Transfer PGS subtitle format in ffmpeg:
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PGS subtitles are different from the more common SRT subtitle format, where SRT are text, PGS are bitmap (aka images) that are overlayed with the video, that cant be edited with most software.
PGS can still be coped (without any modification) from one video to a converted one in ffmpeg though this method:

ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -hwaccel_device /dev/dri/renderD128 -i "A.mkv" -map 0  -acodec flac -compression_level 8 -ac 2 -c:v hevc_vaapi -c:s copy  "B.mkv"

The -map 0 means try to transfer all streams, with -c:s copy means copy the subtitles (either SRT or PGS) to new video/audio file.
There is a glitch where this could fail to start if you have a thumbnail image with the -map 0 flag, so be careful.

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Sources:
https://superuser.com/questions/1418612/copy-multiple-pgs-subtitle-tracks-with-ffmpeg#1420068
https://stackoverflow.com/questions/62976902/ffmpeg-extract-hdmv-pgs-subtitles-from-mkv-to-srt

Date: July 17th, 2021