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hdr-in-png-requirements.md

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Adding support for HDR imagery to the PNG format

Editors: Chris Blume, Pierre-Anthony Lemieux, Chris Seeger, Leonard Rosenthol

Status: Draft

Problem to be solved

The gAMA chunk of the Portable Network Graphics (PNG) format (specified in [PNG]) parameterized the transfer function of the image as a power law. As such, it cannot model the Reference PQ or HLG OOTFs specified in [BT2100], which are commonly used for HDR images.

An existing W3C group note, [BT2100-in-PNG] specifies an approach which is limited: it supports signaling of only the ITU BT.2100 PQ EOTF and uses magic values in the iCCP chunk to signal color spaces.

Basic requirements

  • does not break current implementations
  • extensible signaling of color space based on H.273
  • does not require the presence of iCCP chunk and embedded ICC profiles

Non Requirements

  • bit-identical serialization of the H.273 color space parameters as used by other raster image formats (eg. JPEG, AVIF)

Strawman approach

Two new PNG chunks are proposed, the cICP chunk and the iCCN chunk.

The cICP chunk acts as a color space label (much like the existing sRGB chunk), specifying the color space to which the pixels within the PNG file conforms. The iCCN chunk (much like the existing iCCP) contains an embedded color profile.

A PNG may contain multiple chunks with color space information. A PNG viewer should use the highest priority color space chunk that it can honor, ignoring the others. The priorities are from highest to lowest:

  • cICP
  • iCCN
  • sRGB
  • iCCP
  • gAMA & cHRM

If the PNG decoder knows the display / surface cannot use the cICP code point, it should ignore the cICP chunk and continue down the priority list.

cICP chunk

This chunk SHALL come before the IDAT chunk.

H.273 specifies a controlled vocabulary for the parameterization of color space information.

Define a cICP chunk that contains the 4 bytes necessary to carry the H.273 color space parameters:

  • COLPRIMS, 1 byte, One of the ColourPrimaries enumerated values specified in Rec. ITU-T H.273 | [ISO/IEC 23091-2]
  • TRANSFC, 1 byte, One of the TransferCharacteristics enumerated values specified in Rec. ITU-T H.273 | [ISO/IEC 23091-2]
  • MATCOEFFS, 1 byte, One of the MatrixCoefficients enumerated values specified in Rec. ITU-T H.273 | [ISO/IEC 23091-2]
  • VIDFRNG, 1 byte, Value of the VideoFullRangeFlag specified in Rec. ITU-T H.273 | [ISO/IEC 23091-2]

NOTE: While these are inspired from recent JPEG standards (eg. JPEG-XL) that incorporate these color space parameters, this specification follows H.273.

NOTE: ITU-T Series H Supplement 19 summarize combinations of H.273 parameters corresponding to common baseband linear broadcasts and file-based Video-on-Demand(VOD) services.

iCCN chunk Embedded ICC profile (updated)

This chunk SHALL come before the IDAT chunk.

Structure

The iCCN chunk contains:

Name Size
Profile name 1-79 bytes (character string)
Null separator 1 byte (null character)
Compression method 1 byte
Compressed profile n bytes

The profile name may be any convenient name for referring to the profile. It is case-sensitive. Profile names shall be encoded as UTF-8. Leading, trailing, and consecutive spaces are not permitted. The profile name shall not contain a zero byte (null character).

The compression method shall be method 0 (zlib datastream with deflate compression). The compression method entry is followed by a compressed datastream of an ICC profile as defined in [ICC] or [ICC-2010].

The ICC profile shall either be an output profile (Device Class = prtr) or a monitor profile (Device Class = mntr). Decompression of this datastream yields the embedded ICC profile.

NOTE: This is exactly the same as iCCP except:

  • iCCP is ICCv2 (although many decoders treat it as ICCv4) while iCCN is explicitly ICCv4
  • the profile name is UTF-8 instead of Latin-1. Analogous to tEXt vs. iTXt

Encoder

If the iCCN chunk is present, the image samples conform to the colour space represented by the embedded ICC profile. The colour space of the ICC profile shall be an RGB (RGB ) colour space for colour images (PNG colour types 2, 3, and 6), or a greyscale (GRAY) colour space for greyscale images (PNG colour types 0 and 4). A PNG encoder that writes the iCCN chunk is encouraged to also write gAMA and cHRM chunks that approximate the ICC profile, to provide compatibility with applications that do not use the iCCN chunk.

Decoder

The iCCN chunk should be interpreted according to [ICC] or [ICC-2010] as appropriate.

References

[ICC] ISO 15076-1:2010, Image technology colour management – Architecture, profile format and data structure — Part 1: Based on ICC.1:2010

[ICC-2010] Specification ICC.1:2010-12 (Profile version 4.3.0.0) Image technology colour management - Architecture, profile format, and data structure

ITU-T Series H Supplement 19. Series H: Audiovisual and multimedia systems - Usage of video signal type code points

[BT2100] Recommendation ITU-R BT.2100, Image parameter values for high dynamic range television for use in production and international programme exchange

[ITU-T H.273] Technical Document ITU-T H.273, Color Independent Coding Points for Images

[PNG] Portable Network Graphics (PNG) Specification (Second Edition). Tom Lane. W3C. 10 November 2003. W3C Recommendation. URL: https://www.w3.org/TR/PNG

[BT2100-in-PNG] Using the ITU BT.2100 PQ EOTF with the PNG format

[ISO/IEC 23091-2] ISO/IEC 23091-2