Reencoding to save ~15% file size seems pretty pointless to begin with. Better to just leave the video bitstream alone if possible.
Back when Doom9 was strapping and new, there were utilities that would reduce MPEG-2 encodes by 1-2 QP while preserving motion vectors, which was a pretty simple and very fast way to trim off up to maybe 20% in file size. Has anyone ever tried to do something similar for H.264?
For Blu-ray, there are a lot of restrictions on the bitstream. So if one didn't need to preserve BD player compatibility, I can see a system that would just reencode B and b frames to save bits, passing through IDR and P frames, taking advantage of non-hierarchical B-frames to save some bits.
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Ben Waggoner
Principal Video Specialist, Amazon Prime Video
My Compression Book
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