Example command line (0:0 is the video's stream ID and 0:2 is the sub's stream ID):įfmpeg -i input.mkv -filter_complex 'scale2ref overlay' -map '' -map 0:a -c:v libx264 -c:a copy -y output.mkv In both cases, I had to scale the subtitles to fit the existing dimensions of the video, so both operations simulate not scaling the video at all.and in both cases, scale2ref worked very well. My original file had subs that were already the same dimensions as the video (720p), so I created two files: one upscaled the video to 1080p and one downscaled to 320x'-1' (without touching the sub frame size, so they were still 720p) to see what happened with a larger/smaller native subtitle frame compared to the main video. I just wanted to confirm scale2ref's behaviour with picture-based subtitles, as I already knew that it worked for other inputs like standard images (which are basically the same thing as picture subs anyway). I still have yet to test a bunch of other sub types (and rip some UHD/BRD samples to test with, which is time consuming), so this is just preliminary. I checked the output ts files, they contains the subtitle I want.ĭid a quick preliminary test with an anamorphic DVD (so, VOBSUBs) just to see if scale2ref worked as I figured it would with easy picture-based subs. Rerun the command, and it works, fps is around 37. That first convert the original video and subtitle to 1080P, and overlay them together I tried to learn what does this mean, then figured out that the subtitle steam first converted to a 4K size steam and overlay with the original, finally converted to 1080P,

I met a situation that my server can transcode 4K HEVC video to 1080P smoothly (40~80fps), but cannot when subtitle enabled (15fps)Īfter the investigation, I found the difference of ffmpeg commond is the filters.

Ffmpeg scale