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How can I use ffmpeg to reduce the size of a video by lowering the quality (as minimally as possible, naturally, but I need it to run on a mobile device that doesn't have much available space)?

I forgot to write one thing yet. When the video can use subtitles (*.srt or *.sub) I'd like to convert them too to fit the parameters of converted video file.

See this answer. Quoted below for convenience:

Calculate the bitrate you need by dividing 1 GB by the video length in seconds. So, for a video of length 16:40 (1000 seconds), use a bitrate of 1000000 bytes/sec:

ffmpeg -i input.mp4 -b 1000000 output.mp4

Additional options that might be worth considering is setting the Constant Rate Factor, which lowers the average bit rate, but retains better quality. Vary the CRF between around 18 and 24 — the lower, the higher the bitrate.

ffmpeg -i input.mp4 -vcodec libx264 -crf 20 output.mp4

Unless you're looking for a specific bitrate, I'd recommend the -crf option. This is the most commonly used for x264 encoding: http://slhck.info/articles/crf

In short: a CRF of 23 would make "DVD" quality movie (~700MB-1GB) and lower CRF values would be higher quality (larger files).

You mentioned wanting to reduce filesize to fit more videos on a mobile device, which is my usecase as well. All the answers here are for reducing the compression quality but nobody has mentioned reducing video frame size. It's a lot quicker, from about 3 to 5 times quicker than recompressing in my experience. See the ffmpeg docs on scaling for more info.

ffmpeg -i input.mkv -vf "scale=iw/2:ih/2" half_the_frame_size.mkv
ffmpeg -i input.mkv -vf "scale=iw/3:ih/3" a_third_the_frame_size.mkv
ffmpeg -i input.mkv -vf "scale=iw/4:ih/4" a_fourth_the_frame_size.mkv

You'll need to use 2-pass encoding to "fit" a video within a designated file size (bitrate), without reducing the quality too drastically. This is quite a detailed topic: http://www.mpabo.com/2014/12/14/ffmpeg-and-x264-encoding-guide/

I have a recipe I originally forged for myself in order to convert the Motion JPEG videos my old camera generates (they are very large videos, since each frame is an entire JPEG image) to h264. Here's an adaptation for other kinds of videos (courses, etc).

I'm not using ffmpeg, but mplayer and mencoder. First, We have to demux the audio with mplayer:

mplayer -vo null -ao pcm:fast:file=<audio_pcm.wav> <video>

• The -vo null and -ao null parameters tells mplayer to not extract video.

In the next steps, we'll do a 3-pass compression with mencoder. At the first pass we'll choose a suitable Constant Quality Mode compression (crf parameter) as a start point:

mencoder <video> -ovc x264 
 -x264encopts ratetol=100:preset=veryslow:crf=<value>:pass=1 
 -nosound -o video1.h264

• You can add slow_firstpass parameter to the -x264encopts if you are paranoid with the final quality of the video. Mencoder manual says that this option disable some parameters that “significantly improve encoding speed while having little or no impact on the quality of the final pass”. So, use it only at the last step.




• You should try several values for crf —
  try starting from 25 and goes on increasing it until you note artifacts at the resulting video (higher values compresses more). Remember subsequent encoding passes will improve the quality you have choosed for crf.




• Alternatives for the veryslow preset are slower, slow, medium etc. See mencoder manual for the complete list.




• ratetol controls the bitrate variation — I'm not sure if I'm doing the right thing here, but I set it to the maximum value in order to let total freedom to mencoder to choose the right bitrate for each scene.

After the first pass, you'll note that the last line gives you the average bitrate you will use at the next steps:

(...)
x264 [info]: kb/s:526.43

Change the crf parameter, recommended at the first pass, to bitrate, required at the subsequent passes:

mencoder <video> -ovc x264 
 -x264encopts slow_firstpass:ratetol=100:preset=veryslow:bitrate=526:pass=3 
 -nosound -o video2.h264

This second pass encoding will read the statistics generated at the first pass (divx2pass.log and divx2pass.log.mbtree) in order to optimize the compression.

• Note you'll use the same video input, not the generated by the first pass — first pass' output video is only useful to check the initial quality.




• Note also that the pass=3 (not pass=2) will generate a new statistics file, so you can repeat the last step as many times you want. I usually do pass=3 twice, always paying attention to the result bitrate.

Meanwhile, you can compress the audio too, using lame or oggenc:

oggenc -q<n> <audio_pcm.wav>

Finally, we'll remux audio and video

mencoder -audiofile <audio>.ogg video2.h264 -oac copy -ovc copy 
 -of lavf -lavfopts format=mp4 -o <video>.mp4

• The -of lavf -lavfopts format=mp4 generates mp4 file format using the lavopts muxers.

Note that it seems that ffmpeg already performs some optimization when ran without options, so before trying to use settings you don't understand or deciding to explicitly lose information, give a try to a default conversion :

ffmpeg -i input.mp4 output.mp4

In my case it reduced the bitrate of both the video and audio (you can check and compare the input and output file by running ffprobe on them), transforming a 700 Mb video into a 60 Mb one of seemingly similar quality.