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.