File Layout Specification


OpenRaster files have the file name extension .ora. The data is stored within a Zip file wrapper.

Technical note: Zip files can use a different compression algorithm on a per-file basis. Only DEFLATED and STORED should be used for OpenRaster files. The file names within OpenRaster “zip files” are case-sensitive and must be UTF-8 encoded. If you decide to write non-ascii file names, be aware that the Zip format has an UTF-8 flag for each file name. Make sure this flag is set, otherwise the content might get decoded as cp437. Note also that the Zip file format has a 64 bit extension which must be enabled to write files larger than 4GB.

This file format is hereafter referred to as an “OpenRaster file” in this specification. It has a number of required and optional standard files and subdirectories within it:

example.ora  [considered as a folder-like object]
 ├ mimetype
 ├ stack.xml
 ├ data/
 │  ├ [image data files referenced by stack.xml, typically layer*.png]
 │  └ [other data files, indexed elsewhere]
 ├ Thumbnails/
 │  └ thumbnail.png
 └ mergedimage.png

Required Files


The first file in the archive must be called mimetype, without a file name extension. It must be STORED without compression. This file must contain the string image/openraster, with no whitespace or trailing newline.


This is the UTF-8 encoded XML file from the Layers Stack Specification.


This directory is (by convention) where data files are usually stored. Files must be referenced in stack.xml by their full path relative to the OpenRaster file’s root, e.g. data/layer2.png.

All files inside this directory should be referenced from somewhere. Well known file name extensions (like .png) should be lower case.


An OpenRaster file must have a thumbnail.png in order to allow file browser software to render the thumbnail efficiently. It must be a non-interlaced PNG with 8 bits per channel of at most 256x256 pixels. It should be as big as possible without upscaling or changing the aspect ratio. Any aspect ratio is permitted. It should not contain any frame or decoration.

The thumbnail file must not be referenced in any .xml file.


An OpenRaster file must have a mergedimage.png in order to accommodate viewer software. It must contain the final rendered image without any frame or decoration. This file must be a PNG file with 8 or 16 bits per channel.

Changed in version 0.0.2: mergedimage.png was optional; it is now mandatory.