#jbig — Public Fediverse posts

Not actually back yet, just wanted to toot this for fun... ;)

Raw (TGA/Targa format) 1200ppi US Letter scan: 381MiB
PNG version: 27MiB
After contrast enhancement: 2.3MiB
After reducing to 10 colors: 1.2MiB
After re-doing reduction, first converting to grayscale: 1001KiB
Reducing to 4 shades of gray (from original 256-shade image): 727KiB
Reducing to monochrome (no perceptible loss in quality): 556KiB
Conversion to PDF + OCR with aggressive JBIG2 compression: 264KiB

Overall compression factor from original scan: 1480.6:1

Overall compression factor from uncompressed monochrome version of scan: 61.7:1

Overall lossless compression factor (uncompressed monochrome to PNG monochrome): 29.2:1

Lossless compression factor for full-color image: 14:1

Data compression is a heck of a thing. ;)

#DataCompression #Compression #PNG #JBIG #JBIG2

@natevw

To your first point, you're absolutely right. Thresholding yeilds far more than an 8:1 compression because PNG is far more able to crunch bilevel graphics vs. grayscale.

To your second point, you're describing the #JBIG lossy compressor for scanned documents and monochrome images, and yeah, that's super cursed. I'd be surprised if that's what ocrmypdf is doing, but it's possible? ¯\_(ツ)_/¯

@Screwtapello

The fax compression algorithms are very limited, designed for a time where RAM was scarce. They basically compressed a couple rows of pixels at a time, nothing more, AFAIK.

There is #JBIG, which is a LOSSY 1-bit monochrome image compression algorithm. Yes, it is exactly as cursed as that sounds, there have been many cases where numbers and figures were changed by JBIG because a 6 looked like an 8. Horrifying. XD

I'm not totally sure what happened in this example, because I realized after I posted this that the pdfimages utility is converting whatever the PDF stored the images as into PNGs, not just extracting any embedded PNGs it finds.