DRAFT PNG Working Group Charter

The mission of the PNG Working Group is to maintain and develop the Portable Network Graphics (PNG) specification.

Join the PNG Working Group.

This draft charter is available on GitHub. Feel free to raise issues.

Charter Status See the group status page and detailed change history.
Start date [dd monthname yyyy] (date of the "Call for Participation", when the charter is approved)
End date [dd monthname yyyy] (Start date + 2 years)
Chairs Chris Blume (W3C Invited Experts)
Team Contact Chris Lilley (0.1 FTE)
Meeting Schedule Teleconferences: topic-specific calls may be held as needed.
Face-to-face: we may meet during the W3C's annual Technical Plenary week; additional face-to-face meetings may be scheduled by consent of the participants, usually no more than 3 per year.

Motivation and Background

PNG Third Edition, which is a W3C Recommendation, was a major update. It is widely implemented. PNG Fourth Edition is planned to be a smaller update, focussed on improving HDR & Standard Dynamic Range (SDR) interoperability. In parallel, the group will do longer-term work on researching compression updates (including parallel encoding and decoding) for PNG Fifth Edition.

Scope

Maintain the PNG specification, folding in solutions to reported errata.

Add selected new features which will be broadly useful, are demanded by users, and have significant implementer interest. High Dynamic Range (HDR) and improved compression/decompression (both in filesize and in time taken, including parallel processing) are of particular interest, as is the addition of useful metadata aligning with other industry standards.

Work to ensure the PNG specification is well tested and widely adopted.

When developing any new features, the stability and interoperability with existing image files and image creation and consuming software will be a primary consideration.

Out of Scope

The following features are out of scope, and will not be addressed by this Working Group.

    Changes that will invalidate existing files, editors, or viewers that conform to Portable Network Graphics (PNG) Specification (Third Edition).

Deliverables

Updated document status is available on the group publication status page.

Draft state indicates the state of the deliverable at the time of the charter approval. Expected completion indicates when the deliverable is projected to become a Recommendation, or otherwise reach a stable state.

Normative Specifications

The Working Group will deliver the following W3C normative specifications:

PNG Fourth Edition

This specification will define improvements to HDR & SDR interoperability for PNG.

Draft state: Editor's Draft

Expected completion: Q3 2026

Initial Text: Portable Network Graphics (PNG) Specification (Third Edition)
https://www.w3.org/TR/2025/REC-png-3-20250624/
24 June 2025

PNG Fifth Edition

This specification will define improvements to the compression and decompression of PNG.

Draft state: No draft

Expected completion: Q4 2028

Initial Text: Portable Network Graphics (PNG) Specification (Third Edition)
https://www.w3.org/TR/2025/REC-png-3-20250624/
24 June 2025

Standards

The Working Group will maintain the following W3C standards:

Portable Network Graphics (PNG) Specification (Third Edition)

This document describes PNG (Portable Network Graphics), an extensible file format for the lossless, portable, well-compressed storage of static and animated raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-colour, greyscale, and truecolour images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits.

Latest publication: 2025-06-24

Other Deliverables

Other non-normative documents may be created such as:

  • Use case and requirement documents;
  • Test suite and implementation report for the specification;
  • Primer or Best Practice documents to support web developers when designing applications.

Timeline

  • February 2026: FPWD for PNG Fourth Edition
  • September 2026: Candidate Recommendation for PNG Fourth Edition
  • September 2026: Requirements and Use Cases for PNG Fifth Edition
  • September 2027: FPWD of PNG Fifth Edition

Success Criteria

In order to advance beyond Candidate Recommendation, each normative specification is expected to have at least two independent interoperable implementations of every feature defined in the specification, where interoperability can be verified by passing open test suites. In order to advance beyond Candidate Recommendation, each normative specification must have an open test suite of every feature defined in the specification.

There should be testing plans for each specification, starting from the earliest drafts.

To promote interoperability, all changes made to specifications in Candidate Recommendation or to features that have deployed implementations should have tests. Testing efforts should be conducted via the Web Platform Tests project.

Each specification should contain separate sections detailing all known security and privacy implications for implementers, Web authors, and end users.

Each specification should contain a section on accessibility that describes the benefits and impacts, including ways specification features can be used to address them, and recommendations for maximising accessibility in implementations.

This group is expected to be guided by the following documents:

Coordination

For all specifications, this Working Group will seek horizontal review for accessibility, internationalization, privacy, and security with the relevant Working and Interest Groups, and with the TAG. Invitation for review must be issued during each major standards-track document transition, including FPWD. The Working Group is encouraged to engage collaboratively with the horizontal review groups throughout development of each specification. The Working Group is advised to seek a review at least 3 months before first entering CR and is encouraged to proactively notify the horizontal review groups when major changes occur in a specification following a review.

Additional technical coordination with the following Groups will be made, per the W3C Process Document:

W3C Groups

Color on the Web Community Group
Ensure that PNG meets the needs for Wide Color Gamut (WCG) and High Dynamic Range (HDR) lossless raster imagery.

External Organizations

Libpng maintainers
Development will be undertaken in close cooperation with the maintainers of the libpng library, particularly regarding implementability, backwards compatibility, stability and security.
ISO/TC 42 Photography
Coordination in the area of High Dynamic Range (HDR) imagery, via the existing Class C liaison.
Society of Motion Picture and Television Engineers
The PNG Working Group will work with SMPTE in the area of High Dynamic Range (HDR) imagery.

Participation

To be successful, this Working Group is expected to have 6 or more active participants for its duration, including representatives from the key implementors of this specification, and active Editors and Test Leads for each specification. The Chairs, specification Editors, and Test Leads are expected to contribute half of a working day per week towards the Working Group. There is no minimum requirement for other Participants.

The group encourages questions, comments and issues on its public mailing lists and document repositories, as described in Communication. The group also welcomes non-Members to make technical contributions for ongoing work, provided they agree to the terms of the W3C Patent Policy.

The Chairs should periodically look through the non-Members who have contributed to the Working Group and consider whether each one should be invited to participate as an Invited Expert. If a non-Member contributor would like to participate in meetings, they are encouraged to apply to be an Invited Expert.

Participants in the group are required (by the W3C Process) to follow the W3C Code of Conduct.

Communication

Technical discussions for this Working Group are conducted in public: the meeting minutes from teleconference and face-to-face meetings will be archived for public review, and technical discussions and issue tracking will be conducted in a manner that can be both read and written to by the general public. Working Drafts and Editor's Drafts of specifications will be developed in public repositories and may permit direct public contribution requests. The meetings themselves are not open to public participation, however.

Information about the group (including details about deliverables, issues, actions, status, participants, and meetings) will be available from the PNG Working Group home page.

Most PNG Working Group teleconferences will focus on discussion of particular specifications, and will be conducted on an as-needed basis.

This group primarily conducts its technical work on GitHub issues. The public is invited to review, discuss and contribute to this work.

Decision Policy

This group will seek to make decisions through consensus and due process, per the W3C Process Document (section 5.2.1, Consensus). Typically, an editor or other participant makes an initial proposal, which is then refined in discussion with members of the group and other reviewers, and consensus emerges with little formal voting being required.

However, if a decision is necessary for timely progress and consensus is not achieved after careful consideration of the range of views presented, the Chairs may call for a group vote and record a decision along with any objections.

To afford asynchronous decisions and organizational deliberation, any resolution (including publication decisions) taken in a face-to-face meeting or teleconference will be considered provisional. A call for consensus (CfC) will be issued for all resolutions (for example, via email, GitHub issue or web-based survey), with a response period from one week , depending on the chair's evaluation of the group consensus on the issue. If no objections are raised by the end of the response period, the resolution will be considered to have consensus as a resolution of the Working Group.

All decisions made by the group should be considered resolved unless and until new information becomes available or unless reopened at the discretion of the Chairs.

This charter is written in accordance with the W3C Process Document (Section 5.2.3, Deciding by Vote) and includes no voting procedures beyond what the Process Document requires.

Patent Policy

This Working Group operates under the W3C Patent Policy (Version of 15 May 2025). To promote the widest adoption of Web standards, W3C seeks to issue Web specifications that can be implemented, according to this policy, on a Royalty-Free basis. For more information about disclosure obligations for this group, please see the licensing information.

Licensing

This Working Group will use the W3C Software and Document license for all its deliverables.

About this Charter

This charter has been created according to section 3.4 of the Process Document. In the event of a conflict between this document or the provisions of any charter and the W3C Process, the W3C Process shall take precedence.

Charter History

The following table lists details of all changes from the initial charter, per the W3C Process Document (section 4.3, Advisory Committee Review of a Charter):

Charter Period Start Date End Date Changes
Initial Charter 14 September 2021 31 August 2023 (none)
Charter Extension 14 August 2023 30 November 2023 none
Rechartered 23 October 2023 30 October 2025

Added PNG 4th Edition

Rechartered (this proposed charter) [dd monthname yyyy] [dd monthname yyyy]

Added PNG 5th Edition

Change log

Changes to this document are documented in this section.