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M8: a cleared black-and-white scan is stored as Group 4, or JBIG2 again, when smaller

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Redaction wrote every cleared copy as Flate, so a redacted Group 4 or JBIG2 scan
page grew several times over until the optimizer ran. A cleared one-bit image of one
component is now stored as the smallest of Flate, CCITT Group 4 and, when it was
JBIG2, JBIG2 (PDF 1.4, which such a file already is), as stored, dictionary included:
a tiny image can lose by Group 4's parameters. Both are coded from the
cleared samples, so nothing of the original's code is carried over; rows Group 4 or
JBIG2 can't code (too wide) leave Flate. Images of more bits stay Flate.

Tests: a cleared one-bit Flate scan of ragged word blocks (where Group 4 is smaller
than Flate) comes out Group 4 and an eight-bit image Flate, each decoding to the
cleared samples (the whole buffer); a tiny image whose Group 4 code is shorter but whose
stream isn't, and rows wider than Group 4 codes, stay Flate and clear; the redacted
JBIG2 scan of #5461's test comes out JBIG2, smaller than
Flate, with no globals and no stream holding the original code. The tests' image
helpers decode through Document::decoded_image, as image codecs need.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Grzegorz Martens
2026-10-30 19:08:32 +01:00
co-authored by Claude Opus 5.5
parent 78fc1e5568
commit 7f9554f29b
5 changed files with 157 additions and 14 deletions
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parity/acrobat-features.toml
+2 -2
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@@ -5804,8 +5804,8 @@ milestone = "M8"
status = "shipped"
commands = ["redact.apply"]
tools = ["redact_apply"]
evidence = ["crates/redact/src/tests.rs::images_are_removed_or_have_their_pixels_cleared", "crates/redact/src/tests.rs::redaction_clears_pixels_intersected_without_covering_their_centres", "crates/redact/src/tests.rs::a_fax_image_has_its_covered_pixels_cleared_instead_of_being_removed", "crates/redact/src/tests.rs::group_3_on_bytes_is_removed_whole_and_other_fax_images_cleared", "crates/redact/src/tests.rs::an_image_whose_predictor_is_given_by_reference_has_the_covered_pixel_cleared", "crates/redact/src/tests.rs::a_jbig2_image_has_its_covered_pixels_cleared_instead_of_being_removed", "crates/redact/src/tests.rs::a_jbig2_image_that_cannot_be_cleared_for_certain_is_removed_whole", "crates/redact/src/tests.rs::a_redacted_jbig2_scan_keeps_neither_its_code_nor_its_globals"]
notes = "intersected pixel footprints are cleared conservatively in a copy, CCITT fax and JBIG2 images (scans stored as Group 4 or JBIG2, with their /JBIG2Globals) included; DCT/JPX images, Group 3 on bytes (which the bootstrap renderer may draw as other rows than the decoder reads), and a JBIG2 image whose page isn't its size, removed whole. The copy is Flate (larger than a Group 4 or JBIG2 scan) and refers to no globals; globals another image still uses stay, and may hold the shapes of marks under the area (not where they were)"
evidence = ["crates/redact/src/tests.rs::images_are_removed_or_have_their_pixels_cleared", "crates/redact/src/tests.rs::redaction_clears_pixels_intersected_without_covering_their_centres", "crates/redact/src/tests.rs::a_fax_image_has_its_covered_pixels_cleared_instead_of_being_removed", "crates/redact/src/tests.rs::group_3_on_bytes_is_removed_whole_and_other_fax_images_cleared", "crates/redact/src/tests.rs::an_image_whose_predictor_is_given_by_reference_has_the_covered_pixel_cleared", "crates/redact/src/tests.rs::a_jbig2_image_has_its_covered_pixels_cleared_instead_of_being_removed", "crates/redact/src/tests.rs::a_jbig2_image_that_cannot_be_cleared_for_certain_is_removed_whole", "crates/redact/src/tests.rs::a_redacted_jbig2_scan_keeps_neither_its_code_nor_its_globals", "crates/redact/src/tests.rs::a_cleared_black_and_white_scan_is_stored_as_group_4"]
notes = "intersected pixel footprints are cleared conservatively in a copy, CCITT fax and JBIG2 images (scans stored as Group 4 or JBIG2, with their /JBIG2Globals) included; DCT/JPX images, Group 3 on bytes (which the bootstrap renderer may draw as other rows than the decoder reads), and a JBIG2 image whose page isn't its size, removed whole. The copy is Flate, or for a one-bit image of one component Group 4, or JBIG2 when it was JBIG2, whichever is smallest as stored, and refers to no globals; globals another image still uses stay, and may hold the shapes of marks under the area (not where they were)"
[[feature]]
id = "protect.redact-vectors"