Contributing to OpenImageIO

Code contributions to OpenImageIO are always welcome, and nearly 250 people have done so over the years. Please review this document to get a briefing on our process.

General Tips for Open Source Development

GitHub's Open Source Guides
- Especially the guide on How to Contribute to Open Source

Mail List and Slack

oiio-dev For developers of the OpenImageIO code itself, or users who are really interested in the OIIO internals. This is where we mostly discuss the code (including bug reports), but are also happy to answer user questions about use or working of OIIO. You can sign up for the mail list on your own using the link above.
ASWF Slack has an openimageio channel. Sign up for the Slack on your own, then under "channels", select "browse channels" and you should see the openimageio channel (among those of the other projects and working groups).

How to Ask for Help

If you have trouble installing, building, or using OpenImageIO, but there's not yet a solid reason to suspect you've encountered a genuine bug, start by posting a question to the oiio-dev mailing list. This is the place for questions such has "How do I...".

Bug Reports and Issue Tracking

We use GitHub's issue tracking system for reporting bugs and requesting enhancements: https://github.com/AcademySoftwareFoundation/OpenImageIO/issues

If you are merely asking a question ("how do I..."), please do not file an issue, but instead ask the question on the oiio-dev mailing list or on the Slack channel.

If you are submitting a bug report, please be sure to note which version of OIIO you are using, on what platform (OS/version, which compiler you used, and any special build flags or other unusual environmental issues). Please give a specific, as-detailed-as-possible account of

what you tried (command line, source code example)
what you expected to happen
what actually happened (crash? error message? ran but didn't give the correct result?)

with enough detail that others can easily reproduce the problem just by following your instructions. Please quote the exact error message you received. If you are having trouble building, please post the full cmake output of a fresh VERBOSE=1 build.

Suspected security vulnerabilities should be reported by the same process.

If confidentiality precludes a public question or issue for any reason, you may contact us privately at security@openimageio.org.

Contributor License Agreement (CLA) and Intellectual Property

To protect the project -- and the contributors! -- we do require a Contributor License Agreement (CLA) for anybody submitting changes. This is for your own safety, as it prevents any possible future disputes between code authors and their employers or anyone else who might think they might own the IP output of the author.

Corporate CLA : If you are writing the code as part of your job, or if there is any possibility that your employers might think they own any intellectual property you create. This needs to be executed by someone who has signatory power for the company.
Individual CLA : If you are an individual writing the code on your own time, using your own equipment, and you're SURE you are the sole owner of any intellectual property you contribute.

Please note that it is extremely common (nearly ubiquitous in the US and Canada, and maybe other places) for full-time employees of technology and entertainment companies to have IP clauses in their employment agreement (in that pile of paperwork you sign on your first day of work and then promptly forget about) that give the company rights to any code you write, even on your own time. The OpenImageIO project expects you to know and follow the rules of your employer and to have them sign a corporate CLA if necessary.

The easiest way to sign CLAs is digitally using EasyCLA. There are detailed step-by-step instructions about using the EasyCLA system for corporate CLAs and individual CLAs.

Companies who prefer not to use the online tools may sign, scan, and email the executed copy to manager@lfprojects.org.

The corporate CLA allows a company to name a "CLA Manager" (who does not need signatory power) who has the ability to use the online system to add or delete individual employees of the company who are authorized to submit pull requests, without needing to get an executive to amend and sign the agreement each time.

Please note that these CLAs are based on the Apache 2.0 CLAs, and differ minimally, only as much as was required to correctly describe the EasyCLA process and our use of a CLA manager.

Contribution sign off

This project requires the use of the Developer’s Certificate of Origin 1.1 (DCO), which is the same mechanism that the Linux® Kernel and many other communities use to manage code contributions. The DCO is considered one of the simplest tools for sign offs from contributors as the representations are meant to be easy to read and indicating signoff is done as a part of the commit message.

Here is an example Signed-off-by line, which indicates that the submitter accepts the DCO:

Signed-off-by: John Doe <john.doe@example.com>

You can include this automatically when you commit a change to your local git repository using git commit -s. You might also want to leverage this command line tool for automatically adding the signoff message on commits.

Commit messages

Summary heuristic

Look at the commit history of the project to get a sense of the style and level of detail that is expected in commit messages.

General guidelines and expectations

The first line of the commit message should be a short (less than 80 characters) summary of the change, prefixed with the general nature of the change (see below).

The rest of the commit message should be a more detailed explanation of the changes. Some commits are self-explanatory and don't need more than the summary line. Others may need a more detailed explanation. Hallmarks of a good commit message include:

An explanation of why the change is necessary and what you hope to accomplish with it.
A description of any major user- or developer-facing changes that people should be aware of: changes in APIs or behaviors, new features, etc.
An explanation of any tricky implementation details or design decisions that might not be immediately obvious to someone reading the code.
Guideposts for somebody reviewing the code to understand the rationale and have any supporting background information to fully understanding the code changes.

Remember that at some point in the future, a developer unfamiliar with your change (maybe you, long after you've forgotten the details) might need to understand or fix your patch. Keep that person in mind as your audience and strive to write a commit message that explains the context in a way that saves them time and effort. Be the hero in the story of their future debugging effort!

Using "conventional commits" prefixes

We strive to follow the recommendations known as conventional commits, which means that we would like merged commit messages to have their first line start with one of the following prefixes:

feat: new features
fix: bug fixes
perf: performance improvements
api: changes to the public APIs
int: changes to code internals that don't affect the public API
build: changes to the build system
test: changes to the test suite or unit tests
ci: changes to the CI system
docs: changes to the documentation
refactor: code refactoring
style: formatting or other stylistic non-functional changes to the code
admin: project administration or policy changes
revert: reverting a previous commit

Obviously, some (if not most) PRs may qualify for more than one of these categories (for example, a new feature may also introduce a new API call, add tests, and include new documentation). If that is the case, use your best judgment to pick the category that best captures the essence or most important aspect of the change. When ambiguous, consider the list above to be a priority ranking (e.g., a change that fixes a bug and adds a new test should be listed under fix:, because that appears first in the list).

It is also encouraged, when it makes sense to do so, to put a subcategory in parenthesis after the prefix, like fix(exr): or feat(IBA):. It's ok to use obvious abbreviations for major classes or subsections: IB=ImageBuf, IBA=ImageBufAlgo, IC=ImageCache, TS=TextureSystem, etc. If there is no clear single format or class that is the man focus of the patch, then you can omit the subcategory.

API or ABI-breaking changes should additionally be marked with an exclamation point at the end of the prefix, like feat!: or api!: to make it easily identifiable as a breaking change from the first line of the commit message.

Pull Requests and Code Review

The best way to submit changes is via GitHub Pull Request. GitHub has a Pull Request Howto.

All code must be formally reviewed before being merged into the official repository. The protocol is like this:

Get a GitHub account, fork AcademySoftwareFoundation/OpenImageIO to create your own repository on GitHub, and then clone it to get a repository on your local machine.
Edit, compile, and test your changes. Run clang-format (see the instructions on coding style below).
Push your changes to your fork (each unrelated pull request to a separate "topic branch", please).
Make a "pull request" on GitHub for your patch.
If your patch will induce a major compatibility break, or has a design component that deserves extended discussion or debate among the wider OIIO community, then it may be prudent to email oiio-dev pointing everybody to the pull request URL and discussing any issues you think are important.
All pull requests automatically launch CI jobs on GitHub Actions to ensure that the build completes and that the tests suite runs correctly, for a variety of platform, compiler, library, and flag combinations. The status of the CI tests for your PR will be displayed on the GitHub PR page. We will not accept PRs that don't build cleanly or pass the existing testsuite.
The reviewer will look over the code and critique on the "comments" area. Reviewers may ask for changes, explain problems they found, congratulate the author on a clever solution, etc. But until somebody says "LGTM" (looks good to me), the code should not be committed. Sometimes this takes a few rounds of give and take. Please don't take it hard if your first try is not accepted. It happens to all of us.
After approval, one of the senior developers (with commit approval to the official main repository) will merge your fixes into the main branch.

Please see the Code Review document for more explanaton of the code review process.

Coding Style

File conventions

C++ implementation should be named *.cpp. Headers should be named .h. All headers should contain

#pragma once

All source files should begin with these three lines:

// Copyright Contributors to the OpenImageIO project.
// SPDX-License-Identifier: Apache-2.0
// https://github.com/AcademySoftwareFoundation/OpenImageIO

as a comment in the syntax of whatever source code is used in that file.

Occasionally a file may contain substantial code from another project and will also list its original copyright and license information. Do NOT alter that notice or copy it to any new files, it really only applies to the particular file in which it appears.

Formatting

We use clang-format to uniformly format our source code prior to PR submission. Make sure that clang-format is installed on your local machine, and just run

make clang-format

and it will automatically reformat your code according to the configuration file found in the .clang-format file at the root directory of the OIIO source code checkout.

One of the CI test matrix entries runs clang-format and fails if any diffs were generated (that is, if any of your code did not 100% conform to the .clang-format formatting configuration). If it fails, clicking on that test log will show you the diffs generated, so that you can easily correct it on your end and update the PR with the formatting fixes.

If you don't have clang-format set up on your machine, and your patch is not very long, you may find that it's more convenient to just submit it and hope for the best, and if it doesn't pass the CI test, look at the diffs in the log for the "clang-format" CI run and make the corrections by hand and then submit an update to the patch (i.e. relying on CI to run clang-format for you).

Because the basic formatting is automated by clang-format, we won't enumerate the rules here.

For the occasional non-clang-format regions of code, NEVER alter somebody else's code to reformat just because you found something that violates the rules. Let the group/author/leader know, and resist the temptation to change it yourself.

Each line of text in your code, including comments, should be at most 80 characters long. Exceptions are allowed for those rare cases where letting a line be longer (and wrapping on an 80-character window) is actually a better and clearer alternative than trying to split it into two lines. Sometimes this happens, but it's extremely rare.

We prefer three (3) consecutive empty lines between freestanding functions or class definitions, one blank line between method declarations within a class definition. Put a single blank line within a function if it helps to visually separate different sequential tasks, but don't put multiple blank lines in a row within a function, or blank lines right after an opening brace or right before a closing brace. The goal is to use just enough white space to help developers visually parse the code (for example, spotting at a glance where new functions begin), but not so much as to spread it out or be confusing.

Identifiers

In general, classes and templates should start with upper case and capitalize new words: class CustomerList; In general, local variables should start with lower case. Macros should be ALL_CAPS, if used at all.

If your class is extremely similar to, or modeled after, something in the standard library, or something else we interoperate with, it's ok to use their naming conventions. For example, very general utility classes and templates (the kind of thing you would normally find in std) should be lower case with underscores separating words, as they would be if they were standards.

template <class T> shared_ptr;
class scoped_mutex;

Names of data should generally be nouns. Functions/methods are trickier: a the name of a function that returns a value but has no side effects should be a noun, but a procedure that performs an action should be a verb.

Class structure

Try to avoid public data members, although there are some classes that serve a role similar to a simple C struct -- a very straightforward collection of data members. In these, it's fine to make the data members public and have clients set and get them directly.

Private member data should be named m_foo (alternately, it's ok to use the common practice of member data foo_, but don't mix the conventions within a class).

Private member data that needs public accessors should use the convention:

void foo (const T& newfoo) { m_foo = newfoo; }
const T& foo () const { return m_foo; }

Avoid multiple inheritance.

Namespaces: yes, use them!

Third-party libraries

Prefer C++17 std rather than other libraries, where both can do the same task.

Please do use Imath vector, matrix, and utility classes where applicable. Don't write your own vector or matrix code!

Use these libraries for OIIO internals, but please DO NOT require them in any of our main external APIs unless it's been thoroughly discussed and approved by the group.

Comments and Doxygen

Comment philosophy: try to be clear, try to help teach the reader what's going on in your code.

Prefer C++ comments (starting line with //) rather than C comments (/* ... */).

For public APIs we tend to use Doxygen-style comments (start with ///). They looks like this:

/// Explanation of a class.  Note THREE slashes!
class myclass {
    ....
    float foo;  ///< Doxygen comments on same line look like this
}

Miscellaneous

Macros should be used only rarely -- prefer inline functions, templates, enums, or "const" values wherever possible.

Prefer std::unique_ptr over raw new/delete.

Bottom Line

When in doubt, look elsewhere in the code base for examples of similar structures and try to format your code in the same manner, or ask on the oiio-dev mail list.

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