If you're still there
I formatted my post incorrectly, this should work try it:
ffmpeg -i path/to/video.mkv -vf scale=1280:720 -c:a copy path/to/output.mkv
i didn't put the equals sign last time
'-c:a copy' copies the audio stream to the new file
Thanks. I finally used this
ffmpeg -i MyMovie.mkv -vf scale=-1:720 -c:v libx264 -crf 18 -preset veryslow -c:a copy MyMovie_720p.mkv
The video and audio remains excelent and now it plays perfectly, but the file size has like 100mbs extra.
Can someone explain why?
must be a higher video bitrate, because the audio track is literally the same with your command. many good encoders have really specific settings. libx264 comes with default profiles, so the default profile for 720p may already have a higher bitrate than a professionally encoded 720p video might have. then you just about doubled the default bitrate with the manual -crf setting. this constant rate factor tries to enforce a certail level of quality (which adjusts the bitrate).
-crf 23 is the default value. roughly every six steps lower doubles the video bitrate.
bitrate and file size are directly linked (bitrate x length = file size).
>Can someone explain why?
you asked ffmpeg to enforce a pretty high video quality and it did. that's the sucky (or awesome, depending on how you look at it) part with FFmpeg - it does exactly what you tell it to do, even if that's not very smart.