Re: [EXTERNAL] Re: [PNG] Cancel upcoming meeting?

On Fri, Feb 28, 2025 at 3:31 PM Seeger, Chris (NBCUniversal) <
Chris.Seeger@nbcuni.com> wrote:

> I think I mis-read and thought it was diffuse white metadata.  I blame
> myself :-)
>

Very old fashioned :-)  21496 is at DIS stage so maybe you had an earlier
WD?   My assumption about the ISO DIS stage is that it's all over apart
from the shouting; IRC it goes to industry representatives at the DIS
stage, or maybe just before.  Annex-C contains general "binary metadata
payload" definitions (C.2)

There are a couple of interesting things about it that can be deduced with,
I think, reasonable reliability from the ToC.  First is that Annex A
describes (informatively) the derivation of the gain map from _two_ input
images; an HDR and the desired SDR (the "Alternate" image).  So a cynical
person like me would just describe this as a way of transmitting the SDR
with high compression.  Annex A is informative so it is also possible, in
theory, to derive the gain map directly, possibly computationally, from the
original HDR; computational tone-mapping or, in photographic terms, a
computational approach to dodging and burning.

The second is that the gain map is a set of scale factors for linear RGB
(3.3 and 3.4) to convert the HDR to/from the SDR and, also, between the
endpoints of the two colour spaces (from the "Note 1" in 3.4).  This
implies that the standard encodes some amount of chromatic correction; the
white points of the HDR and SDR may be different.  There is more discussion
in Annex B of "colour conversion" but in general the requirements of
chromatic adaptation cannot be handled completely just by scaling R/G/B.
Well, IRC; simple scaling is the original Von Kries approach isn't it
ChrisL?  (I might very well have got that bit of arithmetic wrong!)

John Bowler