Yes, that is exactly what I am doing.
I am able to target any input to match any white point or the mid grey XYZ values by adjusting the output diffuse white XYZ values.
I did not think this would work, and still not 100% that this is the “correct” way to do it.
But it is extremely effective which pushes my confidence into the 95-99%, especially with the match between ACES 1.2 SDR and HDR.
I first tried only adjusting the Luminance level (La/L_A) but it did very little to affect the XYZ output, I think it has strongest role in the full or partial adaptation to another white point but no affect on where the targeted white point lands in XYZ upon output.
I have done many more tests doing exactly this: SDR=>HDR and HDR=>SDR for ARRI Reveal, ACES 1.2, and of course the Current ACES 2.0 candidate (035).
These tests are interesting are interesting because they can demonstrate different approaches to matching SDR/HDR output. They also become more interesting with the 10 stops over/ 10 stops under colorchecker image.
If there is one particular comparison that someone is curious about I likely already have an image and maybe some data as well and could provide a post.
What I wanted to understand first is what should an appearance match look like between two different luminance levels (according to current CAMs), and how do the values change both inside and outside the model when that happens.
Regarding input/output parameters. If the input and output match there is no effect, so the current model, which does everything inside, is not “wrong” in any way by using Dim in and Dim out, and works as intended.
I used Average in/Average out as a baseline/placebo, since I was transforming Scene Linear ACES values directly to Dim and Dark conditions to see what those scene linear values should look like under those conditions, so that some objective targets could possibly emerge, especially regarding saturation.
The close correspondence with Average in and Dark out with ACES 1.2 (SDR and HDR) demonstrates that this is probably a reasonable approach.
I don’t want to get to ahead of what these tests already show, but to if you wanted to fully take advantage of a CAM based DRT, a single “ideal” render could designed around “Dark” surround output, but by changing the output to Dim, it would automatically transform the output and adjust the contrast and saturation for Dim conditions.
The same could be done with target white point, but both those features might only be practical for HDR since SDR is less flexible where max white lands and the highlight rolloff should still be carefully crafted to maximize the limited dynamic range.
That is why I only consider these tests as more of a sanity check (which I think was overdue, at least for me) to (re)understand what should be targeted and how that can happen inside the model, I hope they can serve that function at least a little.
This sentiment also very much applies to myself, and the approach I have taken with these tests.
It is all a bit hacky, especially the trying to compare scene 1.0 limited HDR with SDR on an SDR display, but I think it is a pretty good hack to at least provide a few talking points or references to help limit the confusion when discussing these transforms.