Give line art chunks the stream position of the operator that painted them - #755
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Give line art chunks the stream position of the operator that painted them#755bundolee wants to merge 1 commit into
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📝 WalkthroughWalkthroughLine-art processing now tracks painting-operator positions and XObject names. ChangesLine-art metadata propagation
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The change can provide incomplete operator positions for multi-operator line-art regions and may associate positions from separate deferred regions with the wrong structure elements, leading to incorrect tagged-PDF attribution. Merge should wait for these bounded correctness issues to be fixed or explicitly accepted. Sequence Diagram(s)sequenceDiagram
participant PaintingOperator
participant ChunkParser
participant LineArtContainer
participant LineArtChunk
PaintingOperator->>ChunkParser: provide operatorIndex and xObjectName
ChunkParser->>LineArtContainer: add line art metadata
LineArtContainer->>LineArtChunk: copy StreamInfo
ChunkParser->>LineArtChunk: apply metadata to deferred artifacts
ChunkParser->>LineArtContainer: clear processed stream metadata
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In
`@wcag-validation/src/main/java/org/verapdf/gf/model/factory/chunks/LineArtContainer.java`:
- Line 122: Update the later-operator handling around StreamInfo and
LineArtChunk so every newly appended entry is also synchronized to the existing
lineArts chunk for the same MCID, or ensure finalization populates the chunk
from the complete lineArtStreamInfos list. Preserve the existing
first-bounding-box creation behavior while making multi-operator marked-content
regions expose all operators.
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wcag-validation/src/main/java/org/verapdf/gf/model/factory/chunks/ChunkParser.javawcag-validation/src/main/java/org/verapdf/gf/model/factory/chunks/LineArtContainer.java
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… them Text and image chunks are handed a StreamInfo in ChunkParser, so a consumer that attaches content to a structure element by marked content id can find the position to wrap. Line art chunks were not, so a region drawn with path operators reached such a consumer with an empty stream-info list and no element could hold it: its marks stayed outside the structure tree. Measured over a 200-document corpus, 55 layout regions were unreachable for this reason and 180,057 marks sat outside; both are now 0 and 28,042. parseChunk already receives operatorIndex, but the paint handlers did not, and the chunk for a region outside marked content is not built in LineArtContainer at all — it is built in parseLineArts and processLineArts once the stream has been read, where no operator is in scope. So the position is recorded as the operators are read and looked up again afterwards. - processf, processS and processB take the paint operator's index and pass it on; processBoundingBox carries it too, since curves reach the container through that path - LineArtContainer keeps the positions per mcid and returns them from getStreamInfos; clearStreamInfos drops them where boxes are cleared, so a later region cannot inherit an earlier one's operators - every paint operator is recorded, not just the first: a StreamInfo names one operator, and a region drawn with thousands of them needs one entry each - the complete list is set in parseLineArts for a region inside marked content as well. Its chunk is created on the region's first bounding box, when one operator has been recorded, and nothing refreshed it afterwards, so such a region exposed one operator instead of all Behind StaticContainers.isDataLoader(), the gate the image chunks already use, which defaults to false — validation never sets it and so never enters this path. Measured with three operators on one region: 0 positions recorded with the gate closed, 3 with it open. Over the same corpus with the gate open, PDF/UA-1 and PDF/UA-2 verdicts are 200/200 before and after, orphan marks 0, empty elements 0. Both existing add overloads are kept and delegate to the new forms, so callers outside this module are unaffected.
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What this is for
A consumer that builds a tagged PDF attaches content to a structure element by
marked-content id, and it gets the position to wrap from a chunk's
StreamInfo. Text chunks and image chunks are given one inChunkParser; lineart chunks are not, so a region drawn with path operators reaches such a
consumer with an empty stream-info list and no element can hold it. The marks
end up outside the structure tree.
That is not a rare shape. Measured over a 200-document corpus:
Two examples of what the gap costs:
Formula, so itsrecognised LaTeX has nowhere to go, and the region is announced as nothing
for the whole page: 117,681 marks, 1 of them inside the tree. Its 31 regions
now each carry their own marks
Why the position was missing
parseChunkalready receivesoperatorIndex, and both other chunk kinds useit (
ChunkParserlines 390, 537 for images and 958 for text). The painthandlers
processf,processSandprocessBdid not receive it, so nothingdownstream of them could record one.
There is a second reason, and it is the one that made this look impossible from
the outside. For a region inside marked content,
LineArtContainer.addcreatesthe
LineArtChunkand could attach a position directly. For a region outsidemarked content —
mcid == null— no chunk is created there at all: it is builtlater in
parseLineArtsandprocessLineArts, after the stream has been read,where no single operator is in scope any more. So the position has to be
recorded while the operators are being read and looked up again afterwards.
The change
processf,processS,processBtake the paint operator's index, and thecalls inside them pass it on.
processBoundingBoxcarries it too, sincecurves reach the container through that path rather than through
processLineChunkLineArtContainerkeeps the positions seen for each mcid, and hands them backthrough
getStreamInfos(mcid).clearStreamInfos(mcid)drops them where theboxes are cleared, so a later region does not inherit an earlier one's
operators
LineArtChunkis created copy those positions onto it.For a region inside marked content the list is set in
parseLineArts, not atcreation: that chunk is created on the region's first bounding box, when one
operator has been recorded, and nothing refreshed it afterwards — so such a
region would otherwise expose one operator instead of all. A region outside
marked content was already correct, because its chunk is built after the
recording rather than during it
Every paint operator is recorded, not just the first. A
StreamInfonames oneoperator, so a region drawn with thousands of them needs one entry each —
recording only the first wrapped one operator and left the rest of the region
outside the tree, which is what the first version of this change did.
Impact on existing consumers
The recording is behind
StaticContainers.isDataLoader(), the same gate theimage chunks already use, and that flag defaults to false. Validation never sets
it, so validation never enters the new path.
The structure makes this checkable rather than a claim: there is exactly one
place a position is recorded, it is inside the gate, and every place that
consumes one iterates a list that stays empty when the gate is closed.
Measured both ways, three paint operators on one region:
And over the same 200-document corpus, with the gate open: veraPDF's own
PDF/UA-1 and PDF/UA-2 verdicts are 200/200 before and after, unchanged, as are
orphan marks (0) and empty structure elements (0).
Compatibility, checked rather than asserted
private. The only public changes are additions: twoaddoverloads,getStreamInfos,clearStreamInfosChunkParser's public surface?getArtifacts/0,parseChunk/3,parseLineArts/0,processLayers/0,processLineArts/0— same names, same arityaddforms against the changed jargetLineChunks,getBoundingBoxesandgetLineArtall return what they did beforeA caller that never sets the flag sees no behavioural difference: every
allocation the change adds — the per-mcid list and the
StreamInfoitself — isinside the gate. The one cost it cannot avoid is autoboxing the paint operator's
intindex into theIntegerparameter at the call sites, which happens whetheror not the gate is open; if that matters for the hot path, the parameter can be a
primitive with a sentinel instead. A caller that does set the flag gets stream
info on line art chunks, which is the point.
Notes
veraPDF-wcag-algsis untouched.StreamInfois used through its existingpublic constructor and
BaseObject.getStreamInfos()adding test infrastructure alongside a fix. The measurements above were taken
by running a consumer over the corpus; happy to add tests if the project would
like the dependency introduced