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 the
W3C 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 the
Third 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 and
Content Light Level
Information can be provided.
This Group Note is endorsed by
the Timed Text Working Group, but is not endorsed by
W3C itself nor its
Members.
This is a draft document and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to cite this document as other
than work in progress.
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does not carry any licensing requirements or commitments on this
document.
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 in
BCP 14
[RFC2119] [RFC8174]
when, and only when, they appear in all
capitals, as shown here.
2. Introduction
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 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 iCCP chunk 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 iCCP chunk 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
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.