Black bars from noisy sources are also a lot worse from both a compression efficiency and quality perspective. Even a little bit of noise in there can require a lot of bits to avoid blocking on LCD displays. Cropping them out thus saves some noise reduction tweaking and processing time as well.
Getting letterboxing to be mod16 is really important for quality with MPEG-2, since it doesn't have any good way of dealing with a really sharp edge mid-block.
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Ben Waggoner
Principal Video Specialist, Amazon Prime Video
My Compression Book
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