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M11: say damaged Group 3 with end-of-line codes is stored as the decoder recovers it

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With end-of-line codes, the CCITT decoder picks a bad row up again at
the next code and reports no damage (an invalid code, a row of too many
pixels: probed as a black box), so such an image is stored again as
Group 4 the way it decodes. Without them, damage stops the decoding,
too few rows come out, and the image is left alone. Telling damage
apart would take a Group 3 parser of our own; the optimize README and
the mono-compression parity note say so instead.

From reviews with the pdfcraft-review skill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P6yUtvWGPsJpMLyh6pxSXG
This commit is contained in:
Grzegorz Martens
2026-10-29 20:48:03 +01:00
co-authored by Claude Opus 5.5
parent cb1e573c08
commit 740bb874a4
2 changed files with 7 additions and 4 deletions
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crates/optimize/README.md
+6 -3
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@@ -68,9 +68,12 @@ let report = optimize_with_progress(&mut doc, &settings, &mut |stage| keep_going
smaller, which only a resampled image or one packed from more bits may be: a 1-bit image that
is only stored again becomes Group 4 or stays. Group 4 alone is only decoded to be resampled;
Group 3, or Group 4 inside another filter such as `ASCII85Decode`, is stored again as Group 4
alone when that is smaller. With Retain, a resampled image keeps its kind of compression (CCITT,
or Flate) and nothing else changes; an image of more bits is then left to the grey or colour
settings. Every change is kept only when it is smaller as stored.
alone when that is smaller. Damaged Group 3 with end-of-line codes is stored as the decoder
recovers it, a bad row picked up again at the next code, as it reports no damage there; without
them, damage stops the decoding and the image is left alone. With Retain, a resampled image
keeps its kind of compression (CCITT, or Flate) and nothing else changes; an image of more bits
is then left to the grey or colour settings. Every change is kept only when it is smaller as
stored.
- Apart from the downsampling it is lossless: the samples keep their meaning, so the colour
space, decode array and `/Mask` stay, but for packed RGB, which becomes grey with one decode
pair. Group 4 wins on scans, whose ragged edges defeat Flate; on clean computer-drawn 1-bit