[guillehr2]

What this changes

Part of #1201: the "Optimization has a more serious problem" item (DCT /ColorTransform lost by Optimize). The /Default* colour spaces and the renderer's own CMYK/YCCK gap in #1201 are separate and not touched here.

Optimize read JPEG samples with the image crate's decoder. That decoder converts three components from YCbCr to RGB unless the JPEG itself says otherwise, and the rewritten image then lost its /DecodeParms. So an RGB JPEG stored untransformed (/ColorTransform 0, no Adobe marker) was converted anyway, and came out in other colours whenever Optimize touched it. That covers recompression, resampling, and merging page pictures printed as strips. In the test fixture the colours moved by 75 levels on average.

ISO 32000-2 §7.4.8 says an Adobe APP14 marker's transform flag decides, overriding /ColorTransform. Without a marker, /ColorTransform decides, and its default is 1 (YCbCr) for three components. pdfcraft-optimize (images.rs) now follows that:

• Reading. pixels finds the transform (ycc_encoded): it reads the APP14 flag from the JPEG's segments before the first scan, and /ColorTransform from /DecodeParms. It then decodes with zune-jpeg directly, the decoder image and the renderer already use (already in the lock; pdfcraft-create depends on it the same way). YCbCr samples are converted to RGB, and untransformed samples are taken as stored.
• Writing. Nothing changes. The new image is a plain YCbCr JPEG (no Adobe marker) or Flate, which is exactly what a missing /DecodeParms means, so dropping it stays correct.
• Left as it is when readers can disagree:
  • the marker and /ColorTransform contradict each other. The standard lets the marker win, but PdfCraft's renderer lets /ColorTransform 0 win over a YCbCr marker;
  • either holds a value other than 0 or 1;
  • /DecodeParms isn't paired with the filter as §7.4 pairs them (a dictionary for a filter name, a one-element array for [/DCTDecode]);
  • the decoder reads the samples otherwise than the PDF says. That means components named R, G and B, which decoders take as stored, while the default /ColorTransform says YCbCr;
  • the JPEG's size or component count doesn't match the dictionary.
• Fragments. decodable, the merge-fragments pre-check, applies the same rule, so a strip that would be refused doesn't join a run. pixels is shared, so merged strips get the fix too.

CMYK and YCCK JPEGs were already left alone (Optimize only handles grey and RGB colour spaces). That doesn't change, and a test now pins it for every marker and /ColorTransform combination.

How you verified it

• cargo fmt --check: clean.
• clippy with -D warnings on the crates I changed: cargo clippy -p pdfcraft-optimize --all-targets -- -D warnings, clean.
• tests pass on the crates I changed: cargo test -p pdfcraft-optimize, 22 passed in optimize.rs and 3 in the new jpeg_color_transform.rs.
• cargo xtask layers (OK, 37 crates, no layering violations), cargo xtask assets (ok), cargo xtask parity (ok).

Tests (crates/optimize/tests/jpeg_color_transform.rs). Each one draws the page with PdfCraft's own renderer (pdfcraft_render::PageRenderer) before and after Optimize (Reduce File Size settings) and compares the mean difference per colour component. The tolerance is 4 levels; the re-encode itself moves about 2.4.

• optimize_keeps_the_colours_of_an_rgb_jpeg_whatever_its_colour_transform covers 12 RGB cases:

  • no marker, a YCbCr marker or an untransformed marker;
  • with no /ColorTransform, 0 or 1;
  • /ColorTransform 0 in a filter array;
  • components named R, G, B.

  For each case it also checks the outcome: re-encoded as a plain JPEG without /DecodeParms, or left byte for byte. It first checks that the fixture tells the two readings apart (by far more than the tolerance).

• optimize_leaves_cmyk_jpegs_as_they_are_with_or_without_an_adobe_marker covers 7 four-component JPEGs (no marker, CMYK marker, YCCK marker; with and without /ColorTransform). They are drawn at 288 ppi, so they would be resampled if supported, and must stay identical and render identically.

• merging_jpeg_strips_keeps_their_colour_transform draws a page picture printed as 16 JPEG strips with /ColorTransform 0. They are merged into one image, which must show the same colours.

I checked that the tests fail before the fix by reverting images.rs and fragments.rs to main in place:

• the strips test fails: colours moved by 74.7;
• the RGB test fails on 5 of 12 cases:
  • no marker with /ColorTransform 0, and the same in a filter array: colours moved by 74.7;
  • a YCbCr marker with /ColorTransform 0: re-encoded, colours moved by 74.7 (the renderer shows that file untransformed);
  • an untransformed marker with /ColorTransform 1, and R, G, B components without /ColorTransform: re-encoded where they should now be left alone (colours within tolerance in PdfCraft's renderer, but readers differ on these files);
• the CMYK test passes before and after, as expected.

With the fix restored, all three pass.

Not run locally: the wasm check and the workspace-wide sweep (CI runs both). The engine uses only pdfcraft-optimize's public API, which is unchanged.

Never-crash

• No new unwrap/expect/panic!/unreachable!/todo! outside tests. The crate's #![deny(...)] and clippy pass.
• No indexing or slicing with input-derived positions:
  • the segment walk uses get and checked additions, and every step advances or returns;
  • the decode is capped at the crate's MAX_PIXELS, and the JPEG's size and component count must match the dictionary before anything is decoded;
  • the output length is checked.
• No unsafe.
• No crash fixed; this is a colour-fidelity fix.

Provenance

• No asset from an Adobe product, and no Adobe icon recreated, traced or imitated. The test JPEGs are generated in the test.
• No font files.
• No GPL/AGPL code.
• Behaviour comes from ISO 32000-2 §7.4.8 (Table 13, ColorTransform), the JPEG segment layout (ITU T.81 Annex B) and the Adobe APP14 segment layout. The four-component test JPEG writer is adapted from crates/create/src/tests.rs.
• No new assets, so no ATTRIBUTION.toml change. zune-jpeg is already in the lock (one version, 0.5.15); only the dependency edge is new.

Security

• No paths, MCP, control channel or network touched.

Parity and reach

• parity/acrobat-features.toml: optimize.recompress-images has the new test as evidence. Its notes say JPEGs are decoded with their colour transform and that a contradictory one is left as it is. Status unchanged (partial: no JPEG 2000).
• The existing doc_optimize and doc_reduce tools go through the changed path.
• Docs: crates/optimize/README.md and the crate docs, plus a ROADMAP.md progress-log line (estimates unchanged).

Anything a reviewer should look at first

• Contradictions are left alone, in both directions. For "untransformed marker, /ColorTransform 1" the standard and PdfCraft's renderer agree (the marker wins, samples as stored), so that case could be re-encoded. I chose the simpler rule, "if the marker and the entry disagree, don't touch it", because a reader that lets the entry win would show the original differently. This is a one-line change in ycc_encoded if you'd rather re-encode it.
• The renderer's own reading differs from the standard for "YCbCr marker, /ColorTransform 0": hayro-syntax's filter/dct.rs lets /ColorTransform 0 win. That is renderer behaviour, out of scope here (the other PdfCraft PDF Color/Image Semantics Audit #1201 work covers the renderer). Optimize just doesn't rewrite such files.
• New direct dependency zune-jpeg = "0.5" on pdfcraft-optimize. Encoding and resampling still use image.
• Overlap. M11: make PDF optimization safe and add a lossless profile #1282, fix(optimize): reject hostile image dimensions safely #2321 and Optimize: give a resized image its own soft mask instead of overwriting a shared one (#1812) #1873 edit optimize/src/images.rs (pixels/process) and tests/optimize.rs. M11: make PDF optimization safe and add a lossless profile #1282 also adds a JPEG size check in pixels, which this change covers too (size and component count must match the dictionary). The new tests are in their own file to stay out of tests/optimize.rs. Print production: Convert colours (grayscale, RGB, CMYK by the device formulas) #1998 (Convert colours) adds recolour_image in the same file, which calls pixels(s, n). After this change that call becomes pixels(doc, s, n) (its doc is in scope), and it then gets the colour-transform fix too. Whichever lands second needs that small rebase.

Summary by CodeRabbit

• Bug Fixes
  • Preserved colors when optimizing RGB JPEGs stored without a color transform, including when merging image strips.
  • Left JPEGs unchanged when color-transform information is conflicting, unsupported, or ambiguous, preventing unintended color shifts.
  • Improved color handling for JPEGs with Adobe color-transform markers or /ColorTransform settings, while keeping estimates unchanged.

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• ROADMAP.md
• crates/optimize/README.md
• crates/optimize/src/fragments.rs
• crates/optimize/src/images.rs
• crates/optimize/tests/jpeg_color_transform.rs
• parity/acrobat-features.toml

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📝 Walkthrough

The optimizer now uses Adobe APP14 markers and /ColorTransform to determine JPEG sample interpretation. It decodes only JPEGs with readable, consistent transform information, and leaves ambiguous or unsupported cases unchanged. New tests cover color preservation, malformed headers, strip merging, and cancellation.

Changes: JPEG color-transform optimization

Layer / File(s)	Summary
Resolve JPEG transform metadata crates/optimize/Cargo.toml, crates/optimize/src/images.rs	The optimizer reads /ColorTransform and Adobe APP14 metadata to determine three-component JPEG interpretation. It leaves JPEGs with conflicting, malformed, or unsupported transform values undecoded for optimization.
Decode and route image samples crates/optimize/src/images.rs, crates/optimize/src/fragments.rs	Document-aware pixel extraction uses bounded JPEG decoding. Image, soft-mask, and fragment paths pass the document to pixel extraction.
Validate and document color handling crates/optimize/tests/jpeg_color_transform.rs, crates/optimize/README.md, crates/optimize/src/lib.rs, parity/acrobat-features.toml, ROADMAP.md	Tests cover RGB and CMYK JPEGs, strip merging, malformed headers, and cancellation. Documentation and parity notes describe transform handling and decoding support.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Optimize as Optimizer
  participant Metadata as PDF image metadata
  participant Decoder as zune-jpeg
  Optimize->>Metadata: Read ColorTransform and image dictionary
  Optimize->>Optimize: Scan JPEG APP14 markers and resolve transform
  Optimize->>Decoder: Decode samples when transform is readable and consistent
  Decoder->>Optimize: Return decoded samples

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Possibly related PRs

• storytold/pdfcraft#1772: Both changes define handling for RGB JPEGs with /ColorTransform 0; this PR handles optimization, while that PR handles partial redaction.

Suggested reviewers: ahmadmuj, aiongg

Merge Risk: ⚪ Minimal · up to afb25

The earlier JPEG header-scan performance issue is fixed, and no remaining actionable merge risk was established. The change is ready for normal merge checks.

Security architecture risk: 🔵 Low · up to afb25

The decoder change remains behind existing optimization entrypoints. Ambiguous inputs are skipped, size checks remain in place, and replacements are prepared before installation. No material security regression was identified in the inspected paths, but the assessment is static and does not establish deployment-wide resource isolation.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — Attacker-supplied PDF image bytes and dictionaries reach the changed decoder through existing optimization paths. The demonstrated scope is the caller-supplied document and its processing host; broader service, tenant, or environment exposure cannot be determined from the supplied deployment context.

Trust Boundaries and Controls

• observed — Document metadata influences sample interpretation, not additional authority. Unsupported or contradictory metadata returns no samples before image replacement, and a failed fragment extraction retains the original content operators.

Resilience and Maintainability Implications

• observed — Image and soft-mask replacements are fully prepared before installation. Exclusive mutable document access prevents concurrent mutation through these APIs. Repeated lossy recompression is not byte-idempotent, but that behavior and the partial-cancellation contract predate this PR.

Pre-merge checks: 5

Check name	Status	Explanation
Title check		The title clearly and concisely identifies the primary change: preserving JPEG colour-transform handling during Optimize.
Description check		The description follows the repository template and covers the change, verification commands, tests, safety checks, provenance, security, parity, documentation, and reviewer considerations.
Docstring Coverage		Docstring coverage is 85.29% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 34 functions across 4 files. (3 skipped: 3 …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 85.29% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 34 functions across 4 files. (3 skipped: 3 unsupported.)

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Inline comments:
Review comments at @crates/optimize/src/images.rs:
- Around line 299-304: Update the header scan loop in `adobe_transform` to
return `None` immediately when the byte at `i` is not `0xFF`, rather than
advancing and scanning subsequent bytes. Preserve the existing handling of valid
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[ahmadmuj]

The saved colour samples, refusal cases, strip merge and downsampled soft mask pass, including an independent Poppler/libjpeg check.

I reproduced the header-scan concern on 21909d4e through optimize_with_progress. One 128x8 JPEG has 8,000,000 stray bytes after SOI and is drawn 64 times at the same position. The first cancellation callback arrives after 4.92 seconds in the test profile. Bypassing only the new eligibility scan reduces that to 0.53 ms. Both runs then return Cancelled at Images { done: 0, total: 1 }.

Could we bound or reuse this scan and add a cancellation regression for repeated references? The delay occurs before pixel decoding.

The FreeBSD aarch64 job fails during Rust installer setup, before Cargo starts. I could not test that platform locally.

[guillehr2]

Thanks for reproducing it. Fixed in 833d654. The header scan now treats a stray byte where a marker should start as an unreadable header. The image is then left as it is, as for any header the scan can't read. The scan also stops after 4,096 markers and fill bytes, so a call costs at most the segment headers, not the stream.

cancelling_works_from_the_first_call_with_a_jpeg_drawn_many_times is your case: a 128×8 JPEG with 8,000,000 stray bytes after SOI, drawn 64 times. It is cancelled at the first progress call and checks for Cancelled at Images { done: 0, total: 1 }. a_jpeg_header_that_takes_more_than_a_few_steps_to_read_is_left_as_it_is fails without the fix. Here that test binary went from 3.45 s to 0.04 s.

I also merged current main (c9c6c85). Main's hardened pixels (u32 sizes, the decoded-pixel count) and this PR's JPEG sample handling are combined there.