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https://issues.apache.org/jira/browse/PDFBOX-5518?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17743421#comment-17743421
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Andreas Lehmkühler commented on PDFBOX-5518:
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Please attach a sample pdf containing threads
> "Threads" array in Document Catalog should be an indirect reference
> -------------------------------------------------------------------
>
> Key: PDFBOX-5518
> URL: https://issues.apache.org/jira/browse/PDFBOX-5518
> Project: PDFBox
> Issue Type: Bug
> Components: PDModel
> Affects Versions: 2.0.26
> Reporter: Christian Appl
> Assignee: Andreas Lehmkühler
> Priority: Major
> Attachments: image-2022-09-23-09-50-30-766.png,
> image-2022-09-23-10-03-15-070.png, image-2022-09-23-10-54-31-618.png
>
>
> *TL;DR:*
> When using either of the methods "getThreads" or "setThreads" in class
> PDDocumentCatalog and saving the resulting document: Adobe Preflight is
> reporting an issue with the resulting "Threads" array in the document catalog
> and claims it should have been an indirect object reference instead of a
> direct object.
> My claim: The COSWriter should be able to create indirect objects for
> COSArrays when required.
> *Checking PDF-32000-1:*
> In table 28 "Entries in the catalog dictionary" we can find the following
> definition:
> !image-2022-09-23-09-50-30-766.png!
> *Determining reasons:*
> 1. The mentioned get and set methods create a COSArray for the entry
> "Threads" of the catalog dictionary
> 2. The COSWriter is assuming, that COSArrays should always preferably be
> written as a direct substructure of a dictionary.
> This may be entirely true for other arrays, but in this case is is cause for
> a syntactical error in resulting documents. (It is plausible and possible -
> but has not been checked - whether this causes issues for other structures
> aswell.)
> The COSWriter provides the means to create indirect objects for
> COSDictionaries, it however does (as far as I can see) not provide the means
> to flag a COSArray for the same handling.
> *Possible solutions:*
> As far as I can see the COSWriter would be entirely capable of creating
> COSObjects for any of the COSBase types, the only thing missing is the
> ability to mark a COSArray to be written indirectly and the matching handling
> by the COSWriter.
> Adding something like:
> !image-2022-09-23-10-03-15-070.png!
> at the right places in the COSWriter (similar to the handling of indirect
> COSDictionaries) seems to do the trick and resolves the issue.
> *Important issue?:*
> I fixed this on our end and hence it is not a pressing issue, also "Threads"
> is not as important and common as other structures and hence most documents
> and users won´t encounter this issue at all.
> However - It would be nice, should this be fixed.
> *Concerning a possible patch:*
> I could provide a patch making the required changes, but would have to adapt
> it for the current PDFBox 2.0.27-SNAPSHOT as I developed it rather as a
> hotfix for our mirror of the library.
> And concerning that patch I should mention:
> As can be assumed - a "isDirectArray" and "setDirectArray" method have been
> added to the COSArray - which is a quick and dirty solution, as it would be
> preferable for COSArray to use the already existing "direct" field, that
> other COSBase types (COSDictionaries) already use.
> As stated - the solution is quick and dirty and for a final solution in the
> PDFBox library a cleaner approach would be preferable. Hence I did not
> provide that patch for now.
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