1.ssa $1.m4v
參考資料
ffmpeg help
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...
Getting help:
-h -- print basic options
-h long -- print more options
-h full -- print all options (including all format and codec specific options, very long)
See man ffmpeg for detailed description of the options.
Print help / information / capabilities:
-L show license
-h topic show help
-? topic show help
-help topic show help
--help topic show help
-version show version
-formats show available formats
-codecs show available codecs
-decoders show available decoders
-encoders show available encoders
-bsfs show available bit stream filters
-protocols show available protocols
-filters show available filters
-pix_fmts show available pixel formats
-layouts show standard channel layouts
-sample_fmts show available audio sample formats
Global options (affect whole program instead of just one file:
-loglevel loglevel set libav* logging level
-v loglevel set libav* logging level
-report generate a report
-max_alloc bytes set maximum size of a single allocated block
-y overwrite output files
-n do not overwrite output files
-stats print progress report during encoding
-bits_per_raw_sample number set the number of bits per raw sample
-croptop size Removed, use the crop filter instead
-cropbottom size Removed, use the crop filter instead
-cropleft size Removed, use the crop filter instead
-cropright size Removed, use the crop filter instead
-padtop size Removed, use the pad filter instead
-padbottom size Removed, use the pad filter instead
-padleft size Removed, use the pad filter instead
-padright size Removed, use the pad filter instead
-padcolor color Removed, use the pad filter instead
-vol volume change audio volume (256=normal)
Per-file main options:
-f fmt force format
-c codec codec name
-codec codec codec name
-pre preset preset name
-map_metadata outfile[,metadata]:infile[,metadata] set metadata information of outfile from infile
-t duration record or transcode "duration" seconds of audio/video
-fs limit_size set the limit file size in bytes
-ss time_off set the start time offset
-timestamp time set the recording timestamp ('now' to set the current time)
-metadata string=string add metadata
-target type specify target file type ("vcd", "svcd", "dvd", "dv", "dv50", "pal-vcd", "ntsc-svcd", ...)
-frames number set the number of frames to record
-filter filter_list set stream filterchain
-reinit_filter reinit filtergraph on input parameter changes
Video options:
-vframes number set the number of video frames to record
-r rate set frame rate (Hz value, fraction or abbreviation)
-s size set frame size (WxH or abbreviation)
-aspect aspect set aspect ratio (4:3, 16:9 or 1.3333, 1.7777)
-bits_per_raw_sample number set the number of bits per raw sample
-croptop size Removed, use the crop filter instead
-cropbottom size Removed, use the crop filter instead
-cropleft size Removed, use the crop filter instead
-cropright size Removed, use the crop filter instead
-padtop size Removed, use the pad filter instead
-padbottom size Removed, use the pad filter instead
-padleft size Removed, use the pad filter instead
-padright size Removed, use the pad filter instead
-padcolor color Removed, use the pad filter instead
-vn disable video
-vcodec codec force video codec ('copy' to copy stream)
-timecode hh:mm:ss[:;.]ff set initial TimeCode value.
-pass n select the pass number (1 to 3)
-vf filter list video filters
-b bitrate video bitrate (please use -b:v)
-dn disable data
Audio options:
-aframes number set the number of audio frames to record
-aq quality set audio quality (codec-specific)
-ar rate set audio sampling rate (in Hz)
-ac channels set number of audio channels
-an disable audio
-acodec codec force audio codec ('copy' to copy stream)
-vol volume change audio volume (256=normal)
-af filter list audio filters
Subtitle options:
-s size set frame size (WxH or abbreviation)
-sn disable subtitle
-scodec codec force subtitle codec ('copy' to copy stream)
-stag fourcc/tag force subtitle tag/fourcc
-fix_sub_duration fix subtitles duration
-spre preset set the subtitle options to the indicated preset
參考指令
假設video.avi有一個視頻軌、兩個音頻軌，取其中的視頻軌和第二個音頻軌加sub.ass做成video.mkv而不重新編碼：
ffmpeg -i video.avi -i sub.ass -map 0:0 -map 0:2 -map 1 -c:a copy -c:v copy -c:s copy video.mkv
如果要重新編碼（字幕合成到視頻圖像）：
ffmpeg -i video.avi -map 0:0 -map 0:2 -c:a copy -c:v <視頻編碼庫，比如libx264> -vf ass=sub.ass video.mkv
