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BT.1886 (not?) and ACES Rec 709 Output Transform

https://forum.blackmagicdesign.com/viewtopic.php?f=21&t=75869

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BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Fri Jun 29, 2018 4:00 pm
by David Cherniack
I'm starting this new thread because I've discovered that Resolve almost certainly outputs Rec 709 as straight gamma 2.4, not BT.709 when working in ACES color science. (I would hazard a guess it's the same in BMCM). This has had me confused for some time but today I ran some tests that are pretty conclusive. Hopefully someone will jump in and correct me if I'm wrong but some clarity from Blackmagic would help.

Settings:
Settings.JPG
Settings.JPG (103.55 KiB) Viewed 2493 times


Using Calman I calibrated two very accurate 3D Color Viewer LUTS for my Dell P2715Q UI monitor: Rec 709 BT.1886 and Rec 709 gamma 2.4. The gamma curves are different as expected

BT1886
Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG
Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG (35.05 KiB) Viewed 2493 times


gamma 2.4
Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG
Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG (38.46 KiB) Viewed 2493 times


I then captured SMPTE bars in the Color Page viewer after separately applying the two LUTS to the 3D Color Viewer slot.

CONTINUED IN POST FURTHER DOWN TO INCLUDE INLINE IMAGES

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Fri Jun 29, 2018 4:17 pm
by David Cherniack
I can't seem to add more than 3 jpegs within the post. Is there a limit? Size restrictions? I can't find any info.

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Fri Jun 29, 2018 4:24 pm
by Jean Claude
David Cherniack wrote:I can't seem to add more than 3 jpegs within the post. Is there a limit? Size restrictions? I can't find any info.


Yes : only 3 images per post. it's a limit (open a next post, etc...)
For size, I think limite is <1MB (not tested the max) :)

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Fri Jun 29, 2018 4:49 pm
by David Cherniack
contuned from above
BT.1886
3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG
3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG (105.12 KiB) Viewed 2487 times


gamma 2.4
3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG
3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG (94.52 KiB) Viewed 2487 times


I then output a tiff and captured it in Windows 10 Pro's Photo Viewer
Tiff output ACES OT 709 .JPG
Tiff output ACES OT 709 .JPG (129.84 KiB) Viewed 2487 times


cONTINUED NEXT MESSAGE

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Fri Jun 29, 2018 4:52 pm
by David Cherniack
If the images appear in this post as I think they're supposed to (they don't show in the preview) it's pretty clear from the near black chips in SMPTE bars that the Rec 709 output matches the 2.4 calibration much more closely. Given the variables of screen cap and jpeg they're probably a precise match. Here they are lined up and enhanced equally in Photoshop:

Black Chips.jpg
Black Chips.jpg (27.88 KiB) Viewed 2487 times


This would appear to indicate that if one is using the 709 transform to output from an ACES project it should be graded on a monitor calibrated to 2.4, not 1886. It may well be the same with BMCM.

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Sat Jun 30, 2018 2:43 pm
by Cary Knoop
The question is why do you need (to master in) BT.1886?

Re: BT.1886 (not?) and ACES Rec 709 Output Transform

PostPosted: Sat Jun 30, 2018 4:14 pm
by David Cherniack
Cary Knoop wrote:The question is why do you need (to master in) BT.1886?


I don't. I do need to know what calibration to apply to my monitor in order to grade properly. Without knowing what gamma curve is applied by Resolve on Rec 209 output it's a bit of a guess.