Paging @grahamh as this might be in some way related to previous work done on getting unscaled SD video output and the “Use HDMI AVI signalling” option. (But it also might not ).
Here’s a link to a small test video: https://we.tl/t-owZN8i9SeB
This is intended to test deinterlacing, but it’s also a useful way of spotting when there’s some scaling going on when there shouldn’t be. I’m playing it with hardware acceleration turned off, with the scaling set to Nearest Neighbour, and output resolution set to 480p/59.94.
As the video resolution is 720x480 and the output resolution is also set to 720x480, there shouldn’t be any scaling going on. But every few seconds there’s some distortion in the left wedge which strongly suggests there is some horizontal scaling happening - in other words, the output is not a pixel-perfect copy of the video, despite the resolutions being the same.
Setting scaling to Bilinear is more visually pleasing, but I think there’s still some stray scaling happening - the lines get more or less sharp at different moments.
Playing in hardware looks much the same as software + bilinear, but as there’s no way to turn off bilinear scaling in hardware I can’t be sure.
Could well be a Kodi issue, of course.
Link to some debug logs: https://paste.osmc.tv/pijejekese