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https://issues.apache.org/jira/browse/PDFBOX-5683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17766766#comment-17766766
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Patrick Dalla Bernardina commented on PDFBOX-5683:
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Let me report my experience on trying to extend some classes:
First I've tryed to make a Wrapper Class that implements ResourceCache and
would delegates the resource resolution to the default resource cache, and if
nothing is found, would get the resources from other alternate source.
Then I've created a Wrapper Class to PDDocument so the document would change
its resourceCache to use this Wrapper implementation.
Then, I extended the PDFParser, to return this Wrapped implementation of
PDDocument.
The problem is that the some low level implementation of COS make some checks
before the ResourceCache is really used. ColorSpace, for example, if it is
saved as an array, checks if the second element of the array is a Stream,
trying to load it from REF position. As it is overwritten, it cannot load it
and throws an exception before ResourceCache implementation can give an
alternative.
> Inconsistent/incomplete PDF rendering
> -------------------------------------
>
> Key: PDFBOX-5683
> URL: https://issues.apache.org/jira/browse/PDFBOX-5683
> Project: PDFBox
> Issue Type: New Feature
> Components: Parsing
> Reporter: Patrick Dalla Bernardina
> Priority: Minor
> Labels: Forensic
> Attachments: pdf1.pdf
>
> Original Estimate: 72h
> Remaining Estimate: 72h
>
> We have integrated tika and its default parsers in a Forensic Tool (IPED). As
> a forensic tool, it tries to recover/carve deleted PDF files, some of which
> can be partially recovered.
> PDFParser throws Exception if there is no PDF version header on the parsed
> content, avoiding any further content parsing.
> Commenting this exception, the parser still throws the exception "Missing
> root object specification in trail" in initialParse method, as this root
> object is normally at begin of a PDF file.
> Although, I could made some simple effort to build a "fake" root
> COSDictionary and build the PAGES entry with the recoverable PAGEs, searching
> them from document.getXrefTable();
> {code:java}
> protected void initialParse() throws IOException
> {
> COSDictionary trailer = retrieveTrailer();
>
> COSDictionary root = trailer.getCOSDictionary(COSName.ROOT);
> if (root == null)
> {
> // rebuild root from xref recovered info
> root = new COSDictionary();
> root.setItem(COSName.TYPE, COSName.CATALOG);
> trailer.setItem(COSName.ROOT, root);
> // identify recovered pages from xref to mount COSName.PAGES
> Map<COSObjectKey, Long> xrefTable = document.getXrefTable();
> COSArray kids = new COSArray();
> for (Entry<COSObjectKey, Long> e : xrefTable.entrySet()) {
> COSObject o = document.getObjectFromPool(e.getKey());
> if (o.getObject() instanceof COSDictionary) {
> COSDictionary d = (COSDictionary) o.getObject();
> COSName type = d.getCOSName(COSName.TYPE);
> if (type != null) {
> if (type.equals(COSName.PAGE)) {
> kids.add(d);
> }
> }
> }
> }
>
> COSDictionary pages = new COSDictionary();
> pages.setItem(COSName.TYPE, COSName.PAGES);
> pages.setItem(COSName.COUNT, COSInteger.get(kids.size()));
> pages.setItem(COSName.KIDS, kids);
> document.setDecrypted();
> root.setItem(COSName.PAGES, pages);
> initialParseDone = true;
> return;
> // throw new IOException("Missing root object specification in
> trailer.");
> }
> // in some pdfs the type value "Catalog" is missing in the root object
> if (isLenient() && !root.containsKey(COSName.TYPE))
> {
> root.setItem(COSName.TYPE, COSName.CATALOG);
> }
> // check pages dictionaries
> checkPages(root);
> document.setDecrypted();
> initialParseDone = true;
> }
> {code}
> This simple effort was enough to show the recoverable pages on PDFDebugger,
> export XMP metadatas, text, and get the rendered pages buffered images to use
> on my OCR module.
> So, it would be very useful if PDFBOX already have some optional
> parameterizable mode to open/recover inconsistent/incomplete pdf file, with
> at least the implementation above or further recover actions.
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