yes, this is my first post ever to aces central.
(couldn’t resist - middle grey is my favorite topic.)
(it is also, for the moment, my last post to aces central.)
the mapping of 0.18 in scene space to 0.10 in sdr output referred space
has a long history. in film, a “perfect” 18% grey onset exposure
resulted in a “LAD” (laboratory aim density) on the negative, and this
was printed to a “LAD” of approximately 1.00 over base on the print.
as density == log10(1/transmittance, a density of 1.00 allowed exactly
(wait for it…) 1/10 of the light through the film print -
thus resulting in 0.10 of peak luminance in the theatre.
this 10% peak luminamce worked out fine for sdr rec709 home video as well.
one of the most amazing confirmations of this being a “good choice”
had to do with the STEM material - this was a bunch of material shot by
ASC DP’s as an early test for the assessment of digital cinema projection.
this wasn’t shot and printed with grey charts - it was lots of different
setups (day/night/warm/cool/etc) that were all graded to “look really good”.
a few years later, lars borg did an analysis of the digital cinema output
referred STEM material - and he calculated that the average linear floating
point RGB pixel values for the ENTIRE stem material was 0.11 0.09 0.11.
- this is really frakking close to 0.10
- it’s slightly magenta because the DCI white point is slightly green -
so this green bias was graded out - just by eye - so that the
results “look really good”.
so that target of 0.10 peak luminance for traditional output referred
displays happens to be…well…let’s say “correct”.
(and the fact that we’re calling 0.18 reflectance the “average scene
luminance” is another talk for another time…)
now we come to the horror^H^H^H^H^H^H joy which we call “HDR”:
- there is no standard “peak luminance” - you can get so-called HDR
displays that peak anywhere from 200 to 3000 nits. and the image is
supposed to “look right” on all of these. - the creative community is split - one half wants their HDR to look
basically the same as their SDR with some extra highlight (and occasionally
shadow) detail, while the other half wants to flaunt and revel in
the total HDR sandbox (“if you’re gonna give me that space to work in,
i’m gonna take full advantage of it!”)
so we have a conundrum. the “7.2” and “15” nit targets mentioned by scott
dyer elsewhere in this discussion were essentially a compromise between
the two creative strategies outlined above. and this may be the best we
can do right now - arrive at a “compromise” that will allow the creatives
to get to where they want to go without too much effort.
hopefully, HDR displays will coalesce around some nominal peak luminance,
and then we would be able to discuss “Where should mid-gray end up through
an Output Transform?” for the moment, we should keep 10% of peak luminance
for SDR. maybe once we have a large number of HDR releases of different
subject matter to analyze, lars borg can repeat his analysis and we can
thus determine where the creatives think mid grey should end up in HDR.
-joshie