ITK Data

This page documents how to add test data while developing ITK. See our CONTRIBUTING and Upload Binary Data guides for more information.

While these instructions assume that the required data will be contained in binary files, the procedure (except that related to the content link file generation) also applies to any other data contained in a text file that a test may require, if any.

If you just want to browse and download the ITK testing images, browse the ITKData Datalad repository.


The workflow below depends on local hooks to function properly. Follow the main developer setup instructions before proceeding. In particular, run



Our workflow for adding data integrates with our standard development process. Start by creating a topic. Return here when you reach the “edit files” step.

These instructions follow a typical use case of adding a new test with a baseline image.

Add Data

Copy the data content link file into your local source tree.

mkdir -p Modules/.../test/Baseline
cp ~/MyTest.png.cid Modules/.../test/Baseline/MyTest.png.cid

Add Test

Edit the test CMakeLists.txt file and reference the data file in an itk_add_test call. Specify the file inside DATA{...} using a path relative to the test directory:

edit Modules/.../test/CMakeLists.txt

   itk_add_test(NAME MyTest COMMAND ... --compare DATA{Baseline/MyTest.png,:} ...)
  • Files in Testing/Data may be referenced as DATA{${ITK_DATA_ROOT}/Input/MyInput.png}.

  • If the data file references other data files, e.g. .mhd -> .raw, follow the link to the ExternalData module on the right and read the documentation on “associated” files.

  • Multiple baseline images and other series are handled automatically when the reference ends in the “,:” option; follow the link to the ExternalData module on the right for details.


Continue to create the topic and edit other files as necessary. Add the content link and commit it along with the other changes:

git add Modules/.../test/Baseline/MyTest.png.cid
git add Modules/.../test/CMakeLists.txt
git commit



For the test data to be downloaded and made available to the tests in your build tree the ITKData target must be built. One may build the target directly, e.g. make ITKData, to obtain the data without a complete build. The output will be something like

   -- Fetching ".../ExternalData/CID/..."
   -- [download 100% complete]
   -- Downloaded object: "ITK-build/ExternalData/Objects/CID/..."

The downloaded files appear in ITK-build/ExternalData by default.

Local Store

It is possible to configure one or more local ExternalData object stores shared among multiple builds. Configure for each build the advanced cache entry ExternalData_OBJECT_STORES to a directory on your local disk outside all build trees, e.g. “/home/user/.ExternalData”:

cmake -DExternalData_OBJECT_STORES=/home/user/.ExternalData ../ITK

The ExternalData module will store downloaded objects in the local store instead of the build tree. Once an object has been downloaded by one build it will persist in the local store for re-use by other builds without downloading again.


An ITK test data file is not stored in the main source tree under version control. Instead the source tree contains a “content link” that refers to a data object by a hash of its content. At build time the the ExternalData.cmake module fetches data needed by enabled tests. This allows arbitrarily large data to be added and removed without bloating the version control history.

For more information, see CMake ExternalData: Using Large Files with Distributed Version Control and the InterPlanetary File System (IPFS).