You are looking at multiple hours (>2) for a 2-hour movie from my experience. One way to reduce this is to spin up a DigitalOcean droplet (a $80 per month droplet) that'll give you a lot of processing power, finish your encoding in 2-3 hours,and shut it down. It'll save you a lot of headache and time.
Not sure I answered your question, but, hope that helped!
In principle, would it be possible to only re-encode the macroblocks affected by the subtitles, leaving the rest of the frame bit-for-bit identical?
It's been a long time since i read a video encoding specification, so i don't know if things still work this way.
Can possibly work, but depending on movie content might not save all that much encoding.
Much faster to encode. Can be toggled. But players need to support it.
https://trac.ffmpeg.org/wiki/Hardware/QuickSync
Even the most low-end processors like the Celeron, if they have embeded GPUs, have this encoding acceleration built in and it makes a huge difference versus using the general purpose CPU portion. The generation of CPU will determine the encoders/decoders available.
I learned about this from the below post, and have been using the functionality where ever I can as it applies many of the popular transcoding sofwares out there, for example handbrake also.
https://forums.serverbuilds.net/t/guide-hardware-transcoding...
You're going to have to reencode it, which is a lossy and CPU-intensive process (unless you manage to get hardware encoding working with your video card, which is possible but very fiddly IME).
> Is it a super cpu intensive process that takes a long time for let's say a typical 2 hour movie?
There are a lot of compute/filesize tradeoffs that you can make. If you don't mind a file that's 2-3x larger than your original then you can do it fairly quickly (say 1/3 of realtime). If you don't mind an effectively uncompressed video (so tens or hundreds of gigabytes) then you can do it as fast as your disks will write. If you want something similar to the original without sacrificing too much quality then it'll probably be slower than realtime.
It can do 300fps+ (for simply transcoding. Burning in subtitles probably will slow it down a little ibt) already on my very dated GPU, likely would be even faster on newer ones.
Edit: Obviously it's more complicated than this. But I think this is one of the reasons behind the need for things like Netflix's VMAF
In any case, they massively lag behind software encoding.
But I guess you can call it "firmware" or "software" dependant too, because they don't really use/support the same version of NVENC to begin with.
And I doubt the result is significantly different, if any, for software encoders like x264 - of course assuming you use the exactly same version and parameter.
The above line will hardcode the subtitles onto the video and stop after five minutes. This should give you an idea of how slow re-encoding is going to be on your machine, and what the quality of the result is going to look like.
What you ideally want is that your rendered video has approximately the same bitrate as the original, and the same visual quality. You can play with the -crf parameter to get the quality the same, and the -preset parameter to get the bitrate the same.
For reference, when I'm rendering 720p video I'm getting about 4x speed on my 10th gen Core i5. 1080p should take about twice the time.
I've played around a bit with using the gpu-accelerated encoders in ffmpeg, but I could never get the same quality or bitrate as the cpu encoder, it seems to me that they're tuned for quickly turning raw capture into something that can realistically be streamed and handled.
See eg: https://mutsinzi.com/add-srt-subtitles-to-quicktime/
TL;DR:
ffmpeg -i yourVideo.mp4 -i yourSubtitle.srt -c:v copy -c:a copy -c:s mov_text -metadata:s:s:0 language=eng yourOutputVideo.mp4
I'm uncertain if there are some metadata you could toggle to hint at default subtitle language and/or default to subtitles on (or off). I generally use vlc, so this isn't much of a problem for me.
To make the first subtitle track default:
-disposition:s:0 default
-disposition:s:0 forced
-disposition:s:1
At my job we have an API that lets you do this to "burn in" subtitles onto a video.
A demo for this: https://transloadit.com/demos/video-encoding/add-subtitle-to...
See https://support.plex.tv/articles/200250377-transcoding-media...
What's also impressive is it can stream your media at a lower quality, say 4k to 720p to your phone, also in near real time, on a very old laptop that I repurposed into a NAS.