PNG (Portable Network Graphics) Register and Extensions

The PNG Specification supports a backward-compatible extension mechanism that need not involve changes to the core specification itself. The documents on this page provide a registry and a specification of such public extensions. (A similar mechanism exists for private extensions, but, with few exceptions, they are not intended to become public and, in any case, are not included here.)

The Register of PNG Public Chunks and Keywords contains a list of all public chunks and text keywords, including their registration dates and indication of where they are specified (i.e., the core PNG specification or the PNG extensions document).

The Extensions to the PNG Specification (also often referred to as the "PNG extensions document") contains a formal specification of those chunks and text keywords that were either not deemed appropriate for inclusion in the core PNG spec or have not yet completed the (lengthy) process for such inclusion.

Note that Chapter 11 of PNG: The Definitive Guide complements the Register and the Extensions document by describing most PNG chunks and keywords in a somewhat less formal, more expository manner. (The remaining chunks are described in Chapter 8 and Chapter 10.) The book has no formal standing, however, and in the case of discrepancies, the documents below are considered definitive (as it were).