The render oracle put four JBIG2 fixtures at the top of its disagreement list against Poppler, and
the obvious reading was that hayro-jbig2, which carries local patches, had a bug. Investigating it
found the reverse.
The 96 `bitmap-*.pdf` corpus files are a conformance suite: one small drawing encoded through every
corner of JBIG2 -- each generic-region template, custom AT pixels, TPGDON and TPGRON, refinement,
halftone with and without skip, symbol dictionaries with Huffman tables, MMR, the composition
operators, random access, unknown-height segments. Those features change how the bytes are
compressed, not what the picture is, so a correct decoder renders all 96 identically. That is a
real test with no reference image and no external oracle: the suite checks itself.
We render 1 distinct image across all 96. Poppler renders 8: it draws a blank page for the seven
halftone-skip and symbol fixtures, and its output changes when TPGRON is switched on, which a
correct decoder's cannot. So roughly a fifth of Poppler's JBIG2 output is wrong, the top of the
oracle's worst list is its bug rather than ours, and Poppler is not a usable oracle for JBIG2.
Watched failing by admitting one unrelated file to the set: the test fails and names it as the odd
one out rather than merely reporting a count.
The expectation holds of the pinned corpus. Whoever re-pins it to a newer pdf.js should re-check
this rather than assume it still holds, which the test's own documentation says.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
