Colorgrading in P3 - gamma shift

Hi, I'm currently grading a feature doc in 4K and P3 color space. I exported a temporary version without credits, which has been validated in a theater (after converson to DCP). I'm now exporting the credit sequence and for the life of me I can't figure out what's going on, the blacks are not at zero any more . Some time has gone by, I'm actually reopeing a project which I haven't worked on in a few months. I have NOT done any upgrades, still working on Resolve 16.2.6 on a 2014 MacPro running MacOS 10.13.6 (High Sierra).
I am exporting to DPX. MY original export is fine, i can reimport it, the blacks are fine, but my original timeline, which I exprted from has the blacks at 4 or 5%. And when I export my title sequence and reimport it, the blacks are clearly different from my previous export. I'm monitoring my signal through HD-SDI and an external Ultrascope. I've checked my settings, my timeline and output are in P3. To make things worse, when I set my monitor (a Flanders CM240 to P3 the image has a green cast to it). Also, Resolve is crashing quite regularly (yes I know I'm pushing it with this old mac...).
Anyhow, if someone thiks they may have insight to what's going on here...
Cheers,
Damien

To make things worse, when I set my monitor (a Flanders CM240 to P3 the image has a green cast to it).

Not surprised. You used your FSI monitor set to P3 to grade this? Not something I'd ever do.

Not surprised. You used your FSI monitor set to P3 to grade this? Not something I'd ever do.

Sadly, this is true. We always do theatrical trim passes in a theater, even if we just rent the room for a couple of days. It is true that DCI-P3 is 6300°K rather than the usual 6500°K of Rec709 (a bit greener), and I believe DCI calls for 2.6 gamma. From Wikipedia:

DCI-P3 uses a pure 2.6 gamma curve, a white luminance of 48 cd/m2, and a whitepoint with the same correlated daylight temperature as D63 but greener. DCI-P3 has a 25% larger color gamut than sRGB.


I react differently to big images on a screen vs. the monitors we use in post -- even a big 55" OLED screen doesn't have the impact of a 16' or a 20' projector -- and I tend to see little mistakes very easily in projection. I also find when they translate a Rec709 grade to P3 DCI, I tend to chase black level problems more than an anything else: it's not a predictable change.

If I were you, I'd make a copy of the timeline, try a global using a final node added to every shot as a "compensation correction," and see if that's a better starting point for DCI.