import filters for vector graphic formats in libreoffice: the reverse- and straight engineering fun

LGM 2012 - LibreOffice Graphic Import filters

LibreOffice import filters for vector graphic formatsThe fun of reverse- and straight engineering

Fridrich Štrba – [email protected] Document FoundationSoftware Engineer, SUSE

mailto:[email protected]

2LGM 2012 - LibreOffice Graphic Import filters

Who Am I?

Software Engineer in SUSE LibreOffice Team

Diverse background

FLOSS enthusiast

Working in free time on various projects


Agenda

Vector graphic import filters resulting from the work of LibreOffice community

How we did itFramework usedMissing file-format documentationCollaboration patternsIncremental reverse-engineering


Why do we handle legacy file-formats?


Legacy formats out there

ODF is the future of the humanityNevertheless, humanity does not know about it as of now

Other de facto standardsSome people use other Office Suites and graphic applications :(

Hard-disks full of bad teenage poetry and indecent drawings in funny formats

LibreOffice offers the freedom to read that pile of …


Pure intellectual exercise

Allows to program for LibreOffice without having to understand its internals

Pretty stand-alone functionality communicating with LibreOffice over well defined interfaces …… almost

Happy users will reward youYou will be the hero of the people who can now read their documents...… and they will get on your nerves listing features that are not converted.


Import filters available to all resulting from the work of LibreOffice community


Vector graphics import filters based on libwpg

WordPerfect Graphics import filter and libwpgStarted by Marc Oude-Kotte and yours faithfulGoogle Summer of Code by Ariya Hidayat in 2006

MS Visio import filter and libvisio

Google Summer of Code by Eilidh McAdam in 2011Guest appearance of re-lab's Valek Filippov

Corel Draw import filter and libcdrWork in progress (kind of) started basically at the end of 2011Will be in LibreOffice 3.6Check http://dev-builds.libreoffice.org for preview fun

http://dev-builds.libreoffice.org/


Future directions

I prefer to speak about future when it becomes a featureToo many projects with declarations of intentions and nothing at the arrivalCode speaks louder then press releases

Google Summer of Code 2012An attempt at MS Publisher file-format

Valek's personal pet file-formatMacromedia Freehand

Trying to crowd-source the development


How we did it?


Minimize the count of reinvented wheels

Reuse, embrace and extendODF as interchange format

Way import filters communicate with LibreOfficelibwpg's application programming interface

Reusing OdgGenerator class implementing this interface

Speedy developmentNo need to write any boilerplate code

LibreOffice import filter itself about 100 LOCThe core written as a standalone library

Faster testing


Graphic Document Representation

namespace libwpg{ class WPGPaintInterface

{ public: virtual ~WPGPaintInterface () {} virtual void startGraphics (const ::WPXPropertyList &propList) = 0; virtual void endGraphics () = 0; virtual void setStyle (const ::WPXPropertyList &propList, const ::WPXPropertyListVector &gradient) = 0; virtual void startLayer (const ::WPXPropertyList &propList) = 0; virtual void endLayer () = 0; virtual void startEmbeddedGraphics (const ::WPXPropertyList &propList) = 0; virtual void endEmbeddedGraphics () = 0; virtual void drawRectangle (const ::WPXPropertyList& propList) = 0; virtual void drawEllipse (const ::WPXPropertyList& propList) = 0; virtual void drawPolygon (const ::WPXPropertyListVector &vertices) = 0; virtual void drawPolyline (const ::WPXPropertyListVector &vertices) = 0; virtual void drawPath (const ::WPXPropertyListVector &path) = 0; virtual void drawGraphicObject (const ::WPXPropertyList &propList, const ::WPXBinaryData &binaryData) = 0; virtual void startTextObject (const ::WPXPropertyList &propList, const ::WPXPropertyListVector &path) = 0; virtual void endTextObject () = 0; virtual void startTextLine (const ::WPXPropertyList &propList) = 0; virtual void endTextLine () = 0; virtual void startTextSpan (const ::WPXPropertyList &propList) = 0; virtual void endTextSpan () = 0; virtual void insertText (const ::WPXString &str) = 0; };

} // namespace libwpg


Key Classes

OdgGenerator.?xxImplementation of libwpg::WPGPaintInterface

OdfDocumentHandler.hxxAbstract SAX interface to receive ODF documentCode that serializes the SAX calls into file (flat ODF and zip-based ODF)

*SVGGenerator.?xxEach library has an internal SVG Generator (suboptimal)New libwpg will make the SVG Generator part of the public API


Advantages

Generating ODF is not trivialSettingsStylesAutomatic stylesContent

Provide a linear interfaceReuse instead of copying the existing ODF generatorsDeveloper does not waste time designing interfaceSpeeds up development by focusing on the essentials


File-format documentation

Almost noneFor libvisio

Marginally useful user and developer documentation of MSDNPossibility to save using the VDX (xml) file-formatBasically XML dump of the binary (the same concepts)

For libcdrDocument explaining a bit the CMX exchange format (similar concepts)

Reverse engineeringBased on re-lab's work

oletoy


Development method

Focus on getting “some” result earlyFirst embedded raster images

Libreoffice is able to render them without further processing

Next graphic primitives“Everything is just a path”

Develop tools along the implementationIntrospection tool improved constantly

Driven by the need of the implementationReflecting growing understanding of file-formatDon't solve problems that don't exist


The team

Valentin Filippov Fridrich Štrba Eilidh McAdam


Development method II

Design the software as you go

Some code is better then an abstract designPossibility to find and fix real bugsLittle communication overhead

Communication by code in gitLearning by doing mistakes and fixing

Release soon, release oftenA release every 2-3 weeksGood to have intermediary targets


Extending the file-format coverage

Incremental reverse-engineeringNobody reinvents a wheel completely

Two subsequent versions of the same file-format will have some common DNA

Try to parse lower or higher version using the existing parserFix issues as they appearImportance of a small number of reference documents covering many featuresImportance of having a working parser for other versions

Experience makes differenceDifferent ways of encoding the same informationDifferent ways of structuring


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Q&A and Stoning session

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