User:Mjb/FFmpeg

Extract a portion of a video

Let's say you just want to take the portion from 37:07.5 to 41:30.

 ffmpeg -i inputfile -vcodec copy -acodec copy -ss 37:07.5 -to 41:30 outputfile

Rotate 180 degrees

When you hold a camera phone the wrong way, it will just put a 180° rotation flag in the metadata, which not all players will support. To rotate the actual video, chain the hflip and vflip filters:

ffmpeg -i inputfile -vf "vflip,hflip" outputfile

The rotation flag will not be changed when you do this, so you can set it afterward (assumes video is stream # 0):

ffmpeg -i inputfile -c copy -metadata:s:v:0 rotate=0 outputfile

Or you can do both at the same time (untested):

ffmpeg -i inputfile -vf "vflip,hflip" -metadata:s:v:0 rotate=0 outputfile

Here's a more robust example (worked for me):

 ffmpeg -i input.mp4 -metadata:s:v rotate="0" -vf "hflip,vflip" -c:v libx264 -acodec copy output.mp4

The c:v libx264 means to output H.264 video, which is what the input will be if it is from an iPhone. For more H.264 ffmpeg tips, see https://trac.ffmpeg.org/wiki/Encode/H.264

References:

• https://stackoverflow.com/questions/3937387/rotating-videos-with-ffmpeg
• https://stackoverflow.com/questions/15335073/can-i-set-rotation-field-for-a-video-stream-with-ffmpeg
• http://thornelabs.net/2013/06/04/correct-smartphone-video-orientation-and-how-to-rotate-ios-and-android-videos-with-ffmpeg.html

Transcode to a specific frame size & bitrate

My iPhone records video at 1920x1080. The audio doesn't take up much space at all, but the video is H.264 at about 17 Mbps, so it uses up 125 MB per minute. Here is a way to get it down to a more manageable size and more portable container format, along with the 180° rotation ("vflip,hflip") mentioned above:

ffmpeg.exe -i inputfile.mov -acodec copy -b:v 2000k -vf "vflip,hflip,scale=1024:-1" outputfile.mkv

This makes it be about 15 MB/minute: 2 Mbps, 1024 width, -1 means whatever height will preserve the aspect ratio.

Reference:

• https://trac.ffmpeg.org/wiki/FilteringGuide#Scaling

Replace audio in a DVD file

Music videos in MPEG-2 format (.vob or .mpg files) sometimes come with bad source audio and I will want to replace the audio with a good copy of my own. Of course, I have to pay close attention and make sure that the audio in the video is the same; videos often use custom edits or they overdub other sounds on top. Assuming I have suitable audio in a lossless format like FLAC, here's what I do to replace it:

First, extract the source audio to a WAV file:

• ffmpeg -i input.vob output.wav

Note: if the original audio was lossy, the resulting WAV will probably be bigger because it includes encoder delay & padding, possibly also decoder delay (i.e. a bunch of silence at the beginning, and a little bit at the end). It's best if you can figure out how much there is and trim it. However, I don't know a good way to do that!

Next, use a wave editor (I use Audition) to create a new WAV that is perfectly time-aligned with the old. There are different ways of doing this. Here's one way:

• Convert the replacement to the desired output sample rate.
• Pick a non-silent spot at the beginning and end of the original file to be the anchor points. You are looking for spots that you can find in both the old and new files. Set a marker at each spot. (In the original, markers at samples 28936 and 8591175; in the replacement, at samples 4568 and 8560470).
• How many samples are in between the markers? (8591175-28936=8562239 and 8560470-4568=8555902) Your goal is to change the replacement to match the original.
• What's the original:replacement ratio? (8562239/8555902=1.0007406583198358279466034089685)
• Do a pitch shift on the replacement with a target duration of that number multiplied by the current duration, in samples. (8659287*1.0007406583198358279466034089685=8665700)
• Check how many samples are in between the markers. (8566809-4571=8562238) It should be really close to the original now. If not, figure out what you did wrong and try again.
• Pad or trim silence from the beginning so that the first marker is at the same location as the first marker in the original. (28936-4571=24365 padding to add). If the beginning is offset or not silent, apply a fade-in beforehand.
• Fade and/or pad the end, so that the total duration is the same as the original.

Now you need to mux them together. What is supposed to work is this:

• ffmpeg -i input.vob -i new.wav -map 0:1 -map 1:0 -vcodec copy -acodec copy out.vob

-map 0:1 means use stream #1 from file #0 (input.vob), and -map 1:0 means use stream #0 from file #1 (new.wav).

Unfortunately, FFmpeg currently doesn't like to mux PCM audio into a VOB or MPEG-2 container without giving all kinds of packet too large / buffer underflow errors. Supposedly this was fixed, but it's not working for me, so...

The solution is to use MPEG Video Wizard DVD. In that app: Drag the video into the timeline's video bar, right-click on it and choose to mute its audio (otherwise it will mix them together). Drag the audio into the timeline's music bar. Click on Export (looks like a videotape) in the main toolbar, and make sure it's going to do a stream copy.

A more advanced example:

• downloaded clip from YouTube as .mp4 (AVC video + AAC audio)
• demuxed and converted AAC to WAV:
  • ffmpeg -i input.mp4 output.wav
• noted that output.wav had 18345984 samples (6:56.009)
• sized replacement audio to 18340836 samples (6:55.892) (a rough guess as to ideal size)
• used fhgaacenc via foobar2000 to encode replacement audio as .m4a
• muxed original video with replacement audio, bitrate ~2 Mbps, saturation 1.7x:
  • ffmpeg -i input.mp4 -i new_audio.m4a -map 0:0 -map 1:0 -vcodec libx264 -acodec copy -b:v 2000000 -vf "hue=s=1.7" out.mp4

Resulting video seems synced with its audio, but just to see how bad my guess was:

• demuxed and converted AAC to WAV:
  • ffmpeg -i out.mp4 out.wav
• noted that out.wav had 18342912 samples (6:55.939) ... 2076 samples more than I input, but 3072 less than needed!

Oh well.