I'm starting with source video as .ts (MPEG2, 720x576x25) - off-air recording from UK television (as a test source).
If I write to MP4 format (any of MP4, H264 or H265 compression) in any resolution or aspect ratio, the picture is always letterboxed within a frame of the chosen resolution. If I write to MKV format, the full picture area is used during 5-second preview but the full video is letterboxed.
(all of the following videos were played with VLC which has scaled its stills to 1024x576 to give square pixels) https://i.postimg.cc/1XVx6gzq/vlcsnap-2022-03-05-00h18m08s396.png is a still from the source video (720x576). https://i.postimg.cc/SR9S41C3/vlcsnap-2022-03-05-00h17m19s319.png is a still from the converted video (720x576: I saved it to MP4/H264)
Notice how the output is squeezed vertically so it has large black borders top and bottom.
Someone asked this question a while ago but never received an answer about the letterboxing.
