Adding Tests#

This page documents how to add test data while developing VTK with Git. See the README for more information.


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

$ ./Utilities/


Our workflow for adding data integrates with our standard Git 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.

Writing new tests#

All new features that go into VTK must be accompanied by tests. This ensures that the feature works on many platforms and that it will continue to work as VTK evolves.

Tests for the classes in each module of VTK are placed underneath the module’s Testing/ subdirectory. Modules that the tests depend upon beyond those that the module itself depends upon are declared with the TEST_DEPENDS argument in the vtk.module file. Test executables are added to VTK’s build system by naming them in the CMakeLists.txt files in each Testing/ directory. In those CMakeLists, standard add_executable() + add_test() command pairs could be used, but the following macros defined in vtkModuleTesting.cmake are preferable as they consolidate multiple tests together, participate in VTK’s modular build scripts, and ensure consistency:

Tests indicate success to CTest by returning EXIT_SUCCESS (0) and failure by returning EXIT_FAILURE (1). How the test determines what result to return is up to the developer. VTK contains a number of utilities for this task. For example, vtkRegressionTester is a helper class that does a fuzzy comparison of images drawn by VTK against known good baseline images and returns a metric that can be simply compared against a numeric threshold.

Many tests require data files to run. The image comparison tests for example need baseline images to compare against, and many tests open up one or more files to visualize.

The source code and data file versions are kept in sync because the Testing/Data directory contains, instead of the real files, similarly named files which contain only the SHA512 hash of the matching data files. During the build process, when CMake sees that a required data file is not available, it downloads it into the directory defined by the ExternalData_OBJECT_STORES cmake configuration entry. The test executables read all data from there. The default setting for ExternalData_OBJECT_STORES is the ExternalData directory underneath the VTK build tree.

To make a change to VTK that modifies or adds a new test data file, place the new version in the Testing/Data or directory (for input data files) or Module/Name/Testing/Data (for regression test images), and build (or run cmake). CMake will do the work of moving the original out of the way and replacing it with an SHA512 link file. When you push the new link file to Gitlab, git pre-commit hooks push the original file up to Kitware’s data service, where everyone can retrieve it.

Add Test#

  1. Write a new test, e.g.

    $ edit Some/Module/Testing/Cxx/MyTest.cxx
  2. Edit the corresponding CMakeLists.txt file:

    $ edit Some/Module/Testing/Cxx/CMakeLists.txt

    and add the test in a vtk_add_test_... call (which references baselines automatically).

  3. For tests not using such a call, reference the data file in an ExternalData_add_test call. Specify the file inside DATA{...} using a path relative to the test directory:

    $ edit Some/Module/Testing/Cxx/CMakeLists.txt
      NAME ${_vtk_build_test}Cxx-MyTest
              ... -V DATA{../Data/Baseline/MyTest.png,:} ...
  4. Some tests may require additional files not referenced on the command line. For these files, add references to a vtk_module_test_data call (usually in the Testing parent directory). For example, adding Testing/Data/lines.vtp would mean adding Data/lines.vtp entry to the call (the Testing directory is part of the path that is looked in automatically.



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

  • Multiple baseline images and other series are handled automatically when the reference ends in the ,: option. Read ExternalData module documentation for details.

Build and Run the Test#

If you already have a data file, skip to the next step to add it. Otherwise, use the following steps to produce a test baseline image file. We assume a build tree has been previously generated by CMake.

  1. Switch to the build tree:

    $ cd ../VTK-build
  2. Run CMake:

    $ cmake .

    Since we have not yet created the baseline image data file, CMake will warn that it does not exist but proceed to generate the test anyway.

  3. Build

    $ make
  4. Run the test

    $ ctest -R MyTest

    It will fail but place the baseline image in Testing/Temporary.

  5. Switch back to the source tree:

    $ cd ../VTK

Add Data#

Copy the data file into your local source tree.

$ mkdir -p Some/Module/Testing/Data/Baseline
$ cp ../VTK-build/Testing/Temporary/MyTest.png Some/Module/Testing/Data/Baseline

Run CMake#

  1. Switch to the build tree:

    $ cd ../VTK-build
  2. Run CMake:

    $ cmake .

    CMake will move the original file. Keep your own copy if necessary. See below to recover the original file.

    During configuration CMake will display a message such as:

    Linked Some/Module/Testing/Data/Baseline/MyTest.png.sha512 to ExternalData SHA512/...

    This means that CMake converted the file into a data object referenced by a “content link” named like the original file but with a .sha512 extension. CMake also renamed the original file.

  3. Build

    $ make

    During the build, the ExternalData module will make the data file available where the test expects to find it.

  4. Run the test

    $ ctest -R MyTest

    It should pass using the new data file.

  5. Switch back to the source tree:

    $ cd ../VTK


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 Some/Module/Testing/Data/Baseline/MyTest.png.sha512
$ git add Some/Module/Testing/Data/CMakeLists.txt
$ git commit

The local pre-commit hook will display a message such as:

Some/Module/Testing/Data/Baseline/MyTest.png.sha512: Added content to Git at refs/data/SHA512/...
Some/Module/Testing/Data/Baseline/MyTest.png.sha512: Added content to local store at .ExternalData/SHA512/...
Content link Some/Module/Testing/Data/Baseline/MyTest.png.sha512 -> .ExternalData/SHA512/...

This means that the pre-commit hook recognized that the content link references a new data object and prepared it for upload.


Follow the instructions to share the topic. When you push it to GitLab for review using

$ git gitlab-push

part of the output will be of the form:

*       ...:refs/data/...      [new branch]
*       HEAD:refs/heads/my-topic  [new branch]
Pushed refs/data/... and removed local ref.

This means that the git-gitlab-push script pushed the topic and uploaded the data it references.

Options for gitlab-push include:

  • --dry-run: Report push that would occur without actually doing it

  • --no-topic: Push the data referenced by the topic but not the topic itself

Note: One must git gitlab-push from the same work tree as was used to create the commit. Do not git push to another computer first and try to push to GitLab from there because the data will not follow.



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

-- Fetching ".../ExternalData/SHA512/..."
-- [download 100% complete]
-- Downloaded object: "VTK-build/ExternalData/Objects/SHA512/..."

The downloaded files appear in VTK-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 ../VTK

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.


A VTK 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 ExternalData module fetches data needed by enabled tests. This allows arbitrarily large data to be added and removed without bloating the version control history.

The above workflow allows developers to add a new data file almost as if committing it to the source tree. The following subsections discuss details of the workflow implementation.


While CMake runs the ExternalData module evaluates DATA{} references. VTK sets in vtkExternalData.cmake the ExternalData_LINK_CONTENT option to SHA512 to enable automatic conversion of raw data files into content links. When the module detects a real data file in the source tree it performs the following transformation as specified in the module documentation:

  • Compute the SHA512 hash of the file

  • Store the ${hash} in a file with the original name plus .sha512

  • Rename the original file to .ExternalData_SHA512_${hash}

The real data now sit in a file that we tell Git to ignore. For example:

$ cat Some/Module/Testing/Data/Baseline/.ExternalData_SHA512_477e6028* |sha512sum
477e6028...  -
$ cat Some/Module/Testing/Data/Baseline/MyTest.png.sha512

Recover Data File#

To recover the original file after running CMake but before committing, undo the operation:

$ cd Some/Module/Testing/Data/Baseline
$ mv .ExternalData_SHA512_$(cat MyTest.png.sha512) MyTest.png


While committing a new or modified content link the pre-commit hook moves the real data object from the .ExternalData_SHA512_${hash} file left by the ExternalData module to a local object repository stored in a .ExternalData directory at the top of the source tree.

The hook also uses Git plumbing commands to store the data object as a blob in the local Git repository. The blob is not referenced by the new commit but instead by refs/data/SHA512/${hash}. This keeps the blob alive in the local repository but does not add it to the project history. For example:

$ git for-each-ref --format="%(refname)" refs/data
$ git cat-file blob refs/data/SHA512/477e6028... | sha512sum
477e6028...  -

git gitlab-push#

The git gitlab-push command is actually an alias for the git-gitlab-push script. In addition to pushing the topic branch to GitLab the script also detects content links added or modified by the commits in the topic. It reads the data object hashes from the content links and looks for matching refs/data/ entries in the local Git repository.

The script pushes the matching data objects to your VTK GitLab fork. For example:

$ git gitlab-push --dry-run --no-topic
*       refs/data/SHA512/477e6028...:refs/data/SHA512/477e6028...   [new branch]
Pushed refs/data/SHA512/477e6028... and removed local ref.

A GitLab webhook that triggers whenever a topic branch is pushed checks for refs/data/ in your VTK GitLab fork, fetches them, erases the refs from your fork, and uploads them to a location that we tell ExternalData to search in [vtkExternalData][] at build time.

To verify that the data has been uploaded as expected, you may direct a web browser to the location where ExternalData has uploaded the files. For VTK, that location is currently where XXXX is the complete SHA512 hash stored in the content link file (e.g., the text in MyTest.png.sha512).

Publishing Data for an External Branch#

The above workflow works well for developers working on a single machine to contribute changes directly to upstream VTK. When working in an external branch of VTK, perhaps during a long-term topic development effort, data objects need to be published separately.

The workflow for adding data to an external branch of VTK is the same as the above through the commit step, but diverges at the push step because one will push to a separate repository. Our ExternalData infrastructure intentionally hides the real data files from Git so only the content links (.sha512 files) will be pushed. The real data objects will still be left in the .ExternalData/SHA512 directory at the top of the VTK source tree by the pre-commit hook.

The .ExternalData directory must be published somewhere visible to other machines that want to use the data, such as on a web server. Once that is done then other machines can be told where to look for the data, e.g.

cmake ../VTK "-DExternalData_URL_TEMPLATES=

In this example we assume the files are published on a Github Pages gh-pages branch in username’s fork of VTK.

Within the gh-pages branch the files are placed at ExternalData/SHA512/$sha512sum where $sha512sum is the SHA512 hash of the content (these are the same names they have in the .ExternalData directory in the original source tree).