After more than twenty years: Time for a new PNG

The third edition of Portable Network Graphics (PNG) has now been officially published, as announced by the W3C. Three new areas are central: Support for animated images, HDR (High Dynamic Range) support and Exif information. PNG is widely used as a web standard, but the last update to the standard was over twenty years ago. Version 1.2 was released on August 11, 1999, and became the second edition of the W3C recommendation on November 10, 2003.

APNGs officially on board

Although APNG (Animated Portable Network Graphics) support is new, the concept behind it is not: the format emerged as early as 2004 and gained significantly more momentum from 2008, when Mozilla supported it natively in its Firefox web browser. Since 2017, Chromium-based browsers have also been able to display animated PNGs. Officially, however, this was not yet part of PNG, which was explicitly not intended as a GIF replacement at the beginning. Multiple-Image Network Graphics (MNG) was supposed to take over this legacy, but was unable to establish itself.

Behind the HDR support is actually the fact that PNG adds Coding Independent Code Points (CICP) for color space identification with the third edition. CICP is a specification that can be used to specify which color spaces an image uses. However, it does not include how to handle this color space. The advantage is that the implementation is lightweight – CICP stores the relevant information in just four numbers. Interested readers can find technical details on this from Chris Lilley, one of the co-authors of the original PNG standard and currently Technical Director at the W3C. CICP originates from the broadcasting sector, but apart from moving images, it is now also used for other image formats such as JPEG XL.

However, as Chris Blume, Chairman of the W3C PNG Working Group, explains, the new HDR support can also be traced back to the broadcasting sector. The W3C Timed Text Working Group, the BBC, Comcast and NBCUniversal as well as MovieLabs were interested in updating the specification to display subtitles and banners. According to Blume, some of these media companies are already working on implementing these features via HDR. Information on this can be found on his website.

PNGs for photography

Finally, PNG now supports Exif data. The Exchangeable Image File Format can be used to store meta information such as the creator or exposure information for photos in the image. The standard is used in digital photography in particular, but is not limited to it.

In addition to the companies mentioned, Adobe, Apple and Google are also involved in the further development of PNG. Blume states that work on the next update is already underway. While the fourth edition is intended to improve the interoperability of HDR and SDR (Standard Dynamic Range), he promises a major change for the fifth edition: PNG is to receive better compression. The developers are still discussing what this means for compatibility with the previous format.

All information on the new PNG specification can be found at the W3C.

(fo)