This specification defines a mechanism for storing images that use the Reference PQ EOTF specified in [[!BT2100-1]] in the Portable Network Graphics (PNG) format (specified in [[!PNG]]), without modification to existing chunks or the introduction of new chunks.

This specification is intended to be published as a Working Group Note.

Introduction

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 [[!BT2100-1]], 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 [[!BT2100-1]]. 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.

PQ HDR PNG File

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 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
[[!BT2100-1]], 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

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.

This specification does not constrain bit depth.

Privacy and Security Considerations

See Section 13.4 at [[!PNG]].