Thanks for all your input.
All the tests I describe below is just I/O + format conversion in Nuke. I didn’t do anything else with the image.
@Jeremy: I haven’t tried it on mac, on PC I made jpeg and tga seq. I exported tga seq with rec709 colorspace. QT player showed it darker, but when I loaded it back to Nuke, there was no difference from the original.
@Walter: Now everything on mac: I import the original file (ProRes HQ 422)to Nuke with rec709 colorspace and export a test.mov with exactly the same settings (ProRes HQ 422 and rec709 colorspace). I import the test.mov to Nuke again and there is no difference between the original and test.mov. (OK some recoding artifact, but only visible at maximum gamma value)
In QuickTime player test.mov is definitely brighter that the original.
I guess QT player sees the two (almost identical) video differently and I can’t do anything with it.
Thanks again.
Gyuri