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)
PRISM (V9.09) always letterboxes video
in Prism
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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.