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OpenRaster and OpenDocument

OpenRaster is a file format for layered images. The OpenRaster specification is small and relatively easy to understand, essentially each layer is represented by a PNG image, and other information is contained written in XML and it is all contained in a Zip Archive. OpenRaster is inspired by OpenDocument.
OpenDocument is a group of different file formats, including word processing, spreadsheets, and vector drawings. The specification is huge and continues to grow. It cleverly reuses many existing standards, avoiding repeating old mistakes, and building on existing knowledge.

OpenRaster can and should reuse more from OpenDocument.



It is easy to say but putting it into practice is harder. OpenDocument is a huge standard so where to begin? I am not even talking about the OpenDocument Graphics (.odg) specifically but more generally than that. It is best that show it with an example. So I created an example OpenRaster image with some fractal designs. You can unzip this file and see that like a standard OpenRaster file it contains:


fractal.ora  
 ├ mimetype
 ├ stack.xml
 ├ data/
 │  ├ layer0.png
 │  ├ layer1.png
 │  ├ layer2.png
 │  ├ layer3.png
 │  ├ layer4.png
 │  └ layer5.png
 ├ Thumbnails/
 │  └ thumbnail.png
 └ mergedimage.png

It also unusually contains two other files manifest.xml content.xml. Despite the fact that OpenDocument is a huge standard the minimum requirements for a valid OpenDocument file comes down to just a few files. The manifest is a list of all the files contained in the archive, and content.xml is the main body of the file, and does some of the things that stack.xml does in OpenRaster (for the purposes of this example, it does many other things too). The result of these two extra files, a few kilobytes of extra XML, is that the image is both OpenRaster AND OpenDocument "compatible" too. Admittedly it is an extremely small tiny subset of OpenDocument but it allows a small intersection between the two formats. You can test it for yourself, rename the file from .ora .odg and LibreOffice can open the image.

To better demonstrate the point, I wanted to "show it with code!" I decided to modify Pinta (a Paint program written in GTK and C#) and my changes are on GitHub. The relevant file is Pinta/Pinta.Core/ImageFormats/OraFormat.cs which is the OpenRaster importer and exporter.

This is a proof of concept, it is limited and not useful to ordinary users. The point is only to show that OpenRaster could borrow more from OpenDocument. It is a small bit of compatibility that is not important by itself but being part of the larger group could be useful.

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June 2015

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