Hacking ITK-Wasm


Install Podman, and

npm i -g pnpm
pnpm install
pnpm build
pnpm test

and contribute the patch with standard GitHub best practices.


Welcome to the ITK community! ☀️

We are glad you are here and appreciate your contribution. Please keep in mind our community participation guidelines.

We follow standard GitHub contribution best practices: pull requests made from forks are tested with continuous integration tests, and updates are made via code review before integration.

We use the conventional commit standard for our commit message format in the itk-wasm repository.

The C++ core and Docker build environment can be developed independently from the CLI, language-specific libraries, and example packages. The latter are developed as a pnpm workspace. Individual packages in the workspace can be developed independently after pnpm install is executed at the root.

The following sections describe how to contribute to ITK-Wasm’s constituent parts.

C++ core

ITK-Wasm’s C++ core can be developed with native toolchains. The steps are:

  1. Install a C++ compiler toolchain and CMake

  2. Build ITK

  3. Build the WebAssemblyInterface module from this module against ITK

  4. Run the tests

From a command line, these steps in brief are:

# Build ITK
git clone https://github.com/InsightSoftwareConsortium/ITK
cmake -BITK-build -SITK -DBUILD_TESTING=OFF -DModule_MeshToPolyData=ON
cmake --build ITK-build

# Build the WebAssemblyInterface module from this module against ITK
git clone https://github.com/InsightSoftwareConsortium/itk-wasm
cmake -Bitk-wasm-build -Sitk-wasm -DBUILD_TESTING=ON -DITK_DIR=$PWD/ITK-build
cmake --build itk-wasm-build

# Run the tests
ctest --test-dir itk-wasm-build

For additional guidance on C++ development, see the ITK Software Guide.

If changes are needed in the ITK repository, start from the current repository and branch where we store changes that are being pushed upstream.

Testing data

To prevent Git repository bloat, we add testing data through CMake content links of Content Identifiers (CIDs). To add new test data,

  1. Upload the data and download its content link .cid file with the CMake w3 ExternalData Upload Tool

  2. Move the .cid file to itk-wasm/test/Input/

  3. Reference the content link with a DATA{<path>} call in itk-wasm/test/CMakeLists.txt.

Build environment Docker images

Builds of the wasm build environment Docker images or WebAssembly modules require Podman. Note: on Linux, ensure the vfs podman storage driver is not used for best performance.

To pull the latest the build environment Docker images,


To build the latest build environment Docker images from the Docker configuration and local C++ core,

./src/docker/build.sh --with-debug

The --with-debug flag will also build the latest-debug tagged images.

To use these locally built images in the pnpm build, remove (clean) the old build artifacts from the repository first.

# Remove old build artifacts
pnpm clean

pnpm install
pnpm build

Command line interface (CLI)

The itk-wasm command line interface (CLI) is a Node.js / NPM script developed with the PNPM package manager tool. This CLI is included with the itk-wasm package.

Development requires:

  1. Node.js / NPM

  2. Bash, e.g. macOS or Linux terminal, Windows Git Bash or WSL terminal

  3. PNPM, which can be installed with npm i -g pnpm

  4. Chrome and Firefox installed

To build and test the CLI,

git clone https://github.com/InsightSoftwareConsortium/itk-wasm
cd itk-wasm/packages/core/typescript/itk-wasm
pnpm install
pnpm build
pnpm test

Language binding libraries

The language binding libaries:


The development of itk-wasm is described above in the Command line interface section.


To develop the itkwasm Python package, which configured with hatch:

git clone https://github.com/InsightSoftwareConsortium/itk-wasm
cd itk-wasm/packages/core/python/itkwasm

pip install hatch
hatch run download-pyodide
hatch run test

Example packages

Example packages are maintained in the itk-wasm/packages/ directory. These packages contain in the top level directory

  1. their C++ pipelines,

  2. CMake configuration code,

  3. a NPM package.json file to drive the build with pnpm.

To develop these packages, at the top level, run:

npm i -g pnpm
pnpm install
pnpm build
pnpm test

This will build and test the packages in order, according to their dependency topology, and the packages will use local workspace dependencies, including the itk-wasm CLI.

The build and test targets are high-level targets that call other targets. They include targets for direct WebAssembly execution, TypeScript bindings, and Python bindings. These can be called individually either with pnpm filters or by changing into the package directory and calling the targets from there.

Debug builds

In development, it often useful to build the wasm in debug mode. To create a debug build, at the root level, clean the tree, call build:emscripten:debug and build:wasi:debug, then build and test as usual:

pnpm clean
pnpm build:emscripten:debug
pnpm build:wasi:debug
pnpm build
pnpm test

Testing data

Testing data is stored in a package’s test/data directory and shared across the direct wasm and language bindings tests. It is downloaded via the test:data:download target, which is called by the test target.

To add new or modify testing data, add new files into the test/data/input/ or test/data/baseline/ directory, then run:

pnpm test:data:pack

This will generate a new test/data.tar.gz tarball and output its CID.

Upload the tarball to a publicly hosted HTTP location. A few example free services that can serve files include FileBase, web3.storage, and data.kitware.com.

Then, update the CID and HTTP URL in package.json.


The wasm.itk.org documentation sources are found at the docs/ directory.

Documentation for individual packages’s documentation can be found in the typescript/README.md, or python/<dispatch-package-name>/docs directory.

To preview wasm.itk.org documentation changes, three options are available.

Option 0: Edit on GitHub, use the pull request preview

If files are edited with GitHub’s web user interface, the pull request will build a preview of changes with a pull request check called docs/readthedocs.org:itkwasm. Click on the Details link to view the documentation build preview.

Option 1: Build and serve locally

To compile the document locally create a python virtual environment and install the required packages.

For example in Linux / macOS:

cd itk-wasm/docs
python -m venv env
source env/bin/activate
pip install -r requirements.txt

Use make html in this directory to build the documentation. Open _build/html/index.html in your browser to inspect the result.

Option 2: Autobuild and serve locally

To automatically rebuild the website with any input markdown changes and serve the result, use sphinx-autobuild

cd itk-wasm/docs
pip install -r requirements.txt
pip install sphinx-autobuild
sphinx-autobuild -a . _build/html

This will start a server at and rebuild whenever the documentation changes.