PNG, Third Edition:
Implementation Report

2. Coding Independent Code Points (CICP) from ITU-R H.273

Originally developed for color space signalling in video workflows, CICP is also used in image formats such as HEIF, AVIF and JPEG-XL. CICP is more compact, and designed to signal the use of specific mathematically represented color primaries, transfer functions, matrix coefficients, and signal quantizations vs ICC profiles, which were originally created to match color between different mediums (display, print, scan). PNG Third Edition adds support for CICP metadata.

Implemented in Blink-based browsers (Google Chrome, Microsoft Edge, Samsung Internet, etc), and in WebKit-based browsers where it uses the macOS or iOS platform to implement CICP.

Mozilla Firefox does not yet implement CICP in PNG; they do however implement CICP for AVIF images, and have stated they will implement it in PNG as well.

Implemented in FFMPEG-dev for reading and writing PNG:

• The cICP chunk allows the PNG color space to be tagged with any of the enum values in H.273, not just sRGB, and it is defined in the working draft of the PNG specification (3rd edition). libavutil/pixfmt.h uses exact the same values as H.273 so these are translated as-is. This commit also adds support for writing the cICP chunk for video frames tagged as anything that isn't sRGB. It still writes cHRM and gAMA as a fallback.

Implemented in Affinity Photo 2

• Includes 32-bit HDR support with an Alpha Channel (PNG specification - Third edition) for interchanging HDR broadcast imagery in a lossless format. CICP's Full-Range-Flag is supported to differentiate between Full/Narrow Range quantizations. Legacy PNG files exported from Photoshop with CICP data embedded in an ICC profile can be imported.

Implemented in Avid Media Composer

Implemented in MediaInfo from MediaArea

• MediaInfo is used throughout the broadcast industry and via the CLI version within softaware. MediaInfo Parses PNG 3rd Edition cICP, mDCv, cLLi metadata into logical values

Implemented in Pixelmator Pro 3.5 and Photomator 3.3 image editors.

Implemented in Adobe Photoshop, Lightroom and Camera Raw, MacOS ProApps, Compressor, Final Cut Pro

Implemented in libavif for export of AVIF to PNG and import from PNG into AVIF:

• Writes primaries and transfer characteritics info in decoded png

3. Priority of color space information

In the case where more than one source of color space information is provided, PNG Third Edition now defines the priority order.

4. Mastering Display Color Volume

This metadata, which is commonly found with HDR content in the BT.2100-PQ colorspace, describes the color characteristics of the monitor used to create the content.

PNG Third Edition adds the mDCv chunk for this purpose.

Implemented in Blink-based browsers (Google Chrome, Microsoft Edge, Samsung Internet, etc):

• The PNG specification now includes “mDCv” and “cLLi” chunks that can encode mastering and content light level metadata. This change exposes such metadata so that it may be used for tone mapping.

Implemented in FFMPEG v2 for reading and writing PNG:

• This adds support for cLLi and mDVc chunks in the PNG specification

Implemented in a JavaScript PNG file chunk inspector:

Tone-mapping to the consumer display can benefit from knowing this metadata; it is also required for HDR10 compatibility. Currently there is no mandated tone mapping operator and thus the results of tone-mapping cannot be verified in an automatic test.

MediaInfo (see cICP section) also parses the mDCv chunk.

5. Content Light Level Information

This metadata, which is commonly found with HDR content in the BT.2100-PQ colorspace, describes the maximum value of the frame average light level (MaxFALL) and the Maximum Content Light Level (MaxCLL). Knowing this information up-front is required for consistent and timely tone mapping, particularly for long animation sequences, where computing it for all frames in real time is impractical.

PNG Third Edition adds the cLLI chunk for this purpose.

Implemented in Blink-based browsers (Google Chrome, Microsoft Edge, Samsung Internet, etc):

• The PNG specification now includes “mDCv” and “cLLi” chunks that can encode mastering and content light level metadata. This change exposes such metadata so that it may be used for tone mapping.

Implemented in FFMPEG v2 for reading and writing PNG:

• This adds support for cLLi and mDVc chunks in the PNG specification

Implemented in a JavaScript PNG file chunk inspector:

Tone-mapping to the consumer display can benefit from knowing this metadata; it is also required for HDR10 compatibility. Currently there is no mandated tone mapping operator and thus the results of tone-mapping cannot be verified in an automatic test.

MediaInfo (see cICP section) also parses the cLLi chunk.

6. ICC profile version

PNG Second Edition only referenced the then-current v2 of International Color Consortium (ICC) profiles, although it had since become industry practice to also use higher versions such as 4.3.

PNG Third Edition clarified that the iCCP chunk, which contains the ICC profile, can contain profiles conforming to any version of the ICC.1 specification.

(There is also a newer ICC.2 specification, known as ICCMax; this is not currently supported in browsers, and PNG does not currently provide a way to embed ICC.2 profiles).

7. Exchangeable Image File (Exif)

Widely used in other image formats such as JPEG/JFIF, EXIF metadata is also available in PNG. PNG Third Edition moves the eXIf chunk, which was already in the PNG Extensions document, into the main specification.

Exif mostly contains metadata (copyright, lens focal length) which does not affect the display of the image.

The Web Platform places a constraint on Exif image metadata that affects layout, such as image orientation: it is required to be placed before the image data. PNG Third Edition enforces that constraint, for Exif in PNG.

Respecting Exif image orientation for PNG images in CSS is implemented in WebKit-based browsers.

Respecting Exif image orientation for PNG images in Canvas 2D is implemented in WebKit-based browsers and Blink-based browsers.

Blink-based browsers (Google Chrome, Microsoft Edge, Samsung Internet, etc), and Mozilla Firefox implement Exif orientation in CSS for JPEG/JFIF images, and are expected to implement it for PNG as well. (Chrome bug and Firefox bug).