> Yes, I know, and for monochrome images it compresses much better. But
> I'm wondering about the scrambled content and the huge amout of PNG
> files in the ripped data, so it seems to me, that each subtitle pic
> becomes somehow decrypted from many PNG fragments which unfortunately
> fill 10 % of the entire stream.

JPEG isn’t a bitmap format. And those png’s aren’t encrypted, they are regular 
(more or less) black and white bitmaps. On a commercial pressed dvd it would 
have been kind a wireframe that the subtitles are built on using color info 
from the program it’s in. So ~10% isn’t too bad when it’s pulled out as 
something independently decodable. You can ocr the text, but if you are wanting 
to archive the original bitmap based subs, not much that can be done. There’s 
no easy way to weave it back in as subpicture either, not a reversible process.
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