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Contributing to ITK

This article documents how to contribute improvements to ITK.

For a quick start guide, see the ITK Git Cheatsheet.

Setup

Before you begin, perform initial setup:

  1. Register for a GitHub account.
  2. Optionally download our one page PDF desk reference.
  3. Follow the download instructions to create a local ITK clone:
   $ git clone https://github.com/InsightSoftwareConsortium/ITK
  1. Run the developer setup script SetupForDevelopment.sh to prepare your ITK work tree and create Git command aliases used below:
   $ ./Utilities/SetupForDevelopment.sh

This script helps configure your GitHub fork remote, Git client-side hooks, and useful Git aliases. The default Git remote names for your fork and InsightSoftwareConsortium/ITK are origin and upstream, respectively. However, other remote names can be used. Note that ITK defines some useful Git aliases, such as review-push, pr, pr-clean, and prepush, through the setup-git-aliases script for general Git tasks in ITK.

Visit the Pro Git: Setup resource in Git Help for further information on setting up your local Git environment.

Workflow

ITK development uses a branchy workflow based on topic branches. This corresponds to the Fork & Pull Model mentioned in the GitHub flow guide. Our collaboration workflow consists of three main steps:

  1. Local Development
  2. Code Review
  3. Integrate Changes

Update

Update your local master branch:

   $ git checkout master
   $ git pullall

Create a Topic

All new work must be committed on topic branches. Name topics like you might name functions: concise but precise. A reader should have a general idea of the feature or fix to be developed given just the branch name.

To start a new topic branch:

   $ git fetch upstream

For new development, start the topic from upstream/master:

   $ git checkout -b my-topic upstream/master

For release branch fixes, start the topic from upstream/release:

   $ git checkout -b my-topic upstream/release

(You may visit the Pro Git: Basic Branching resource in Git Help for further information on working with branches.)

Edit files and create commits (repeat as needed). Add a prefix to your commit message (see below).

   $ edit file1 file2 file3

(To add data follow these instructions.)

   $ git add file1 file2 file3
   $ git commit

(You may visit the Pro Git: Recording Changes resource in Git Help for further information on making changes and committing snapshots.)

Note: If your change modifies any of the modules in the Modules/ThirdParty directory, please read our Updating Third Party guide.

Breaking Changes

Breaking changes are defined in ITK as those changes that introduce changes to the API of the major version of the toolkit, and as such, make a component of the toolkit no longer backwards compatible. Breaking changes are only allowed in new major releases. Thus, the change may be held up by the toolkit's maintainers to ensure consistency in the toolkit. Before making such changes to the code, and considering other options to keep the code backward-compatible, please either open an issue from the appropriate category or discuss the subject in ITK's Discourse. If the change finally is made into a pull request, cross-reference the issue and/or the discussion with the appropriate link.

Design Changes

Design changes should be discussed in ITK's Discourse. A Design Impact Report can also be opened to keep track of the requested change. Design changes need explicit approval from the toolkit's maintainers.

Commit Messages

Write your commit messages using the standard prefixes for ITK commit messages:

  • BUG: Fix for runtime crash or incorrect result
  • COMP: Compiler error or warning fix
  • DOC: Documentation change
  • ENH: New functionality
  • PERF: Performance improvement
  • STYLE: No logic impact (indentation, comments)
  • WIP: Work In Progress not ready for merge

The body of the message should clearly describe the motivation of the commit (what, why, and how). In order to ease the task of reviewing commits, the message body should follow the following guidelines:

  1. Leave a blank line between the subject and the body. This helps git log and git rebase work nicely, and allows to smooth generation of release notes.
  2. Try to keep the subject line below 72 characters, ideally 50.
  3. Capitalize the subject line.
  4. Do not end the subject line with a period.
  5. Use the imperative mood in the subject line (e.g. BUG: Fix spacing not being considered.).
  6. Wrap the body at 80 characters.
  7. Use semantic line feeds to separate different ideas, which improves the readability.
  8. Be concise, but honor the change: if significant alternative solutions were available, explain why they were discarded.
  9. If the commit refers to a topic discussed in ITK's Discourse, or fixes a regression test, provide the link. If it fixes a compiler error, provide a minimal verbatim message of the compiler error. If the commit closes an issue, use the GitHub issue closing keywords.

Keep in mind that the significant time is invested in reviewing commits and pull requests, so following these guidelines will greatly help the people doing reviews.

These guidelines are largely inspired by Chris Beam's How to Write a Commit Message post.

Share a Topic

When a topic is ready for review and possible inclusion, share it by pushing to GitHub and opening a pull request on the InsightSoftwareConsortium/ITK upstream repository.

Checkout the topic if it is not your current branch:

   $ git checkout my-topic

Check what commits will be pushed to GitHub for review:

   $ git prepush

Push commits in your topic branch for review by the community:

   $ git review-push --force

A URL will be provided in the terminal -- visit this url to review the topic and open a pull request.

Optionally, discuss the change by opening a topic on ITK's Discourse.

Test a Topic

When a topic is submitted, it is tested across the three major platforms before being merged thanks to the Azure DevOps Pipelines CI system, as well as the CDash GitHub Checks, and ITK Coding Style check.

If a platform configuration test failure appears to be a false positive, the test can be re-executed by adding a comment to the pull request with the content /azp run <ConfigurationName>. For example:

    /azp run ITK.Linux

After the topic has been merged, it is tested on many platforms and configurations on the nightly dashboard.

If tests fail on a submitted topic, see the Revise a Topic step on how to submit a revised version. After a topic is merged, please check the next day's nightly dashboard to ensure there are not any regressions. If there are any new warnings or errors, submit a follow-up patch as soon as possible.

Revise a Topic

Usually, a topic goes through several revisions in the review process. Once a topic is approved during GitHub review, proceed to the next step.

Checkout the topic if it is not your current branch:

   $ git checkout my-topic

To revise the most recent commit on the topic edit files and add changes normally and then amend the commit:

   $ git commit --amend

(You may visit the Pro Git: Changing the Last Commit resource in Git Help for further information on revising and rewriting your commit history.)

To revise commits further back on the topic, say the 3rd commit back:

   $ git rebase -i HEAD~3

(Substitute the correct number of commits back, as low as 1.)

Follow Git's interactive instructions.

Return to the Share a Topic step to share the revised topic.

(You may visit the Pro Git: Changing Multiple Commits resource in Git Help for further information on changing multiple commits -i.e. not only the last one, but further back in your history-, and the Pro Git: Rebasing resource on taking all the changes that were committed on one branch and replaying them on another one.)

Merge a Topic

Only authorized developers with GitHub merge permissions execute this step.

After a feature topic has been reviewed and approved in GitHub, ITK maintainers will merge it into the upstream repository via the GitHub user interface.

(If the merge conflicts follow the printed instructions to resolve them.)

For bug fixes that are ready to be included in the next patch release, make a comment on the pull request which states the topic should be merged to the release branch.

Here are the recommended steps to merge a topic to both release and master branches, assuming the topic branch is forked off the release branch:

   $ git checkout release
   $ git merge --no-ff my-topic
   $ git push upstream release

and do:

   $ git checkout master
   $ git merge --no-ff release
   $ git push upstream master

to merge the release branch back to master.

Delete a Topic

After a topic has been merged upstream, delete your local branch for the topic.

Checkout and update the master branch:

   $ git checkout master
   $ git pullall

Delete the local topic branch:

   $ git branch -d my-topic

The branch -d command works only when the topic branch has been correctly merged. Use -D instead of -d to force the deletion of an unmerged topic branch (warning: you could lose commits).

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