[jpegqs]

Great description, but there is a typo in the second row:

    00000000 abcdefgh ijklmnop qrstuvwx
    -------- -------- -------- --------
    00abcdef 00ijklgh 00qrmnop 00stuvwx
    0000ijkl 0000qr00 0000gh00 00mnop00 <-- missing "qr"
    000000qr 00000000 00000000 00gh0000

[jpegqs]

Still can't improve this stage of encoding, it requires a lot of operations. Maybe there is something trivial, but I don't see it.