I was given a H.264 1920 by 1080 video. File is 315 MB..i just need to add subtitles to it. So, as i'm not to lose so much quality in the process I wonder.
I drop the clip in a project automatic set resolution to 1920-1080..seems it can only be render as ProRes...so if I export this it creates a ProRes video of 6 GB..i could use compressor and make it H.264 again...and that's fine.
But..its this back and forth necessary? will i lose to much generation quality with this steps?
my mind this that it the original is 315MB, i could just export the same format once and keep it simple..but FCPX seems to want me to use Prores to edit the clip...
You get a file with a certain resolution in H.264. You don't know if it has a bitrate of 10 Mbps or 15 Mbps (you could check it, if you know the length and you have the size of the file, you know) or 20Mbps or 2, or etc. ...
If you export H.264 out of FCPX it will also have a certain bitrate. Depending on that bitrate, which you can make a custom setting for in Compressor, you would also be able to make the file the same size (more or less) as the on you got.
But to answer your other question:
You will modify the file anyhow (adding subtitles, doing an effect, etc.) , so FCPX will need to render and re-compress anyhow. It uses Prores as a RENDER codec.
Because H.264 is a LONG GOP codec, it will need re-compression even if you just cut it up in different pieces.
That's one of the reasons working with H.264 is not a great idea instead of working with Prores.
But for small pieces, I can imagine that it's handy.
I would suppose you best export to Prores. That will happen the fastest. And then afterwards recompress to H.264. Prores can take much more generations of compression, and the engine in FCPX compresses from H.264 source to Prores pretty quickly. If you export as H.264 directly, it could maybe take a lot longer then going from H.264 and re-exporting as H.264. That's the nature of the codec...