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https://issues.apache.org/jira/browse/PDFBOX-4952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17259084#comment-17259084
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Maruan Sahyoun commented on PDFBOX-4952:
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[~lehmi] also keep in mind the linking mechanism between the free entries
{quote}
There are two ways an entry may be a member of the free entries
list. Using the basic mechanism the free entries in the cross-reference
table may form a linked list, with each free entry containing the object number
of the next. The first entry in the table (object number 0) shall always be
free and shall have a generation number of 65,535; it is shall be the head of
the linked list of free objects. The last free entry (the tail of the linked
list) links back to object number 0. Using the second mechanism, the table may
contain other free entries that link back to object number 0 and have a
generation number of 65,535, even though these entries are not in the linked
list itself.
{quote}
> PDF compression - object stream creation
> ----------------------------------------
>
> Key: PDFBOX-4952
> URL: https://issues.apache.org/jira/browse/PDFBOX-4952
> Project: PDFBox
> Issue Type: New Feature
> Components: PDModel
> Affects Versions: 2.0.21
> Reporter: Christian Appl
> Assignee: Andreas Lehmkühler
> Priority: Major
> Fix For: 3.0.0 PDFBox
>
> Attachments: 102_Spot_to_CMYK_X1a.pdf,
> 102_Spot_to_CMYK_X1a_unc_BAD-3.0.0.pdf,
> 102_Spot_to_CMYK_X1a_unc_GOOD-2.0.22.pdf, image-2020-09-07-09-47-30-172.png,
> image-2020-09-07-10-05-15-631.png
>
>
> I implemented a basic starting point to realize a PDF compression based on
> PDFBox 2.0.22-SNAPSHOT
> I want to use this ticket, to ask if you would be interested in such a
> feature and whether you would be interested to merge it into PDFBox.
> This is sort of a POC, only implementing some very basic functionality, that
> surely must and could be extended further and it does only implement some
> very basic and simplistic Unit Tests.
> However it is able to reduce the size of resulting documents, and creates
> objectstreams as defined in the PDF reference manual.
> *What it currently does:*
> It provides the bundling and compression of objects to objectstreams -and
> further applies simple content compression to a small selection of contents-.
> -To realize content compression, it provides a simple interface and abstract
> class for "ContentCompressor"s which search a document for specific content,
> that could be compressed and do compress that contents.-
> -Currently two content compressors exist:-
> -_ImageCompressor_-
> -Searches for simple images, that could be compressed using DCT.-
> -_UnencodedStreamCompressor_-
> -Searches the document for yet unencoded streams and applies a Flate
> compression where necessary.-
> -Both compressors can be parameterized using a centralized
> "CompressParameters" instance which is passed to a new "saveCompressed"
> method of PDDocument.-
> The compression is based on, modifies and is realized by a set of extensions
> for the "COSWriter" class. Basically it organizes objects, that are passed to
> the COSWriter in objectStreams -and applies content optimization where
> necessary and possible-.
> Currently this does support encryption, but does not support linearization of
> the compressed documents.
> *Caveat:*
> If this feature is interesting to you, then I would not expect you to simply
> merge this fork into 2.0.22. I am expecting that you would like to have some
> details and concepts changed and am ready to implement changes that would be
> required for this to work to your liking.
> *POC:*
> 4 resulting documents can be found in "target/test-output/compression" when
> "COSDocumentCompressionTest" is run.
> *The Pull request can be found on Github at:*
> [https://github.com/apache/pdfbox/pull/86]
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