Using the ITU BT.2100 PQ EOTF with the PNG Format (original) (raw)
Abstract
This specification defines a mechanism for storing images that use the Reference PQ EOTF specified in [ITU-R-BT.2100] in the Portable Network Graphics (PNG) format (specified in [PNG]), without modification to existing chunks or the introduction of new chunks.
Status of This Document
This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in theW3C standards and drafts index at https://www.w3.org/TR/.
Note: This specification is deprecated and should not be used for new applications.
Applications are instead encouraged to consider using theThird Edition of the PNG specification which adds explicit support for HDR imagery, including images encoded according to Rec. ITU-R BT.2100. Signaling of HDR images is achieved using the cICP chunk which uses the common Coding Independent Code Points specified in Rec. ITU-T H.273. For example, images that use the BT.2100 PQ color encoding are signaled with the cICP chunk components values 9 16 0 1. In addition, the Mastering Display Color Volume andContent Light Level Information can be provided.
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This document is governed by the03 November 2023 W3C Process Document.
Table of Contents
- Abstract
- Status of This Document
- 1. Conformance
- 2. Introduction
- 3. PQ HDR PNG File
- 4. Privacy and Security Considerations
- A. References
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words SHALL and SHOULD in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This section is non-normative.
The gAMA chunk of the Portable Network Graphics (PNG) format (specified in [PNG]) parameterizes the transfer function of the image as a power law. As such, it cannot model the Reference PQ EOTF specified in [ITU-R-BT.2100], which is commonly used for HDR images.
This specification uses the existing iCCP chunk to unambiguously signal the color system of an image that uses the Reference PQ EOTF specified in [ITU-R-BT.2100]. It also allows graceful processing by decoders that do not conform to this specification by recommending fallback values for the gAMA chunk, cHRM chunk, and embedded ICC profile.
The file SHALL conform to [PNG].
The image type SHALL be truecolor.
The image SHALL conform to one of the Pixel Encoding Systems specified in the table below.
The file SHALL contain an iCCP chunk, constrained as follows:
- the profile name of the
iCCPchunk SHALL be set as specified by the table below according to the Pixel Encoding System of the image; and - the embedded ICC profile of the
iCCPchunk SHOULD contained one of the ICC profiles specified by the table below according to the Pixel Encoding System of the image; and - if present, the embedded ICC profile SHALL be compressed using Method 0, as specified in [PNG].
The cHRM chunk and the gAMA chunk of the file SHOULD be set as specified by the table below according to the Pixel Encoding System of the image.
Pixel Encoding Systems
| Pixel Encoding System | iCCP Chunk Profile Name | iCCP Chunk Embedded ICC Profile(s) | gAMA Chunk | cHRM Chunk |
|---|---|---|---|---|
| [ITU-R-BT.2100], using the Reference PQ EOTF and full-range quantization | ITUR_2100_PQ_FULL | ITUR_2100_PQ_FULL.icc | 15000 | Parameter Value White point x 31270 White point y 32900 Red x 70800 Red y 29200 Green x 17000 Green y 79700 Blue x 13100 Blue y 4600 |
Note
The gAMA chunk and embedded ICC profile specified in the table above are not accurate, and their processing is not required to reproduce the image as intended by the author. They are provided as fallback for implementations that do not recognize the semantics associated with a iCCP Chunk Profile Names defined by this specification. Future versions of this specification might include additional ICC profiles, e.g. to reflect improvements in ICC profile capabilities.
Note
This specification does not constrain bit depth.
This section is non-normative.
See Section 13.4 at [PNG].
[ITU-R-BT.2100]
Recommendation ITU-R BT.2100-2 (07/2018), Image parameter values for high dynamic range television for use in production and international programme exchange. ITU. 9 July 2018. Recommendation. URL: https://www.itu.int/rec/R-REC-BT.2100/
[PNG]
Portable Network Graphics (PNG) Specification (Third Edition). Chris Lilley; Leonard Rosenthol; Pierre-Anthony Lemieux; Chris Needham; Simon Thompson; Chris Seeger; Chris Blume; Cosmin Truta. W3C. 15 May 2025. W3C Proposed Recommendation. URL: https://www.w3.org/TR/png-3/
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc2119
[RFC8174]
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc8174