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Shit like this is why we need open source printers! (sh.itjust.works)

submitted 2 months ago by einfach_orangensaft@sh.itjust.works to c/science_memes@mander.xyz

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[-] GamingChairModel@lemmy.world 53 points 2 months ago

He's written up his findings in English, for anyone who prefers English over German or text over video.

But basically the JBIG2 image compression algorithm used in those scanners looked for certain repeating patterns, and incorrectly compressed certain portions of the image into "close enough" blocks of pixels. Unfortunately, that meant that scanned number data wasn't guaranteed to be accurate, even when the decoded output clearly looked like a number with no distortion or noise.

It's worth the full read.

[-] kayohtie@pawb.social 21 points 2 months ago

You left out what I feel is the best part: even in the "uncompressed" mode, even when that was disabled, it was still happening sometimes.

[-] GamingChairModel@lemmy.world 12 points 2 months ago

To be precise, the "lossless" compression is still a compression algorithm. They just didn't implement the steps that actually make the compression algorithm lossless.

From the write up:

JBIG2, the image format used in the affected PDFs, usually has lossless and lossy operation modes. Pattern Matching & Substitution„ (PM&S) is one of the standard operation modes for lossy JBIG2, and „Soft Pattern Matching“ (SPM) for lossless JBIG2 (Read here or read the papery by Paul Howard et al.1)). In the JBIG2 standard, the named techniques are called „Symbol Matching“.

PM&S works lossy, SPM lossless. Both operation modes have the basics in common: Images are cut into small segments, which are grouped by similarity. For every group only a representative segment is is saved that gets reused instead of other group members, which may cause character substitution. Different to PM&S, SPM corrects such errors by additionally saving difference images containing the differences of the reused symbols in comparison to the original image. This correction step seems to have been left out by Xerox.