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*letterboxing when input aspect ratio is less (there is a proportionnality to calculate) than the output aspect ratio, because we lost height's pixels for nothing...
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The pixels in the blanking area don't have to be blank, it's just that the aspect ratios you were targetting for the 720x[480/576] image were slightly wrong.
Just think of the 720x576 box as having an aspect ratio of either ~16.4:9 or ~4.1r:3 rather than 16:9 or 4:3. The result would be the same as if you'd applied your original script, then scaled down to 704 width (or 702 for PAL) and added borders back to 720. The image's height would be the same.
Of course this all assumes that the video was scaled down correctly in the first place, which may not be the case...
David