Add PNG dWLm (Diffuse white luminance metadata ) chunk to conform to ISO 22028-5

[digitaltvguy]

ISO 22028-5 High dynamic range and wide colour gamut encoding for still images (HDR/WCG) identifies a very important parameter (Diffuse white luminance metadata ).

This metadata allows us to determine at which point highlight compression might occur during HDR to SDR conversion.

[palemieux]

What is the relationship between diffuse white luminance metadata and reference white specified in Report ITU-R BT.2408?

[digitaltvguy]

Sounds like this will get added for 4th Edition.

[ProgramMax]

It sounds like ISO 22028-5 will be paywalled when published.
Is ITU-R BT.2408 effectively the same (and not paywalled)?

[svgeesus]

[svgeesus]

What is the relationship between diffuse white luminance metadata and reference white

In the new version of 22028-5, what was previously called Reference White is now renamed to HDR Reference White for clarity.

BT.2048 already defines, in section 2.1, HDR Reference White and does so the same as 22028-5.

[svgeesus]

So in practice, the metadata is a way to override the definition in BT.2048, ie when the luminance is not 203

[leo-barnes]

So in practice, the metadata is a way to override the definition in BT.2048, ie when the luminance is not 203

Yep, pretty much. IIRC 203 nits is just a default, but before there was not really any way to signal anything other than the default.